what to use on a bench power supply to tie negative to ground
Every demote needs a ability supply to fully complement the rest of its test equipment. I consider this to exist just below the DMM in essential exam equipment. The question arises, though, as to what type of supply and what features should be incorporated and so equally to get in as universal as possible.
Background
I have designed and built a dozen unlike power supplies over the years, only recently I built the i presented in this commodity. It is the culmination of many years on the test bench, which I feel will cover about situations without going to ridiculous extremes.
When I started this design, I set down certain parameters that I idea were necessary and threw out some that I felt were of too express use to warrant the actress circuitry and expense. I based these decisions on previous supplies I had built. Some of my objectives were the following: smallest physical size possible so every bit to take up minimum demote space; separate analog and digital supplies with floating grounds; at least ii analog supplies with easily variable voltage; full metering on both these supplies; and final simply non least — good regulation and stability.
The final blueprint came out something like this: dual 0-20V @ 1A; 5V fixed @ 1A; no-load/full-load regulation <10 mV; RMS ripple < 0.five mV; and simultaneous voltage and electric current metering on both variable supplies. This gave the capability for design/repair of all depression power control circuits, but still have plenty "oomph" for mid-power circuitry.
From previous feel, I take found that dealing in high-power circuits (which doesn't occur that often), the requirements for that power are all over the place. Information technology would exist difficult and expensive to blueprint a supply to handle all these situations. At present, I have only one high-power supply — a 150 watt, 13.2V unit of measurement for standard mobile use (auto, marine, etc.). For the occasional circuits that I build under the loftier-ability heading, I construct a supply just for that device and its particular needs.
Since all outputs are floating with respect to grounds, this can be ready for ± supply voltages (my preference) for analog circuitry and even so have a digital 5V source available. Also, all the supplies can exist "serial-ed" together to provide over 45V at 1 amp for higher power when needed.
Considering of limited panel space due to the unit's relatively small size, I deleted some features that others might consider essential. One of these was electric current limiting. I have rarely experienced much do good from this. Whenever I accept accidentally shorted something on the breadboard, usually the belch of 'onboard' capacitance will burn a device out in curt social club. With dual metering for voltage and current, I tin can constantly monitor this when the circuit is powered up. Upon initial power-upwardly or whatsoever circuit changes thereafter, I can slowly bring upward the voltage to the desired level while monitoring the current for unusual backlog. As far as protection to the ability supply itself, the IC regulators have built-in overload and thermal shutdown.
In the by, my designs accept incorporated a lot of discrete circuitry for regulation, etc. Since this unit of measurement would have a maximum rating well within the capabilities of readily-available IC regulators on the marketplace, I decided to check them out as to their claims. They all lived upwards to their specs and performed also as anything I had designed previously. Their only shortcoming was the lack of provision for remote voltage sensing, but I figured I could live with that by up-sizing my connexion leads and the fact that I would only be dealing with a maximum of i-amp loads.
This article will deal with the power supply every bit built, but will besides exist interjected with a lot of options and full general guidelines for ability supply design. To offset with, I had the option to get with a linear design or a switchmode design. The linear design pros are, in general, better line/load regulation and less ripple. The switchmode pros are better efficiency and less weight. The advantages of the switcher were of no effect here, every bit we do not need mobility or great efficiency in this situation. The last thing one needs on a exam bench is a source of EMI, which the switchers are prone to produce!
For the analog supplies (0-20V), I chose LM317K regulator chips with a TO-three instance for improved heat transfer power (more than on this later on). Nearly of this design is pretty much right out of the book and you lot can Google LM317 (caput for National Semi on this page) for a lot of boosted information. The 5V stock-still supply is based on simple 7805 pattern.
Theory of Operation and General Guidelines
Most of this discussion on circuit theory, options, and some full general guidelines in linear power supply design volition refer to the schematic in Figure 1.
FIGURE 1.
To begin with, the centre of the supply is the chief transformer(south). In one case the required output voltage and current are determined, pattern of the raw B+ section can proceed. In this unit, I wanted slightly over 20.0 volts at a full load electric current of one amp coming out of the regulator scrap (LM317). These regulators need a minimum of iii volts across them (Vin-Vout) for proper operation. The valleys of the input ripple plus the average DC must remain loftier enough to support the regulator's required three-volt differential, and puts the near strain on the raw B+ at full load current. As well, one must consider line voltage sags which farther necessitates an increase in raw B+ to encompass these situations.
All these factors that require an increment in raw B+ for reliable operation are known as "headroom." Headroom is the available B+ (pure DC component) under worst-case conditions as compared to the B+ needed under ideal conditions. Regulators love a lot of headroom to encompass all situations encountered and nevertheless operate reliably. Regulators also hate a lot of headroom due to the increased voltage across them, which increases wattage dissipation, which increases internal junction temperature and, if it goes loftier enough, could somewhen destroy information technology. Then, as is the usual case with electronic designs, we are faced with tradeoffs.
In this supply, I have stated that nosotros need xx volts of regulated output plus a minimum of 3 volts differential under whatsoever load. This supply also has an RMS ripple of 0.half-dozen volts at full load. That means the negative peak ripple (1.41 x RMS) volition dip the average B+ by 0.85 volts. We as well still have to consider line voltage sags. These days, electric line voltages are pretty stable, then I merely allowed for a 5% reduction hither. Adding all these factors (differential, ripple, and line sag), we need 25.0 volts for worst-case conditions, which would be under total load current, in regards to raw B+. In transformers of the VA range used here, the voltage would ascension almost 20% under no load conditions and will produce a raw B+ of thirty volts at that point. This is the minimum headroom at idle.
A full general rule-of-thumb when designing total wave rectifiers that are feeding a capacitive input circuit is to size the transformer secondary current at one.five times the filter capacitor's average DC output electric current. This is due to the high charging currents needed to maintain the capacitor at peak or near peak voltage. Assuming a one volt drop (or two volt for bridges) across the full wave rectifier, nosotros demand a summit voltage of 31 volts to each rectifier from the secondaries. This would crave a transformer secondary of 22 volts RMS (44 volt center tapped) and a electric current rating of 1.5 10 one amp (the DC output electric current) or 1.v amps.
I was fortunate to have salvaged one of these transformers from one of my obsolete supplies — a 120V master with a pair of 44 volt middle tapped secondaries at one.v amp each. Although these transformers are bachelor, you may find them pricier than the common 'garden diverseness' 24 volt secondary ones. With these (assuming you couldn't find a 48V with a middle tap), you can substitute a full wave span circuit in lieu of the one-half moving ridge rectifiers. Just make certain the electric current ratings are every bit mentioned.
Tin't find one with dual secondaries? And then apply two single transformers — one for each variable supply. Two will take upward more cabinet space, simply not much more. If you choose to make your supply for higher/lower maximum outputs, then size your transformer appropriately. The main idea here is to allow acceptable headroom for the regulator, merely no more than necessary, to reduce wattage dissipation at that point and the subsequent increase in heatsinking.
1 note of Circumspection here! I testify a 35V rating for C1,C2 in the Parts List based on a 22V transformer secondary. If you choose to become with a 24 volt or higher secondary, you will need to increase this rating to 50 volts every bit it puts y'all just over the limit. And while I am on the bailiwick of C1, although I ordinarily figure nigh two,000 µF/amp for input filtering, I went to iv,700 µF on these supplies in gild to reduce ripple and yield more than headroom above the regulator.
The LM317 regulation circuit is pretty much right out of the book, so I will but run through a quick explanation of how it works. The voltage across the output pivot and adjust pivot is internally set at i.25 VDC. A resistor placed across these points volition create a abiding current source at the Adj. pivot junction and that current volition be determined by the value of R6. This abiding current will exist returned to raw B+ through the regulator'southward output pin. The information sheets call for a minimum of five mA quiescent electric current through the chip for reliable operation. Since the current through R6 is, for the about part, the regulator's operating current, R6 was called at 240Ω to allow for a 5.2 mA flow to satisfy that requirement. This is likewise the constant current that will menstruum from ground up through the voltage adjusting resistance P1,P2. A 1% resistor is used here, not so much for accuracy, but for stability. When P1,P2 are added to the circuit at the constant current point, they will vary the voltage by virtue of their chosen resistance setting. The output will track this voltage plus the constant 1.25 volts (adj. pin to output pivot).
However, when P1 and P2 are at naught, the regulator still puts out 1.25 volts. In society to have its output to zero volts, this voltage has to exist cancelled. This is where Z1 comes into play (a precision 1.225 volt Zener) by developing an almost exact voltage of opposite polarity. Its excursion consists of negative half moving ridge rectifier D3, filtered by C2 and current express by R4.
Now, when P1 and P2 are at zero, a negative one.225 volt level is presented at the electric current source point and calculation this voltage to the positive 1.25 volt constant of the regulator equals zero volts on the output or near so. On my unit of measurement, I have 0.015 volts and 0.019 volts, respectively, on the variable outputs at "zero" setting. Going back to the voltage adjusting pots P1,P2, their values are determined by two things: the desired output voltage of the regulator and the constant current source. Again, I'll use this unit of measurement equally an example.
The abiding electric current was set at 5.2 mA as previously described. Also, since I am taking the low cease of the adjustment string to a -one.225 volts which almost nullifies the regulator'southward constant of +1.25, the output voltage will track the exact voltage drib beyond P1 and P2. For a maximum output voltage of twenty.0 volts, we need a 20.0 volt drop beyond P1,P2 and — according to Ohms police — R=E/I = xx/0.0052 = 3850 ohms full.
From past experience, I knew that a unmarried turn pot would non requite a smooth enough voltage adjustment for this nifty of range. It would demand well-nigh x times as much travel for smooth transitioning. I was faced with two choices: a x turn aligning pot or two pots with a 10:i ratio. A 10 turn pot is as well expensive and time-consuming for repeated adjustments, and so the combo unit was the clear selection here. These pots should have a resistance ratio of nearly 10:1. I chose 5K and 500 ohms based on the resistance calculated. In do, the fine pot is unremarkably left at its midpoint and then tweaked subsequently the fibroid pot has set the judge output desired. This now allows for a fine adjust of ±1.25 volts — a very optimum span.
With the fine adust pot (500 ohm) set up at its midpoint and putting 250 ohms into the resistance cord, this is subtracted from the 3,850 ohm full previously calculated and leaves usa with 3,600 ohms to add for P2. Since the nearest standard pot is 5K, it volition have to be shunted to accomplish the value we need. This is achieved with R5 (12K) and is calculated based on a five,000 ohm pot. Bear in mind, most pots take a ±20% tolerance and R5 will take to be adjusted equally required (hence, the asterisk on R5).
With P2 (fibroid) at maximum and P1 (fine) in its mid position, the voltage output will be twenty.0 volts, and P1 will permit for a fine adjust of ±i.25 volts throughout the total range. With P1 and P2 ready at minimum, the voltage output will be zero or nearly so. One additional note is that there is a minor mistake current flowing through the P1,P2 string and into the regulator's adj. pin. This is approximately 50 µA and has been omitted from prior calculations for simplification and considering information technology is of little consequence hither.
C3 is added for input bypassing and stability insurance. D5 is a protection diode and will shunt any external capacitive belch around the regulator rather than through it; in any event, causing a more positive voltage on the regulator output than its input. C4 gives much improved ripple rejection and D6 prevents unintentional C4 discharge through the IC, again shunting these currents around the regulator via D5. C5 and C6 are added for improved transient response.
As for the LCD panel meters, I selected the smallest I could find (simply still maintain adequate character height for readability) due to limited front panel space. They are all 200.0 millivolt meters. The voltmeters are ready for 20.00 volt operation by proper decimal location and input scaling. R7, R8 are the scaling resistors and P4 is added for calibration. These trimmers are modest and inexpensive and are much more than accessible than the panel meter's internal calibration pot.
The current metering presented a small problem. I wanted to measure 0-i amps in milliamp steps. This equates to 100 µA/mA since this is a basic 200.0 mv voltmeter (no decimal brandish is used here). The shunt required for this conversion is 0.i ohms (R1). I could not add together this into the positive output line every bit at that place is no provision to keep information technology in the regulator'due south feedback loop.
This would have presented an increasing regulator load electric current error. For that and other reasons, I did not want it in the negative output either. However, it could be added ahead of the regulator where its voltage drib would exist of no consequence. That location presented one other problem though, and that was, even with no load electric current, there is still the regulator quiescent current of v.2 mA flowing through this point. This had to be dealt with as fifty-fifty with a 5 mA load current, the meter would evidence over 10.2 mA — a 100% error at this indicate.
The solution was to outset the quiescent electric current and that is the part of R2, R3, P3. Since i mA is read for every 100 µV developed across the shunt, a negative 520 µV beginning had to be applied to the panel meter's positive input pb. The values of those components provide just that, so that at no load the quiescent current is exactly cancelled out giving a meter reading of 0.0 mA.
Even though the raw B+ voltage sags at full load and introduces an fault in this divider string, it merely amounted to a ane mA fault at the full load of 1,000 mA. This fault is far beyond the panel meter'south accuracy and tin can be ignored. The second variable supply is not shown in particular on the schematic because it is a carbon copy of the outset one. Just bear in mind, you will need double the components shown to complete both supplies. A diode (D4) to ability a 14V LED console light is attached to one of these supplies to indicate DC ability on/off. Y'all will discover I have incorporated a four pole-single throw switch for all DC supplies and the indicator lite. When I switch off DC power to a circuit, I want but that — no power! Past merely switching off Air-conditioning power, there is a bleeding off DC voltage present at the panel jacks. This could hang on a long fourth dimension for calorie-free loads. Other reasons for divide Air-conditioning and DC power switches are to eliminate transients presenting themselves at the font panel jacks upon Ac ability upwardly/ability down, which can occur at the first few milliseconds of Air-conditioning switching.
Another reason is that when using a negative reference voltage such as Z1(-1.225V) for the regulator and due to unequal charge discharge times of C1, C2, a 1.5 volt step-upwards in voltage can occur upon Air-conditioning shutdown only. This is not an like shooting fish in a barrel problem to overcome to comprehend all situations. Again, DC switches block this undesirable occurrence.
All my power supply designs accept incorporated carve up AC and DC switching for these and also to give you a setup procedure while the DC power switch is off, every bit the metering will withal be active. If one were to but elevate this supply to the test demote for a couple of hours at a fourth dimension and then render information technology, there really wouldn't be any need for an Ac switch — just plug and go! But for my utilize, this supply will become a permanent fixture on my test bench, so I decided it needed Ac switching.
Permit'southward discuss the five volt supplies that are shown in Figure two.
Effigy 2.
There are five of these, simply it's not equally bad as information technology sounds. There are some options hither. The first of these is the 5V at 1A front panel supply. This is and then straightforward, it about needs no explanation. T2, BR1, C1 provide raw B+ to the regulator flake IC5 (7805), which and so does a fine job of regulating it to +5 VDC. T2 has a single ix VAC secondary at 1.5 amps and was called for minimum voltage differential across IC5 for reduced wattage dissipation.
The other 4 five-volt supplies are for the iv forepart panel meters. These are all meters requiring isolated ability supplies. Ii of these supplies could be eliminated for the forepart panel voltmeters, since the meter's negative input is connected to mutual. A 78L05 (TO-92 instance) could be connected to the raw B+ supply and power these meters directly from its positive and common. You lot will need 1 of these for each supply y'all are metering. The meters volition accept to exist the type that operate with common grounds (negative input lead and negative power lead tied to common).
Another choice is to utilize only one meter per supply and switch them from volts to amps, with regards to their input leads. These will crave isolated power supplies, but only two instead of four. I have to admit that the meters used in this projection are by far the greatest price of constructing it. I wanted the luxury of full metering all the time, however, and wouldn't accept done it whatsoever other mode.
The four supplies for these meters accept upward very little room and the total cost for all the components required was but over $five. These supplies demand simply half wave rectification and little filtering since load requirements are on the order of 5 mA each.
Structure
I congenital this unit as minor as possible, only you may want to open up upwards the overall dimensions slightly. The power supply's overall dimensions are 9" wide 10 three-one/2" high x 6" deep. If you alter the design, lay out all your parts starting time to get specific measurements for chiffonier clearances and dimensions needed before starting construction. When I make upwardly an enclosure for circuits requiring a off-white corporeality of estrus dissipation, I unremarkably build it as follows (see Figure 3).
FIGURE iii.
The bottom and back are made of i piece of 1/8" aluminum sail. A 90 degree bend is formed at the intersection of the lesser and back. In my case, this was a sheet 9" x 9-1/2" with the 90 degree bend three-1/2" from the long finish. This part of the enclosure volition likewise double as the heatsink, eliminating a lot of expense and clutter. Pop rivet three ane/2" aluminum angle brackets to the sides and front of this base, flush with their edges. (In my unit, these were 2 6" lengths and i 3-iii/iv" length.) These will exist the attachment points for the forepart panel and top comprehend.
Next comes the front panel. This was a piece of 1/xvi" aluminum cut to three-1/2" x 9". Lay out mounting holes at the bottom and transfer these to the front angle bracket on the bottom plate. Drill and tap these for the front console mounting screws.
Adjacent, the metallic cover is formed. Mine was 16" x six-1/2" with two 90 caste bends, 3-1/2" from the long ends, to form the sides. The 6-1/two" dimension gives the cover a little overhang on the front and back of the enclosure. Drill two mounting holes at the bottom of each side. Marshal the encompass to the base and transfer those holes to the side bending brackets. Drill and tap these for the cover mounting screws.
Some plumbing fixtures may exist necessary equally you are constructing this. Disassemble so later pigment it the color of your option, merely do non pigment the ane/8" bottom canvass. Add together four anxiety (can be self-adhesive) and the bones enclosure is done. Strong, functional, bonny, and unproblematic, to boot!
At this point, the major components tin be installed. Make sure you take allowed ample room and clearance in regards to overall dimensions for the components you have selected. I installed the main power transformer (T1) in the rear centre of the lesser plate. This is a logical mounting indicate and gives the unit of measurement good balance. On either side of T1 are ii excursion boards spaced and stacked on standoffs, and measuring 2-3/eight" wide ten iii-1/ii" deep. (That will be 4 boards full.) To the left side of T1 is the five volt panel supply transformer T2, a low contour type mounted to the bottom board. Standoff mounted, the upper board is one of the 0-20 volt (A) boards.
To the right side of T1 are the five-volt supply regulators along with T3, T4, and is the lesser board. As withthe left side, the mounted collision is the other 0-20 volt (B) board. The rear panel receives ii LM317K ICs mounted directly behind their respective regulator boards. The 7805 (IC5) chip is mounted to the bottom plate near the front end left manus corner of its regulator board. F1, S1, and the line cord input are mounted through the back console wherever it's convenient. The front panel components should be laid out in a nice symmetrical lodge and and so machined. Watch for clearance here to interior components!
For front panel labeling, I have been experimenting with diverse escutcheons. Recently, I have been using some of the CAD programs from PC and front panel manufacturers. Any CAD plan volition work as long as it gives you a reference signal, precise layout cursors, and the ability to add together text. For this console, I used the free CAD program (Front Panel Designer) from Front Panel Express LLC to lay out and label it. From in that location, you have two choices: immediately e-mail this to the manufacturer for product or make up your own.
Figure 4 shows the escutcheon I made up from the CAD program.
FIGURE four.
1 click of a tool push and you tin hibernate lettering and show layout points for machining the console. These files will impress out to exact dimensions. When I brand my own escutcheon, I print out the image of Figure four to matte photo newspaper and then double coat it with Krylon Crystal Clear Finish. Let dry and apply to the front console with 3M spray adhesive. Pigsty trim out can be done before or after. For this project, I treated myself and had the manufacturer make upwards the panel. Pricey, just they do ane cute chore. However, the photocopy method does produce a very professional front panel expect, particularly with blackness impress on a natural aluminum background.
One last note — the particular panel meters I used were chosen for their pocket-sized panel mountain size since panel space was at a premium. I besides chose these with the isolated power choice. For some reason, I seem to become ameliorate low cease stability with isolated power supplying these meters. A 4 pin SIP header was continued to the meter leads via 3" #24 judge wires, and a matching 4 pin SIP socket was soldered to the forepart of the board. This gives easy install/removal of the front panel.
Heatsinking Details
Any article on linear power supplies would not be complete without some give-and-take on heatsinking. A lot of readers are aware of the demand for heatsinking merely are non familiar with the details of requirements. Heat transfer — as simple as it sounds — can be an extremely complex scientific discipline, then I will try to keep this discussion simple.
To begin with, all materials take a given rate of rut transfer, which is their ability to dissipate heat generated within them. This ability is known every bit thermal resistance and is shown as the Greek symbol theta (θ). This volition exist shown hither as Tr (thermal resistance) and actual temperature equally T. The lower this quantity is, the lower the Tr, and hence, the greater rate of heat transfer. The rating is given as degrees C/Watt and is the ascension in temperature in degrees Centigrade for one Watt of dissipation within a given object. The lower the number, the better the dissipation factor. A loose electronic analogy would be the discharge rate in an R/C circuit, with accuse being coordinating to heat and R representing Tr. The lower the R is, the more rapid the discharge rate or oestrus dissipation is.
We outset our calculations with the manufacturer's maximum rating for a semi'south junction temperature. For almost all commercial grade semiconductors, this will be 125 degrees C (150 for war machine). This ways that even operating at worst-case conditions, nosotros must proceed that device under that temperature, preferably under 100 degrees C (100 C is not ever possible or practical).
Ii factors dominate the semiconductor junction temperature rising: the wattage developed across the device and the Tr to the ambient air surrounding information technology. I will use this supply as an example. The offset hurdle we must cantankerous is the Tr of the junction to its case. This is noted as Tr j-c. I chose the TO-three version of the LM317 because it has a lower Tr j-c than the TO-220 package. The manufacturer specifies a Tr j-c of 2.iii degrees C/W. This means that for every watt of dissipation at the junction, the case temperature will ascension by 2.3 C. We always want to look at the worst case here to determine maximum temperature rising. In this example, I will employ a near worst-case example of 5 volts output at a one amp load. The raw B+ at this point is approximately 25 volts, and by Ohms law, we have 25V-5V x ane.0 amp or 20 Watts of rut dissipation within the regulator fleck. The 317 case temperature will rise to 2.3 degrees per Watt or 46 degrees C for our 20 W. This is a given we have to live with, just from this indicate on, nosotros do accept some control.
The side by side hurdle to cross is the Tr of the insulating textile between the case and its mounting surface. This is noted as Tr c-s (thermal resistance of case to heatsink). This is most nix if the case is mounted directly with no insulator. If the case is not electrically isolated from the chip, we have to use an insulator, as is the situation here. I used mica insulators and high quality thermal grease, in which the manufacturer specifies a Tr of 0.35 C/W.
Ane word of caution — NEVER utilise production type self-adhesive insulators as they are terrible and seem to perform even worse with thermal grease. Also, I would similar to mention at this point that the heatsinks mounting surface should be perfectly flat with no burrs from drilled holes. When insulated like this, mounting is accomplished the easiest with nylon screws and nuts.
The last hurdle to cross is the Tr s-a (thermal resistance of the heatsink to ambient air surrounding it). Upwardly to this point, we have a Tr j-c of 2.3 C/Due west and a Tr c-s of 0.35 C/West for a full of ii.65 C/West and a semiconductor dissipation of 20 Due west. The temperature ascension at the heatsink signal-of-contact volition be 2.65 C x xx W or 53 C. Conversely, the semiconductor junction will be 53 degrees C hotter than the heatsink at this betoken, and if nosotros were running the function at the maximum temperature of 125 C, that means this contact point would now be 125 C -53 C or 72 C.
Bench test equipment is normally used in an surroundings not exceeding 25 C (77 F) and this is as low as the heatsink temperature can become. The heatsink cannot be allowed to rise in temperature more 47 C (72 C - 25 C) in guild to maintain the maximum junction temperature. Heatsinks are rated by their thermal resistance Tr s-a (sink to ambient air) which is given every bit degrees C/Watt. Since nosotros are dealing with twenty W and plugging this into the formula, we take 47 C/twenty W or two.35 C/Due west. This is the minimum heatsink Tr we tin get away with. In practice, we would want a heatsink with less Tr for a greater margin of safety functioning, probably on the order of 1.five C/W. This would reduce the maximum junction temperature to 108 C for worst-case situations.
From here, you would look up a manufacturer's heatsink for this rating and it would probably be fairly sizable. Also, y'all need one for each 20V supply. Every bit a thing of interest, the manufacturer cautions against too much heatsinking on the 317 then that they can reach their thermal shutdown temperature when normal overload occurs.
From past experience, I knew the heatsinking capabilities of the chassis used in this unit of measurement would exist adequate, so I didn't bother with precalculations. Post construction tests diameter this out by measuring case temperatures at near worst-case for the variable supplies running individually and normal case tests with all supplies running meantime.
These tests were at full load current and case measurements taken after one hour of operation. The junction temperature was computed by the case temperature, Tr j-c and wattage (20 W) to arrive at a effigy for this. The variable supplies were almost 115 C, which is just about perfect in regards to overload vs. thermal shutdown. The five volt stock-still supply was at 69 C. Even though the TO-220 case of the 7805 has a college thermal resistance, it runs with much less wattage across it, hence the lower junction temperature.
With all supplies running concurrently at 20V-5V-20V and serial up to a one amp load current, the junctions were 65 C - 80 C - 65 C, respectively. This is well under the maximum operating temperatures of 125 C. These were tested with the cover in place and with no ventilation holes. I may add together vents at a later date, which would undoubtedly improve these readings.
The current rating on this unit is very conservative and will really output closer to 1.five amps, before dropping out of regulation. All the same, it wouldn't be advisable to practice this at low voltage settings for more than a couple of minutes due to heat dissipation. At higher voltage settings, it can exist longer (it all relates dorsum to headroom).
Conclusion
This supply has been in service for several months now, and has performed flawlessly. It has also been very user-friendly. If y'all exercise not currently ain a bench supply, build this ane. After a few usages, you lot will wonder how you always got along without 1. I hope that even if you don't build this unit, you will take gained some helpful pointers that will be of use in similar projects. NV
PARTS Listing | |
|---|---|
| ITEM | Description |
| 0-20 VOLT BOARDS | |
| T1 | 120V primary, dual 44V heart tap secondaries @ i.5A |
| D1,2 | 3A, 100 PIV |
| D3,4,five,6 | 1N4002 |
| C1 | 4700 µF/35V * |
| C2 | 100 µF/35V * |
| C3 | 0.22 µF |
| C4 | ten µF/35V |
| C5 | 22 µF/35V |
| C6 | 0.i µF |
| R1 | 0.1Ω 1% 3W |
| R2 | 16Ω |
| R3 | 750K |
| R4 | ii.7K |
| R5 | 12K * |
| R6 | 240Ω ane% |
| R7 | 100Ω |
| R8 | 910Ω |
| P1 | 500Ω |
| P2 | 5K |
| P3 | 200K |
| P4 | 200Ω |
| Z1 | 1.25 volts reference |
| IC1 | LM317K |
| 5 VOLT SUPPLIES Board | |
| T2 | 120V master, 9V [e-mail protected] 1.5A |
| T3,T4 | 120V primary, dual [electronic mail protected] 65 mA secondaries |
| C1 | 4700 µF/16V |
| C2 | 10 µF/16V |
| C3,v,7,9,11 | 0.1 µF |
| C4,6,8,ten | 47 µF/16V |
| BR1 | 2A, 100 PIV |
| D1,2,3,four | 1N4002 |
| IC5 | 7805 |
| IC1,2,iii,4 | 78L05 |
| MISCELLANEOUS | |
| S1 | SPST toggle switch |
| S2 | 4PST toggle switch |
| Panel Meters | 200.0 mV |
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Source: https://www.nutsvolts.com/magazine/article/a_test_bench_power_supply
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