DIY Power Supply Build

[Joshua]

Hello Everyone!

You may have seen my other topic(s) about building a bench supply. I have decided to build a dual supply based on the schematic below.


Here is the full pdf...http://www.elv-downloads.de/Assets/Produkte/2/225/22532/Downloads/22532_Universelle_Netzteilplatine_um.pdf

I have a couple of questions regarding this design. According to the description(translated), by selecting some resistor values, I can either build a 0-15V 4A supply, or a 0-30V 2A supply. Why is the 30V version limited to 2A? If it is not possible to build a 0-30V 4A design, which version would be more useful? I am beginning to learn electronics, but know that I will want to deal with motors, robotics, audio circuits, etc...

To build a dual supply, will I also have to have two transformers? I will want to be able to connect them in series, and in parallel, so I know they have to be isolated from each other. What kind of transformer do I need? When searching for transformer on digikey, I do not know what I need enough to filter the results.

On the bottom left, there is some form of temperature measurement using a 'SAA965' which is actually a KTY81-151. -  Datasheet: http://www.datasheetcatalog.org/datasheet/philips/KTY81-151.pdf
What do I use this to measure the temperature of? The heat sink of the transistor? How do I attach it to what ever I am measuring? Also in the area of the temperature measurement, there is a 5V supply. Where is the 5V supply coming from, the 7805 at the top? And what are the the two lines coming out to the left, labeled 'ST6' & 'ST7' going to?

I will want to have panel meters displaying voltage and current output. I think I want to incorporate possibly a uC into the project, for some 'fanciness' I have in mind, once I have the foundation, so maybe just a LCD is all I need. For the Panel meter, and micro ideas, can I use the 5V supply, that is also the 5V supply for the LM324, or will I need another small transformer, to power these items?

Some people suggested in another thread, that I use a ten turn pot on my supply. I also agree that that is a good idea. In the LM317 datasheet, there is an example(below) about digitally selected outputs. Would this also work on this supply? I think I want a button for 5 and 3.3 volts, and the micro will select the correct resistance. What are the downsides of this?



In Dave's recent video about the HP Supply, he mentioned that it had no 'load switch.' Is that just a normal toggle switch(that can handle the current), to just quickly disconnect the output? Would a beefy switch, or a small switch with a relay be better?

What are the funny looking pots in the top right of the schematic? Why do they have a flat end, instead of an arrow-does this represent something significant?

One last question: What are the transistors with two lines coming out of them? What do the two lines represent?

My goal is to have a nice dual supply, that I can use for years as I continue to learn about electronics. I will greatly appreciate any advice, answers to questions, or just ideas I haven't thought about. Also, some of the above information is not 'set in stone'. I.E. if it is better to use a different design for the supply, than I am willing to use that one instead.


Thanks in advance,
Joshua

[alm]

I have a couple of questions regarding this design. According to the description(translated), by selecting some resistor values, I can either build a 0-15V 4A supply, or a 0-30V 2A supply. Why is the 30V version limited to 2A? If it is not possible to build a 0-30V 4A design, which version would be more useful? I am beginning to learn electronics, but know that I will want to deal with motors, robotics, audio circuits, etc...

Power dissipation in the pass transistors. Max. dissipation with 0-30V, 2A is about 2A*30V(+drop over pass transistors at 30V), 0-15V, 4A is also about 60W. Max dissipation of a 0-30V, 4A supply is about 120W. You would probably need double the pass transistors, plus a heat sink with about half the thermal resistance.

To build a dual supply, will I also have to have two transformers? I will want to be able to connect them in series, and in parallel, so I know they have to be isolated from each other. What kind of transformer do I need? When searching for transformer on digikey, I do not know what I need enough to filter the results.

Two transformers or one transformer with two separate secondary windings (not one center tapped). The transformer would obviously need twice the power in the latter case. Something like 30V/8A (7A) per power supply (extra current because of the rectification and smoothing) in your case. 2x15V/8A (put them in series) or 2x30V/4A (put them in parallel) would also work.

Disadvantage of two secondary windings is that the isolation between secondaries is usually less than the isolation between primary and secondary. Commercial supplies from eg. Agilent are rated for 250VDC between the outputs, I wouldn't count on this for secondaries without any insulation in between.

Transformer selection should be primarily driven by output voltage and current. Input should be your mains voltage (or center tapped windings which can be wired in parallel/series for 115V/230V) at 50/60Hz, this will be true for 95% of the mains transformers you find. I wouldn't worry about finding a transformer with the low-power center tapped 18V winding, you're unlikely to find this. Just get some small 2x9V transformers.

On the bottom left, there is some form of temperature measurement using a 'SAA965' which is actually a KTY81-151. -  Datasheet: http://www.datasheetcatalog.org/datasheet/philips/KTY81-151.pdf
What do I use this to measure the temperature of? The heat sink of the transistor? How do I attach it to what ever I am measuring? Also in the area of the temperature measurement, there is a 5V supply. Where is the 5V supply coming from, the 7805 at the top? And what are the the two lines coming out to the left, labeled 'ST6' & 'ST7' going to?

It's attached to the heat sink with some sort of clamp (Halteklammer / Schelle für den Temperatursensor). It's there to shut down the power supply if the heat sink gets too hot. This can be omitted if there is enough cooling, it's not critical to the operation, it's just an extra safety feature. ST6/ST7 are just solder terminals to connect the temperature sensor to the PCB.

I will want to have panel meters displaying voltage and current output. I think I want to incorporate possibly a uC into the project, for some 'fanciness' I have in mind, once I have the foundation, so maybe just a LCD is all I need. For the Panel meter, and micro ideas, can I use the 5V supply, that is also the 5V supply for the LM324, or will I need another small transformer, to power these items?

Digital panel meters usually need a supply that's floating relative to the voltage they're measuring, so you need a separate secondary windings for each. For a uC, it doesn't have to float, but the input voltage should be within the power supply range of the uC (check datasheet for exact range). If you connect uC ground to the ground of the reference voltage, this should work out fine for current (which is positive in respect to this potential, although only between 0-0.3V or so), but not so well for the voltage, ST9 is negative relative to this, so you would need to bring this within range with something like resistors or the spare op-amp (IC3D).

Some people suggested in another thread, that I use a ten turn pot on my supply. I also agree that that is a good idea. In the LM317 datasheet, there is an example(below) about digitally selected outputs. Would this also work on this supply? I think I want a button for 5 and 3.3 volts, and the micro will select the correct resistance. What are the downsides of this?

Very coarse resolution. Something like a DAC would work fine, although it will probably require some modifications to the circuit. To keep things simple, I would probably first build it with regular pots.

In Dave's recent video about the HP Supply, he mentioned that it had no 'load switch.' Is that just a normal toggle switch(that can handle the current), to just quickly disconnect the output? Would a beefy switch, or a small switch with a relay be better?

Haven't watched the video yet, my guess is that's indeed an output enable/disable switch, which is quite convenient because it saves you from turning of the whole power supply or disconnecting wires each time. Both solutions should work fine. If you're going for digital control, controlling a relay from the uC might have the advantage that it makes it easier to add remote control/programmability later.

What are the funny looking pots in the top right of the schematic? Why do they have a flat end, instead of an arrow-does this represent something significant?

Trimpots. They are for setting the max. voltage/current, the procedure is described under 'Abgleich'.

One last question: What are the transistors with two lines coming out of them? What do the two lines represent?

Darlington transistors (you should be able to look this up in any textbook like AoE or even Wikipedia). Used because of their high current gain.

My goal is to have a nice dual supply, that I can use for years as I continue to learn about electronics. I will greatly appreciate any advice, answers to questions, or just ideas I haven't thought about. Also, some of the above information is not 'set in stone'. I.E. if it is better to use a different design for the supply, than I am willing to use that one instead.

I like this design and I know multiple people who have build it and are happy with it. I'm sure there are other good designs out there, maybe even some documented in English, but I haven't found one yet, although I haven't looked very hard either.

[Joshua]

Alm, thanks for the reply!

If I wanted to build a 30V 4A design, would a pass transistor like this be suitable?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=BDX33CGOS-ND
or this, in a To-3 package for heatsinking
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MJ11012GOS-ND

For the transformer, where do I buy these things at? A simple search on digikey returned a toroid for $100! (I don't want to pay that much for a transformer, especially if I need two of them) Can I get these cheap somewhere or something?


For the temperature sensing, can I just take it out like below? (assuming I use adequate cooling as you mentioned.)





For the panel meters, if I get a small transformer, just for the meters, would this work? Also, how would I measure the current with a panel meter, would I have to put it in series? Is it possible to some how use a resistor, and actually measure the voltage?

In practice, is there a difference between Constant voltage, with current limit; or constant voltage with constant current? If so, which is this supply? How will I set the constant current/limit with a panel meter? I think dave mentioned something in his recent video about shorting the output terminals-how does this apply?

For the BC558 PNP Transistor, will any general-purpose PNP work, or does it need to be the BC558?

For the 5V, and 3.3V buttons I was talking about, what I had in mind, was some trimpots set for 5V, and 3.3V output, and also the regular 10 turn pot for full scale adjustment. The micro would 'select' the 5V resistor when I pressed the 5V button, and likewise for the 3.3V. I could then have the output from the 10 turn pot going to an analog input of the micro. If the micro sensed that the pot was changing, it would then select that resistance,and leave it selected, until one of the buttons was pressed. Would that work?

Thanks again,
Joshua








 

[alm]

If I wanted to build a 30V 4A design, would a pass transistor like this be suitable?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=BDX33CGOS-ND
or this, in a To-3 package for heatsinking
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MJ11012GOS-ND

I expect issues with the SOA of the BDX33. At DC, max I_C with V_CE=30V is slightly over 1A. This should be just OK for this design (1A per transistor in their design at 30V/2A), but is a bit too close for my comfort. The MJ11012 looks like it should work fine, but is a lot more expensive than the TIP142.

For the transformer, where do I buy these things at? A simple search on digikey returned a toroid for $100! (I don't want to pay that much for a transformer, especially if I need two of them) Can I get these cheap somewhere or something?

Someone that lives in the same country as you (wherever that is) is probably going to be more helpful. You can sometimes buy surplus transformers.

For the temperature sensing, can I just take it out like below? (assuming I use adequate cooling as you mentioned.)

Yes. It's basically an OR gate with three 'inputs', and you just remove one of those, which is logically equivalent to the temperature protection always being off.

For the panel meters, if I get a small transformer, just for the meters, would this work? Also, how would I measure the current with a panel meter, would I have to put it in series? Is it possible to some how use a resistor, and actually measure the voltage?

Small transformer will work, remember one secondary winding (so 2x9V can power two panel meters) per DPM. Also need rectification/smoothing/regulation for each.

In practice, is there a difference between Constant voltage, with current limit; or constant voltage with constant current? If so, which is this supply? How will I set the constant current/limit with a panel meter? I think dave mentioned something in his recent video about shorting the output terminals-how does this apply?

I believe current limit / constant current is explained in the Agilent appnote 90B I linked to earlier. Constant current is more useful in my opinion (it allows the supply to operate as a current source), but more complex. Yes, for most cheap supplies, you have to short the outputs (to force it into constant current mode) to set the current limit.

For the BC558 PNP Transistor, will any general-purpose PNP work, or does it need to be the BC558?

Sure, nothing critical, it's just a constant current source.

For the 5V, and 3.3V buttons I was talking about, what I had in mind, was some trimpots set for 5V, and 3.3V output, and also the regular 10 turn pot for full scale adjustment. The micro would 'select' the 5V resistor when I pressed the 5V button, and likewise for the 3.3V. I could then have the output from the 10 turn pot going to an analog input of the micro. If the micro sensed that the pot was changing, it would then select that resistance,and leave it selected, until one of the buttons was pressed. Would that work?

That should work, although it seems overly complex to me.

[david77]

If I wanted more power from that circuit, say 30V/4A, I'd double the number of series transistors, each with their own emitter resistors, of course. That way you can spread the heat dissipation over a larger area of heatsink and you'd still be able to use the cheap TIP142s.
Only thing I'm not quite sure about are R30 & 31. Would they need to be another value?

Btw. I'd leave the overtemp protection in, it certainly doesn't hurt. Powerfull linear supplies like that can double as heaters under some extreme load conditions . You could always glue the sensor to the heatsink using epoxy glue or something similar.

PS. A good decision to use this circuit I reckon. ELV has a very good name in Germany, their kits and other stuff are at the top end of the line. If something in the German description needs clarification - just shout.

[Joshua]

Hey Everybody!

It's been a while (I've been really busy), but I managed to build it up on a breadboard. It took me a couple weeks working on it about 5 minutes at a time.



It KINDA worked. I was using many(somewhat dead) 9V batteries instead of a transformer and rectification. I was able to get some voltage adjustment, like from 20-23v. I thought about trying to see what was wrong with the circuit to get full voltage adjustment, but figured it would be pretty much impossible to troubleshoot it without just starting from scratch. I've decided to just go with this circuit and trust that it works, based on you guys' recommendation. (not that I didn't trust that it works, I just wanted to test it out first just for the fun of it)

I am now on a quest for transformer(s). I would like to build two of these supplies of 30V 4A. I will connect them in series or parallel based on what I desire at the moment. I am in the U.S. so I will need a transformer rated(?) for 120V (110, 115, whatever). My meter actually measures 123.4V at my outlet.

I do not know what kind of transformer I need. Should I get a regular(?) or a toroidal? What should be the ratings I need? Where is the best, cheapest place to get these. I do not want to spend $80 a piece, when I need to transformers. However, I could justify $40 a piece.

Once I get my transformer(s) I will probably build up one supply on perfboard as a test. I will then determine what enclosure I want, and figure out my cooling system. I will then try and lay out my own board, and either etch it, or have one professionally made.


Any advice on the project, or more specifically the transformer, is greatly appreciated!

Thanks,
Joshua

 

[Mechatrommer]

i wonder how to test high power capability of this thing before putting it in pcb?

[Zad]

Car lamps, not halogen ones though, just the usual 20W tail lamp types.

[Mechatrommer]

i mean without burning the tracks/breadboard. and i mean at max 120W (say 12V 10A), including stability and heat analysis.

[Excavatoree]

It looks like one would have to go ahead and design the installation for the heatsink-mounted components, (or at least mock it up to be close enough) as well as the transformer and rectifiers.  The rest of the circuit could still be on the breadboard for testing/tweaking.  Once the design was certain, then a board could be made.

EDIT:  See the post below mine - that's what I'm trying to say.

Interestingly, this is similar in design to the Kepco power supply I've been working with.  I am considering posting a thread similar to the Lamda PS thread about it.

[johnmx]

I’m also designing my own quad bench power supply (4x 20V@3A). The circuit is not the same as yours. It’s based on an Elektor article but with some modifications.

Right now I finished the analog control circuit. The complete power supply will have 2 toroidal transformers. Each one has dual outputs. I added 4 more windings to each transformer which will supply the analog and digital control circuit. Each transformer is rated for 150VA and the biggest advantage over normal laminated steel transformers is the size and weight, almost half.

As you can see in the attached image, the pass transistor (2N3055) is mounted on a heat sink. This is crucial for the tests. The 3A conductors don’t pass in the breadboard, just in terminal blocks. Also note the sensor resistor mounted on a terminal block.

My next step is to replace the potentiometers by two DACs.

Later I can post the schematics if someone would be interested.

[nukie]

Isn't E type core better than toroid in terms of noise coupling? We want our PS output to have least noise as possible right? Can anyone explain this?

[Mechatrommer]

yea! thats it! separate the power line and control line.

[johnmx]

That’s true, but I will use them anyway because I want to reuse them. 0€ cost 

I could use another transformer for the control line, but I want to keep the power supply the small as possible in terms of size. Since it's a quad isolated bench power supply, I need 4x 12Vac + 4x 8Vac lines for the analog and digital section. So I prefer to have all windings in the same transformer and filter the control supply (after being rectified) with an LC filter.

This post explains some differences between both types:
http://www.diyaudio.com/forums/power-supplies/45348-toroidal-e-i-core-transformer.html#post509909

Also check this white paper about noise reduction in toroidal transformers:
http://www.dalitech.com/Resources/Narow%20Bandwidth%20Technology.pdf

[Wim_L]

I'll note that using those large power transistors on a solderless breadboard is not a good idea. Even ignoring the ability of the breadboard to deal with large currents, the pins on those big devices are thicker than most, and can cause permanent damage to the springs in the breadboard. That can lead to bad contact on thinner wires later.

[Joshua]

Hey everyone,

Thanks for the replies! I will get back to them later(I am in the process of re-imaging my computer because of a virus  )

Anyway, I saw these transformers on ebay, and was wondering if they would be 'approved' by you guys for the project. I believe he has 2 available of each one.


150W 110V Toroid Transformer 17V + 17V & 33V + 33V

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190533592999&ssPageName=STRK:MEWAX:IT#ht_1597wt_1139


300W 110V Toroid Transformer 17V + 17V & 38V + 38V

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=190533594073&ssPageName=STRK:MEWAX:IT#ht_1229wt_1139

Which one, if any, should I get. If the 300W is a good deal, and I could build a larger power supply, like a 30V 8A, then I don't care to change the design.

Thanks for the replies so far. Like I said I will get back to them hopefully tonight, I just had to push these transformers to the top because there is only a day left on them.

Thanks,
Joshua

[Excavatoree]

Being a Yank, I'm not familiar with the markings on those transformers - do any of them indicate approval from a reputable agency?

Because this is connected directly to the mains, I'd be concerned about getting something safe. 

I need to do more studying about this.......

[alm]

Being a Yank, I'm not familiar with the markings on those transformers - do any of them indicate approval from a reputable agency?

These transformers look like US spec to me, from which country do you believe the markings are? Or is this the standard 'pardon my ignorance, I'm a product of the US public school system' excuse ?

As far as I know, they indicate isolation class and that they're for indoors (dry) environments only. Not sure about the left one.

One thing to be careful about is the max current per winding. They don't appear to specify it, so it may be something like 2x38V/4A and 2x17V/100mA. 33V, let alone 38V is a bit high for my taste, about 44-45V after rectification. That means lots of heat dissipation for something like 5V. 2x17V should be fine for something like 2x15V or 1x30V, provided it can deliver enough current. Don't forget to divide AC current by two (multiply by 0.6something) to get the correct DC current after rectification.

[Excavatoree]

It's difficult to do a google search for an icon.

I meant agencies such as UL, but I know there are others in the world, and a good part may not have the UL symbol, but still be reviewed by someone.

I speak in words, not pictograms.  Ever hear of people who have trouble with words but can read pictures?  I seem to have the opposite problem.

I have never built a power supply, mainly out of laziness, and because I've been able to accumulate a collection of relatively cheap power supplies.  I guess I need to do a post about my collection.

[johnmx]

You need to know the current rating of each secondary winding. Request this information from the seller.

If the efficiency is 100% (ideal):
Isec(ac)= 150/(17+17+33+33)=1.5A
Isec(ac)= 300/(17+17+38+38)=2.7A

But probably the two windings have different current ratings, it depends of the diameter of the copper wire. From the photograph it looks that the 33V windings are thicker.

[tekfan]

Ideally you want a transformer that will let you deliver rated power under low-line with just enough voltage overhead for your pass transistors which means minmum power dissapation.

A neat idea is to connect a variac and a potentiometer together so that the variac always supplies the filter caps and series pass transistors with a few volts more than the desired output. You can get a really efficient linear power supply that way.

[Joshua]

I'm Back! Sorry for the delays everyone, some things came up, and I ended up having to go out of town, blah, blah, blah.

Johnmx, I would love to see a link/schematic to the psu you are a building! 

Anyway, about the eBay transformers, the seller never responded to my multiple questions - Screw You! I'll give my money to someone else.

Alright, so I've found a couple of transformers that might be viable.


http://www.newark.com/jsp/search/productdetail.jsp?SKU=38K4930&CMP=AFC-OP&CMP=AFC-OP
Datasheet: http://www.farnell.com/datasheets/74774.pdf


TOROIDAL TRANSFORMER
Power Rating:300VA
Secondary Voltages:30
Transformer Type:Toroidal
Secondary Current AAC:5A
Output Current, Secondary Series:5A
Supply Voltage:115V



http://www.antekinc.com/details.php?p=66
Datasheet: http://www.antekinc.com/pdf/AN-3230.pdf

Outputs   2x
Power   300VA
Output Voltage   Current
30V   5.0A
Torroid




Since these are both transformers with dual secondaries, I believe I can actually use just one transformer that will take care of both supplies. Am I correct in that?

Also, they are both rated at 5A. Does that mean 5A total, as in both secondaries together can not pull more than 5A? Or does it mean 5A per secondary, giving a total possible current draw of 10A?

Also, here is a link to the product listing of Antek. Are they a quality brand?

Also, when multiplying 30V x 1.414 I get ~42V peak to peak. Is that to high-should I go with like a 24V secondary output, to get ~33V?

Joshua

[bilko]

Since these are both transformers with dual secondaries, I believe I can actually use just one transformer that will take care of both supplies. Am I correct in that?

Also, they are both rated at 5A. Does that mean 5A total, as in both secondaries together can not pull more than 5A? Or does it mean 5A per secondary, giving a total possible current draw of 10A?

Also, here is a link to the product listing of Antek. Are they a quality brand?

Also, when multiplying 30V x 1.414 I get ~42V peak to peak. Is that to high-should I go with like a 24V secondary output, to get ~33V?

Joshua

 Yes, you can use 1 transformer for both supplies
 Transformer is rated at 300VA, that's 150VA per winding, so at 30V you can draw 5A from each winding

[Joshua]

Well that seems like it might be the 'dope' ! I neglected to think through the ratings logically like you did...

Does anybody have any thoughts on these particular brands?

[alm]

Keep in mind that isolation between secondaries tend to be much less than isolation between secondary and primary. This means that you may get troubles if there is a significant potential difference between the two outputs. The closest commercial lab supply I have allows 350V DC + peak AC between ground and any output. The rating would be much less with just two secondary windings only separated by some enamel.