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Convert 0-30V 3A PSU to 5A or more

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Yes, this is exactly what I ment. For a second I though that something is wrong with me too. I mean, I saw that somebody said that the emmiter of Q1 should be connected to ground and since it is already shown correctly on the schematic that you posted I wondered what the heck is wrong... ;D

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I was just making some libraries for the PCB of this project when I saw the "small" power rating of R7 - "only" 15W. Now, I was checking on different manufactors about this type of resistors since they are not something that I can get here in some local store. This is what I saw in mouser electronics:


I know this is 25W instead of 15W, but this seems to be one of the resistors that has R = 0.27. The thing I do not like about it is that it's low tolerance resistor which adds up to the price even more as you can see. What I like about it is that it is basically built into a heatsink ;D. I am just looking for appropriate resistor so I can finish the library and PCB. I was thinking also of more than one resistors in parallel, but I don't think this will save neither space or money. Any suggestions?

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Hello everyone :)

Im in the process of building this PS.. Using the updated SMARTER kit schematic posted in this thread... I know there has been tons of talk about using higher voltage OP amps and issues with under rated transformers.. Ive read both threads about 1.5 times... - alot of info to digest

Im building this with parts on hand.. which happen to be very close to those specified in the schematic. My OPamps are 44v OP177 types...
now believe me I know they should be rated for more, but im stubborn and want to try with these for now.. if they start smoking ill try something else ;)

Now because im trying to get about 5A... basically the most I can (the actual output will be measured later). I know this entitles using a higher voltage rated transformer, because of all the drops incurred along the way.. I also know that lower VA transformers have bad voltage regulation.. and would float at a much higher voltage when not loaded.

So Im rewinding a microwave oven transformer.. Rated at about 600 - 800 W im guessing (still have to get out the scale to weigh all that iron). Ive removed the secondary and the primary looks real nice. Im going to determine the V/T ratio tomorrow with the original primary.

My first question is:
What do you think a good unloaded secondary voltage would be?
I think 26V AC is the max I can go and stilll keep below the 44v DC OP amp limit, because 26v * 1.5(because of AC RMS to near peak conversion)= 39v +5v(for - supply) = 44v ( ignore the extra .6v or else its over= I like to live dangerously ; )

Will 26v AC unloaded be enough to deliver 30v DC loaded on the output? Keep in mind the transformer core is rated for about 600W

I will be able to change the secondary turns/voltage without too much trouble later on... I just want a good starting point.

Thanks, Chester Lowrey


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Hi Chester,
Welcome to our forum.
On your website I like your propclock and big sparks. Good luck with your 15KW sine-wave inverter project.

This 30V/5A lab power supply:
If your custom-wound transformer produces 28VAC with a light load, then 44V-rated opamps are almost at their limit. Just hope that your mains voltage doesn't increase 5% like mine does at times. The calculations are as follows:
1) The peak voltage of 28VAC is 28 X 1.414 = 39.6V.
2) The rectifier bridge has 2 diodes conducting in series with a resulting voltage drop of 1.4V, so the positive voltage is 38.2VDC.
3) Add the negative supply voltage of 5.6V and the total is 43.8VDC.
You still have a 200mV margin of safety!
The voltage will be slightly over 44V if your 5.6V zener is 5% high.

I don't think that you will get 30VDC/5A regulated though, as follows:
1) Fully loaded, the transformer's output (600VA!) will probably drop to about 27VAC RMS, but its peak voltage of 38.2V will drop further during momentary rectifier current of 25A down to about 37.2V.
So the transformer's output sine-wave will have flattened peaks.
2) The rectifier bridge will probably have a voltage drop of 2.2V when conducting momentary current of 25A, and the voltage is now 35.0V.
3) A 10,000uF capacitor will probably have about 1Vp-p of ripple when fed with full-wave rectification and having a 5A load, so now the voltage is down to about 34.0VDC.
4) The three 2n3055 transistors may have a b-e voltage of 1.2+V with a collector current of 1.67A each, plus the voltage drop across the 0.1 ohm emitter resistors of 0.17V drops the output voltage to 32.6V.
5) The TIP31A for Q2 could have a b-e drop of 0.8V with a collector current of 125mA, dropping the output voltage to 31.8V.
6) If it is only 100 ohms, R15 will further drop the voltage 0.2V down to 31.6V.
7) The saturation voltage of U2 could be as high as 1.6V, dropping the output voltage down to exactly 30.0V. Just makes it? Nope, I'm not finished yet.
8) 5A through R7 (0.27 ohms) will drop the output voltage down to 28.65V. Not enough.

You might get lucky and have a lower voltage drop on your transformer and transistors, but then the maximum voltage output will probably have lots of ripple each time your 'fridge or AC turns on.

Caution! Do not use the negative offset adjustment as shown for U2 on our schematics, with your OP177 opamps. Their offset voltage control must connect to the positive supply, instead of the negative supply like most other opamps.
They have a fantastically low offset voltage anyway, so you don't need to adjust it.
The OP177 has a voltage gain that is 10 or more times less than most other opamps, so your project's regulation and ripple will suffer.

If you want 30VDC/5A that is well-regulated and has low ripple, then maybe you should re-consider using a 30VAC transformer and OPA445AP high-voltage and high-gain opamps.

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Thanks for all the help everyone :)

Good luck with your 15KW sine-wave inverter project.

Thanks, I really need it... trying to switch 350Amps at 48v average is no joke

Caution! Do not use the negative offset adjustment as shown for U2 on our schematics, with your OP177 opamps. Their offset voltage control must connect to the positive supply, instead of the negative supply like most other opamps.

yup, I discovered that by comparing pinouts.. and designed my PCB accordingly.

Just one thing about the OP177, remember when or if they emit smoke so can anything connected to the output also be doing.

Good point.. I assumed I could do some stress testing after it was completed and if it works it works... but youre right, if im running this close to the maximum limits... anything could happen. Id still like to try :P

Maybe ill do some destructive testing on the OP177 to see just how high they will go, anyone for leaving this PS on for 4 days at a time? and then subjecting it to pulsed inductive loads... or something like that.

Since you are winding you secondary, why not make multiple outputs to experiment with?

I actually plan to build two of these supplies.. Using the same transformer core, but I wont build the second until I have the first working reliably, and maybe a fixed 12V and 5V supply also... who knows.

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Wound the transformer, etched and assembled the PCB, scrounged for resistors..

Just got it working a few minutes ago...

The transformer puts out 26.06V AC unloaded and 25.78V AC loaded with 1.2A (on output of regulator)

After the rectifiers = 35.16v DC unloaded and 33.47v loaded with 1.2A

Regulator puts out 28.99V DC unloaded, 28.95v with a 1.2A load.

The 1.2A load is just a halogen bulb I happened to have handy... Soon ill find a better load to test the full capacity, also this was just a test setup... its not in a case or anything yet, and im only using one 2n3055 right now.

Ive ordered samples of the OPA445..

Ive noticed that the current limit just acts like a cutoff switch.. Id assumed it would just lower the voltage to that current, but it seems more like full off full on type of thing.. maybe its just really sensitive? Is this how most variable current PS work? The current limit is just like a cutoff? so you just use the voltage knob to do both? I can see it might be more usefull this way, especially if you have no voltage meters, because the output is either at the set voltage, or its at close to zero and the current limit LED is lit.

I really appreciate all the time you guys have spent revising and improving this circuit.. All the hard work already done ;)


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Hi Chester,
Your rectifiers have a higher voltage drop than I expected. Aren't you using a 10A or more rectifier bridge module?

With the regulator set to 29.99V it loses voltage regulation when its output drops to 28.95V with a 1.2A load. I'll bet that the output has a lot of ripple.

The current regulator isn't working properly. It should be linear and accurate, not just a switch. At first I thought that the incandescent light bulb load was showing its changing resistance with its temperature, since it will draw nearly the same current with its voltage turned down. Catch-22: Current is too high, cut the voltage down. Still too high, cut voltage down even more, etc.

Analysing the current regulator I see an error on the "Smarter" schematic: Pin 7 of U1 should connect to the positive unregulated rail (rectifiers output), not to the output of U3. So the current regulator was dragging-down the positive supply for the voltage reference.

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Thanks for spoting the error Audioguru!, I thought that connection looked funny.. but didn't bother to check it against the original schematic..

The bridge rectifier is a junkbox model CM3501 = 35A 100V It has a nice aluminum case for bolting to a heatsink, measured at .436 volt drop per diode

With the regulator set to 29.99V it loses voltage regulation when its output drops to 28.95V with a 1.2A load. I'll bet that the output has a lot of ripple.

Did you read my reply wrong? or am I missing something?, I was only getting 5 thousandths of a volt difference from no load to 1.2A load. Which could have by the leads going from the regulator output to my load... ( I was measuring on the load, not the regulator output)

I did notice the bulbs almost constant current draw over different voltages.. makes a nice test load in that respect... must be the simplest constant current device there is.

Ive fixed the V+ connection for U1 and the current regulation works great now..

I dont know what the best way to measure ripple on the output is.. My fluke multimeter reads zero AC voltage on the output, with or without a 1.2A load.
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Hi Chester,
I'm glad that the current regulator works great now.
Yeah, I'm sorry that I mis-read your post, but maybe you did too. The voltage regulation is actually quite good, dropping 40mV (not 5mV as you say) and I bet that it is much better if you measure right at the regulator's output.
I'll also bet that you used 1W zener diodes that have their voltage rated at 50mA, didn't you? D8 has only 1.2mA through it, so should be rated at such a low current. A 1W zener will reduce the regulator's max. output voltage. Maybe we should reduce the value of R4 to 1K, to increase the zener current and get it up over its voltage knee.
Never mind the rectifiers' voltage drop. It will take a 'scope to see whether they switch slowly so that the peak voltage from the transformer has past before they begin conducting.
What value is your C1? The ripple rejection is excellent if you can't measure even 1mV, and you can say that it is "unmeasureable".

Do you like the current regulation that is created by an ordinary lightbulb? But it is very slow to react. A lighbulb causes a huge current surge until it has warmed-up.

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Hey.. ya, sorry about that, i got my decimal places mixed up ;D

Im actually using 5.1v 1W zeners, with a regular diode in series, using the foreward voltage drop to bring it up to about 5.6v..

I have a 20mhz DSO, I dont think they are so slow that the peak is missed.. the sine just gets a flat top.

My C1 is 10,000 uf..

I like the lightbulb current regulation just because its sooo simple.. nothing impressive about its performance.

-- still waiting on the HV OP amps..


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Hi Chester,
I am amazed that your re-wound microwave oven transformer produces flat tops on its sine-wave output when the rectifiers conduct. I thought that it would be powerful enough so that its voltage wouldn't sag like that. Maybe your rectfier current is causing your electrical utility's mains voltage to have flat tops!

I still think that your Zener diodes are operating at a voltage that is much less than their rating because your maximum unloaded output voltage is lower than expected. The range of the voltage adjustment pot should be able to set the output voltage well into saturation of the output circuit. If your 10K resistors R5 and R6 are exactly the same then the output voltage of U1 should be exactly 11.2V, if your 5.1V zener diode plus 0.6V regular diode add to 5.6V. If the output gain-setting resistors R11 and R12 also have an accurate ratio then I'll bet that your U1 has an output voltage of only 9V plus change.

I knew it! About 30 years ago I studied the voltage curves of zener diodes with changes of current through them. Today I found an old data book but I can't find those curves on Google. Datasheets don't have curves anymore, just like skinny women.
Our project, with R4 being 4.7K, has the voltage reference zener diode operating at only 1.2mA.
A 1W zener diode such as the 5.6V 1N4734A is rated and measured at a current of 45mA. At 1.2mA it just barely conducts and is about only 4.2V (the normally vertical voltage line has curved to be nearly diagonal, like an ordinary resistor). Your 5.1V zener is even worse!
A 500mW zener diode such as the 5.6V BZY79C5V6 (available worldwide) is rated and measured at a current of 5mA, where it performs very well. It also performs poorly at 1.2mA (5.3V) but is much better than a 1W zener diode.

Therefore we should change R4 to 1K (for a zener current of 5.6mA), and specify using a BZY79C5V6 zener diodes for D7 and D8.
Edited Nov.6/04: in bold.

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