My first power supply - Page 1
Forum section: electronics / projects

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 26, 2019, 01:02:45 am
content: They say you learn a lot from building power supplies. And I am. My first power supply is straight forward linear power supply. Not a ton of power but enough to power small circuits and toys.120vac - 12vdc 500ma center tap to bridge rectifier to smoothing - to regulator -to small caps .Powers on and functions great, still waiting on my fan to come in.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 26, 2019, 02:09:31 am
content: I can't be sure but it seems like you have 2 LDOs in parallelOne will back feed the other due to voltages not being exactly the same which is badalso did you consider a basic voltage adjustment pot? most LDOs are adjustable via a voltage divider circuit which you can make from 1 fixed resistor and 1 pot to allow crude adjustability of the voltage.

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 26, 2019, 02:15:23 am
content: Each are LM7812 and LM7805. Not adjustable. Not intended to have pot on my first one. Fixed voltages

author: OM222O
author_rank: Frequent Contributor
timestamp: April 26, 2019, 02:32:22 am
content: oh! that makes more sense now. just a quick note, for your smoothing stage you have only used a large electrolytic capacitor. LDOs usually specify a 100n ceramic capacitor on their inputs for noise suppression which you can put in parallel with the electrolytic one. Dave also has a great playlist detailing how to make a decent adjustable power supply which you can watch here:

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 26, 2019, 02:42:12 am
content: I added the smaller disc caps to the outputs but if needed i can add to the input as well.And I will review your linkThank you

author: MarkF
author_rank: Super Contributor
timestamp: April 26, 2019, 02:45:23 am
content: You should also have some protection diodes and a minimum load for each regulator.10mA to 15mA for the LED power ON indication works great as a minimum load. "Kills two birds with one stone."

author: Tek Tech
author_rank: Contributor
timestamp: April 26, 2019, 05:01:37 am
content: Quote from: queennikki1972 on April 26, 2019, 01:02:45 amIt's super-cute and serves your project needs. Most importantly: you made it as your first project!Congratulations!BTW, everyone here will give you an idea how to make it still better. You can use the advice for your next project, but I'd suggest that you leave your first baby intact as is.

author: kripton2035
author_rank: Super Contributor
timestamp: April 26, 2019, 06:51:06 am
content: remembering my first project 40 years ago, that was a mains-powered clock... that blew instantly once plugged in !I did not keep itanyway congrats for your first project, hope there are more to come.

author: ocset
author_rank: Super Contributor
timestamp: April 27, 2019, 12:28:22 pm
content: Ive attached a simulation in the free ltspice of a linear power supply.....its always helpful to do a quick sim of a cct before building it, as it helps highlight the little details that can sometimes be forgotten.As you know, SMPS can help you with the efficiencyBy the way, if you wish, here is a link to my free SMPS course (there is also some stuff on linears in there)This is the link from my google drivehttps://drive.google.com/open?id=1cmk81KxXA7-EliHHWr8ZimpEp7HCMIcYPlease tell me if it doesnt work.

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 28, 2019, 07:04:01 pm
content: @MarkF I found the 1n5395 obsolete and expensive. I also found this page that shows alternatives.https://www.el-component.com/diodes/1n5395Can you confirm that for example the RL207 would be a fair replacement for a circuit like the one above?I have many of the 1n4000 series diodes but they do not show to be a replacement

author: ocset
author_rank: Super Contributor
timestamp: April 28, 2019, 07:36:56 pm
content: almost any diode that is rated to at least VIN would be ok to replace that 1N5395....yes rl207 is finehttps://www.rectron.com/data_sheets/rl201-rl207.pdf

author: MarkF
author_rank: Super Contributor
timestamp: April 28, 2019, 07:57:25 pm
content: You can use anything in the 1N539x series. 1.5A diodes with 50V to 1000V rating.For examplehttps://www.mouser.com/ProductDetail/Vishay-Semiconductors/1N5393-E3-54?qs=sGAEpiMZZMtbRapU8LlZD0HbIjlpuZ44OLW52lnUrFQ%3DYou can use diodes from the 1n400x series. But, they are only rated at 1A. The MC78xx and MC79xx regulators are also only rated for 1A. Your choice. I just would not push the maximum for the diodes or the regulators.You can also use a bridge instead of individual diodes. I needed to buy diodes so I used the same value for all of them.1N539x datasheet:https://www.vishay.com/docs/88514/1n5391.pdf

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2019, 08:02:29 pm
content: Quote from: OM222O on April 26, 2019, 02:09:31 amWhen I was about 12 I built a power supply with 3 7812 regulators in parallel, it may not be advisable to do but it did work and I used it regularly for about 20 years before I finally rebuilt it with a switchmode regulator. Ironically that blew up within a year.So the point is, I wouldn't do something like that in a commercial product but sometimes you can get away with things.

author: MarkF
author_rank: Super Contributor
timestamp: April 28, 2019, 08:54:19 pm
content: If you are just adding the protection diodes to your old project, use the 1N400x diodes you already have.

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 29, 2019, 02:06:05 am
content: Yes MarkF, its a small transformer and fairly low current I was thinking that too about using the 1n4000 series for the small power supply, but I did order some 1.5amp diodes today so I'll have them. I am taking notes on everything you guys have suggested. I have no need for the variable pots on this project as I have other power supplies on hand. And I read up on why you have a third binding post for earth ground and that chassis ground (ac ground )should not be same as - negative binding post by default. i hope I said that right.

author: n3vti
author_rank: Contributor
timestamp: April 29, 2019, 02:25:46 am
content: Your power supply looks awesome!Maybe I should get back to rebuilding mine at some point... need to get a bench first.

author: queennikki1972
author_rank: Regular Contributor
timestamp: April 29, 2019, 02:30:05 am
content: Treez the linked work great. Thank you

author: MarkF
author_rank: Super Contributor
timestamp: April 29, 2019, 03:21:26 am
content: Quote from: queennikki1972 on April 29, 2019, 02:06:05 amNot exactly. You want "earth ground" and "chassis ground" to be connected together for safety reasons. Chassis ground being the metal enclosure you put your project in. You don't want any voltage on the metal enclosure. When you bring the earth ground to the front panel, you have the option to tie it to any of the other outputs as a reference.The "common ground" (labelled as such in my earlier drawing) is your circuit reference in most cases. Typically, people call it ground but it is floating and not tied to "earth" or "chassis" ground for most power supplies. If the common is left floating (i.e. the secondary transformer windings are NOT tied to earth ground), you can put multiple power supplies in series to create other voltages. In my triple supply above, you can tie the +5V COM output to +12V output. You would then have +17V, +12V and -12V outputs referenced to the +/- COM connection.

author: m3vuv
author_rank: Super Contributor
timestamp: April 29, 2019, 10:24:55 am
content: why not go down the lm327 route with pass transistors?

author: ArthurDent
author_rank: Super Contributor
timestamp: April 29, 2019, 12:12:31 pm
content: MarkF –QuoteHere is one of the first little supplies I put together a few decades ago that illustrates that above principle. It was just a small linear supply board with transformer that I removed from some piece of scrap equipment that put out a couple of hundred milliamps on each output. It was convenient to test small circuits so I added a switch for +/-12 and +/-15 VDC and built it into this small metal case. As you can see, the common for the 3 outputs is not connected to the earth/line/case ground but the ground terminal is available on the front panel.This project is old enough so it actually uses a line powered neon panel light and not an LED

author: Tek Tech
author_rank: Contributor
timestamp: April 30, 2019, 01:43:50 am
content: Quote from: james_s on April 28, 2019, 08:02:29 pmI've done the same thing: stacking several 78xx in parallel for more Amps and less heat and it worked surprisingly well. I'd suggest, for best results they all should be from the same batch.

author: MarkF
author_rank: Super Contributor
timestamp: April 30, 2019, 02:29:20 am
content: Quote from: Tek Tech on April 30, 2019, 01:43:50 amIf you put several regulators in parallel, you should put a series resistor on each output to limit the current due to small differences in the output voltage of each regulator. Refer to example from the datasheet:

author: rsjsouza
author_rank: Super Contributor
timestamp: April 30, 2019, 02:54:54 am
content: Good project and good looking supply - congratulations! Power supplies are always pretty cool as starting projects - I still remember the one I assembled with my dad in the 80's.Back in the day we did not have access to quality fans, thus we had to rely on large heatsinks for the bypass transistors (large TO-3 2N3055). Hey, at least the power supplies were quiet.As you progress, you may want to pay a visit toTanner Electronicsin Carrolton if you haven't yet been there. It has quite a great selection of parts for projects.BG Microis another local store (in Garland) that also has a good selection - they are an online shop, but you can always save on shipping by arranging local pickup.

author: Tek Tech
author_rank: Contributor
timestamp: April 30, 2019, 05:01:20 am
content: Quote from: MarkF on April 30, 2019, 02:29:20 amYes, I should, and I never did. But it worked quite well, trouble free. However, using chips by different manufacturers would definitely require resistors on the outputs.BTW, I also did the trick building home-made light dimmers long ago. I would connect two triacs in parallel (straight, no resistors) to reduce the heat. It was very effective.

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 21, 2019, 11:07:01 am
content: So, I did go back and add the protection diodes, green banana jacks tied to PE, larger heat-sinks and a larger transformer of 12v 1.8 amps. I did not use the adjustable pots as the LM7812 and LM7805 are not variable. I measured the ripple and it looks good. I am purchasing a electronic dc load after December so Ill be able to test the load. I've tested about .25 amp load and seems to run nice and cool.I've since built a variable supply with LM317T and working on a dual adjustable supply of 1.5-60vdc. With each type I learn more. After I fully understand the linear supplies Ill try some switch-mode supplies or some built using IC's and Transistors.

author: dom0
author_rank: Super Contributor
timestamp: September 21, 2019, 04:33:23 pm
content: Quote from: queennikki1972 on April 26, 2019, 01:02:45 amI would recommend a larger heatsink instead of a fan at these power levels, since the thermal resistance (K/W, kelvin per Watt, calculations just work like electrical resistance) of these tiny heatsinks is pretty bad and doesn't improve that much with a fan.

author: MarkF
author_rank: Super Contributor
timestamp: September 21, 2019, 05:05:26 pm
content: Quote from: dom0 on September 21, 2019, 04:33:23 pmI agree with @dom0. Your heatsinks are really small even with a fan.The LM78xx's are going to get hot if you expect to pull their full power.It looks like you are using a full wave bridge IC. It could use a bigger heatsink too.Here is a picture of a 51mm and two 38mm tall heatsinks on the triple supply circuit I showed earlier.Even these get hot at or near full power.

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 21, 2019, 10:17:39 pm
content: Also I noticed there is a small spike from the fan noise? on the the 12v side, since thats where the fan is connected. I agree about larger heatsinks as well. Is there anything you can put inline with the fan to suppress the spikes? Maybe a larger output capacitor? It is minimal but just wondering. The 5v side is much smoother.

author: xavier60
author_rank: Super Contributor
timestamp: September 21, 2019, 10:34:46 pm
content: Quote from: queennikki1972 on September 21, 2019, 10:17:39 pmIt's difficult to completely remove the spikes by increasing the output capacitor. It could help.A simple series resistor will also reduce the spikes. You could experiment with the value. Larger values will give better filtering but also drop more voltage.A DC choke wold be more effective, but check for excessive spiking on the fan side, A capacitor might be needed directly across the fan also.Extra: A good option is some series resistance plus a capacitor directly across the fan.

author: OM222O
author_rank: Frequent Contributor
timestamp: September 23, 2019, 01:53:23 am
content: using a common mode choke followed with 4 capacitors and an inductor (2 large value electrolytics / tantalums and 2 small value ceramics) in a pi filter configuration should basically drop that switching noise to nothingalthough I think the pi filter alone should be good enough in this case.

author: MarkF
author_rank: Super Contributor
timestamp: September 23, 2019, 02:00:03 am
content: You could also add a series resistor with the fan from the unregulated side (before the LM78xx's). You will have to experiment a little with the resistor value to get the fan voltage down to 12V (and maybe 10 or 11 volts). If the fan still puts out enough air flow, it will be a little quieter with a slightly lower than rated voltage.Also since you are driving both regulators, the current capabilities of both the transformer and the rectifier bridge must be large enough for both (i.e. greater than 2A). The capacitance of the filter cap must be large enough for both also. You might actually be better off with two before the regulators.Edit: I just checked your first post. The transformer output is only 12V CT @ 500mA.You are NOT going to get a lot of power from you supply.Watch that the combined output of both regulators is less than 500mA.At 500mA and larger heatsinks, you probably do not even need a fan.

author: MarkF
author_rank: Super Contributor
timestamp: September 23, 2019, 02:02:10 am
content: Quote from: OM222O on September 23, 2019, 01:53:23 amIt is not a switching power supply. A linear power supply with fan noise on the one output.

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 24, 2019, 11:08:31 am
content: Quote from: MarkF on September 23, 2019, 02:00:03 amI upgraded the transformer to one I had lying around that is 1.8 amps, not 500mA since this is just an experiment for learning..I learned from this project that I needed not only separate smoothing caps, but also at least 2 per side for redundancy. I will probably redo the entire main board to have separate caps and larger surface area for heatsinks.

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 24, 2019, 11:11:05 am
content: Quote from: MarkF on September 23, 2019, 02:00:03 amI don't know why I didn't think of that!!!

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 24, 2019, 11:23:53 am
content: Quote from: MarkF on September 23, 2019, 02:00:03 amBridge Rectifier is 6A and the one large cap is 6800uf . When I redo the new board Ill put 2x2200uf cap 35v per side.

author: mariush
author_rank: Super Contributor
timestamp: September 24, 2019, 11:49:56 am
content: My recommendation would be to replace those green prototyping boards with stripboards like this:https://www.ebay.com/itm/1PCS-DIY-Prototype-PCB-Matrix-Circuit-Board-Stripboard-Universal-8-5-20cm/253234159272?It does restrict you a bit because you'd have to place components like 3 pin regulators in such a way that each lead is on a separate 4 hole strip, but you have the added benefit of those continuous strips which can potentially carry more current that thin wires.You can also tin those continuous traces and floodfill the holes with tin if you wish.You can also place components closer to how they sit on a prototyping board and use short lengths of solid core wire to make connections between 4-hole segments.For capacitance after the bridge rectifier you can sort of estimate how much would be needed based on your minimum desired voltage.Capacitance = Maximum Current / [2 x Mains AC Frequency x (Vdc peak - Vdc minimum desired)]So you're saying you have a 12v AC 1.8A transformer and ou're in US where mains frequency is 60Hz... that means your peak DC current will be approximately 0.62 x Iac = 1.16A but let's round it up to 1.2ALet's say you want minimum 14v dc voltage, because the 7812 regulator will have approximately 2v drop at 1A of current.When your bridge rectifier converts 12v AC to DC you get a peak voltage of sqrt(2) x Vac - 2 x voltage drop on rectifier diode so your Vdc peak is 1.414 x 12 - 2x 0.8v = ~15.37 v ... let's round it to 15.5vSo you can put the numbers in the formula and the minimum capacitance you need to have minimum 14v would be:C = 1.2A / [ 2 x 60 Hz x (15.5v - 14v) ] = 1.2 / 180 = 0.0066666 Farads... so you should have at least a 6800uF 25v rated capacitor there to get your minimum 14v.

author: MarkF
author_rank: Super Contributor
timestamp: September 24, 2019, 05:17:40 pm
content: Personally, I like the green protoboards. Stick with them.Quote from: queennikki1972 on September 24, 2019, 11:23:53 amI think that is a bad idea. Stay with the 6800uF 35V capacitor on the input and use 100uF caps on the output.At full load, you may actually need something bigger than 6800uF depending on the transformer and rectifier bridge voltages.If you already have 2200uF caps, leave room to add one later on if you find that the voltage is too low.I see nothing wrong with your original design other than the transformer is under sized for what is possible.A 3-5A CT transformer would maximize your output. But, what you have is fine.With that, you probably should keep an eye on the 6A rectifier. It may or may not get hot without a heatsink also.I show LEDs on the outputs which provide two functions. First, they indicate the power is ON.Second and more important, they provide a minimum for each regulator to work properly.YOU HAVE NO CURRENT LIMITING. Careful or you may have a BAD day. Not ideal, but okay as minimal supply.Stay with:Or with a CT transformer you could reduce the voltage drop (i.e. heating) of the 5V regulator:

author: MarkF
author_rank: Super Contributor
timestamp: September 24, 2019, 06:43:05 pm
content: F.Y.I.Here are the heatsinks I've used:https://www.digikey.com/products/en?keywords=EA-T220-38E- 1.5" heighthttps://www.digikey.com/products/en?keywords=EA-T220-51E- 2.0" height

author: OM222O
author_rank: Frequent Contributor
timestamp: September 24, 2019, 10:36:12 pm
content: Quote from: MarkF on September 23, 2019, 02:02:10 amone shall ask about noise / EMI reduction and one shall receive answers for noise / EMI reductionit doesn't matter what's switching, a PSU or a fan, that will significantly reduce the sharp edges and fast rise / fall times to basically nothing. Think of it as adding a lot of momentum to a heavy object. small bumps to it here and there won't change how steadily it moves (i.e: how stable the voltage is). the resistor method works fine too, but is less efficient and not as good when it comes to noise reduction

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 25, 2019, 12:01:55 pm
content: Quote from: MarkF on September 24, 2019, 05:17:40 pm

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 25, 2019, 02:08:09 pm
content: The Capacitor across the fan stopped the fan noise almost completely.

author: MarkF
author_rank: Super Contributor
timestamp: September 25, 2019, 07:29:56 pm
content: Quote from: queennikki1972 on September 25, 2019, 12:01:55 pmYou don't have sides unless you are going to use a CT transformer as in my second circuit.In your original circuit, all I was trying to say is you might want to add a 2200uF to the 6800uF since you are at the minimum as shown in @mariush's calculation. I don't think it is necessary for you to disassemble what you have. If you don't expect to pull maximum current, you could stay with just the 6800uF with larger heatsinks.If you want four 2200uF capacitors in parallel, that's okay too.Oh, you don't need LEDs either since your meters will provide the minimum load.You might want to review the calculation for the ripple capacitor value.Quote from: mariush on September 24, 2019, 11:49:56 amWith some visual aids. See also:https://www.electronics-tutorials.ws/diode/diode_6.htmlThe minimum capacitor isC = ILOAD/ (2 * fLINE* VMAX_RIPPLE)

author: MarkF
author_rank: Super Contributor
timestamp: September 27, 2019, 02:24:56 am
content: Quote from: queennikki1972 on September 21, 2019, 11:07:01 amBTWIf you would like to build a DC Load, here is a link to the one I built:https://www.eevblog.com/forum/beginners/struggling-with-dc-load/msg2698044/#msg2698044

author: queennikki1972
author_rank: Regular Contributor
timestamp: September 28, 2019, 03:49:51 pm
content: I did build a different power supply using a positive +12 and Negative -12 with a common in the middle and a center tap transformer. I used the protection diodes, the led indicators and larger heat sinks. Yes I am very interested in building the load. I was actually going to buy one but this sounds fun.

author: MarkF
author_rank: Super Contributor
timestamp: September 28, 2019, 07:13:02 pm
content: Quote from: queennikki1972 on September 28, 2019, 03:49:51 pmYou will want a large heatsink for the MOSFET. I used:https://www.digikey.com/products/en?keywords=RA-T2X-64EIt will definitely will need a fan also.The MOSFET I used was an IRFP064. I reworked the PCB and going to use an IRFP250 that's cheaper.I suggest something in a TO-247 package. Twice the size of a TO-220 for better heat dissipation.Design guidelines:- The (+) op-amp input (and hence the sense resistor) voltage range is 0V to 0.5V.You set the maximum current capability by selecting a sense resistor knowing its max voltage will be 0.5V.In the circuit the sense resistor is 0.200 Ω which results in a 2.5A maximum load current setting.I recommend you select a sense resistor in the range of 0.100 to 0.500 Ω (i.e. 5A to 1A respectively).

author: MarkF
author_rank: Super Contributor
timestamp: October 01, 2019, 10:18:51 pm
content: While building your Electronic Load, this schematic of the HP-6060 may be a helpful reference(It was posted somewhere here but I can't find the reference):