Power Supply

[Andrew Davie]

I thought this would work - connecting the two GND outputs.

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In this picture, one is supplied with 9VAC and the other with 15VAC. When the black wire connecting output GND is not there, I get 13.3VDC and 22.1VDC.
When the black wire is connected, both outputs are 22.1VDC give or take.

So my concept of "common ground" is faulty somewhere.

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[gary]

Where do the 2 AC voltages come from Andrew?

[Andrew Davie]

Where do the 2 AC voltages come from Andrew?

From a single transformer which has multiple output tabs.

I go from 0v on that to both of the rectifier modules.
I go from 9v tab to one of the module's other input
I go from 15v tab to the other module's other input

So, both of the modules I built connect to the same transformer and share a AC 0v /GND on the input side too.

[Steve Anderson]

This doesn't bode well. I suspect you've created a short or some other high-current path here. Seeing as you appear to have used full-wave rectifiers the two windings, one for each supply, need to be separate. No connection between them within the transformer - or use two transformers.

Steve A.

[Andrew Davie]

This doesn't bode well. I suspect you've created a short or some other high-current path here. Seeing as you appear to have used full-wave rectifiers the two windings, one for each supply, need to be separate. No connection between them within the transformer - or use two transformers.

Steve A.

Thanks Steve; could you clarify?

Yes, full-wave rectifier. BR64, 6A.

I assume my "power modules" are OK - they each work in isolation as expected and they're pretty simple, right? What could I possibly do wrong.

What you seem to be saying is that I can't use a single transformer as source - but instead need to use two. And when I use two AND connect the output ground together, things should be OK? I should get the expected output from each.

Do I understand correctly?

[Andrew Davie]

Just one thing, yes you have a ripple free output when you have no load on it, but when you put a load on it things will be different because the capacitor will be discharging between cycles - check the ripple with a dummy load that will be drawing a current at least as much as the real load with be and test the ripple then. I think it will be ok because that's a fairly good sized cap and I doubt you will have too much of a heavy load.

Well, I thought I'd do a bit of measuring and experimenting. I dug up the old LED array and wired in a terminal so I could connect/disconnect easily. This is an array of 8x5 ultra-bright LEDs (strings of 5). I wanted to see how it worked (if at all) with different voltage inputs, and also have a look at that ripple.

Surprisingly, every LED lit (not real bright, but lit up nonetheless) at about 13.2V DC (that is, when I was feeding my power module with 9V AC).
It was, of course, blindingly bright when fed with 22.3V DC (off the 15V AC input).

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The above picture shows the latter - LED module running directly off the power module. The multimeter shows 19.9V (a drop of, what, about 2.4V from unloaded). Not sure what that voltage drop from the unloaded measurement is actually telling me. The oscilloscope is indeed showing some ripple, as we all expected. Using a 10x probe here. Both are connected to the output of the power module.

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[Andrew Davie]

Learning to read my oscilloscope again. The volts/division is set to 0.1V. But we're on a 10x probe, so it's really 1V/div.
The ripple appears to be about 1/2 a division; in other words about 1/2V. The frequency of ripple - each division is 5ms, and a single ripple is therefore 10ms.
But we're doing full-wave rectify of an AC signal, so two "bumps" = 1 AC cycle. Two bumps is 20ms.
So we have an AC source of 1000 (ms) /20 ms Hz ---> 50Hz. Which is correct.


What I am confused about is understanding where "0" is. I have the ripple, but I can't tell at what level it's at (i.e., 22V).

[Steve Anderson]

A 'scope is an electronic graph basically. Here with the x10 probe you're trying to measure around 22V, which means at 1V/div you would need 22 divisions on the screen - which clearly you don't have. If you set the scope to 0.5V/div and with the x10 probe it becomes 5V/div. So disconnect the supply, set the (now) 0V horizontal line to somewhere near the bottom of the screen and re-connect the probe. It should leap up just over 4 divisions with a proportionally lower ripple - but still 0.5V, one tenth now of a division. You won't see 0V again until you disconnect the probe or turn off the power supply.

'scopes are mainly used to view wave-shapes rather than DC work. Measuring video signal amplitudes for example. That's where you'd nominally set the signal to 1.0V p-p which would read garbage on a multimeter. Different tools for different jobs. You don't use a hammer to fix a screw - unless you come from Birmingham. (Hammer - aka a Birmingham screwdriver). Here you may want to measure the ripple which as you say is around 0.5V (don't use fractions) which you've measured on the scope. That's sitting at and average of 22V as measured by your multimeter. That's actually not too bad at all.

But if you're pulling 1A from the +12V regulator, the regulator has to drop 10V from 22V at 1A = 10W - think of a 10W light bulb (globe), they get hot!

As for the other power supply problems you have there's something really wrong there. Without a better insight to what you've built it's hard to get a handle on it.

Steve A.

[Andrew Davie]

Steve, thanks for your feedback. I read it all very carefully!

But if you're pulling 1A from the +12V regulator, the regulator has to drop 10V from 22V at 1A = 10W - think of a 10W light bulb (globe), they get hot!

But I won't be doing that...? The supply to the 12V regulator will be the DC rectified from either 9V AC or 12V AC. It will not be too much above 12V. My original televisor was feeding about 22V DC to the 12V rectifier and it was practically melting the air around it. Yes, hot. Now I know why - which is a great leap forward in my understanding because I had no idea why it was so hot before a few days ago.

The 22V DC is the power feed for the LED array - and this doesn't go through a regulator, as far as I'm aware. Still getting back into the swing of it, but my current thinking/understanding is this: I have two "power modules". One provides roughly 12V DC + a few V to cater for the voltage drop across the regulator, so that the regulator can supply a fairly contstant 12V to the main circuitry. The second "power module" provides a higher voltage (22V or so) to the LED array so that it can support a string of serial LEDs (5 in the current case). That second one is not passing through a regulator.

Thanks too for the review of the scope reading - yes of course this makes sense to me now. Great.

I still have the confusing "common ground" problem. I will try two transformers as source AC and see how that changes things.

[Andrew Davie]

Curiouser and curiouser.

OK, I bought two new AC transformers so that I could test my "power modules" in various configurations. Where I last left off, when I connected the GND of the outputs of my power modules together, the consequent DC voltage BOTH of them output was the highest of the two. I was connecting the GND to get the "common ground" concept going.

Here's my new power transformer test setup..

Note - although 240V AC is coming in from top-left into a connector-strip, I have plugged any likely point of accidental contact with hot glue (including tightening screws) and I think this will be fairly safe sitting on my desk for test purposes.

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So these experiments...

• Individually, each "power module" puts out the expected DC voltage when connected to tabs on any one of the transformers. For example, connecting to 0V/6V then the power module outputs about 9.6V. When on 12V tab it's pumping out about 17V. As expected. Same for the other transformer - I have one power module connected to one, the other to the other one, and both power modules are pumping out the correct, expected, DC voltage.
• In the above configuration (each power module connected to its own transformer) and THEN I connect the output GND together for the two power modules, they remain at the expected correct output voltage for each (i.e, they are different to each other, based on the input AC).
• If I connect BOTH of the power modules' inputs to a single transformer (e.g., one to 0V/6V and the other to 0V/9V) then the output from each power module is as expected.
• If I THEN connect the output GND of the two power modules, I get the original problem - which has now appeared on all three transformers I have tried with - and that is, the output of BOTH power modules is again the same - being the highest of the two values expected.
• All the transformers are manufactured by the same supplier - I am assuming some internal design issue?

This is the "working" configuration. Each power module connected to its own transformer, and the GND connected at the DC output. Each power module produces an independent voltage as expected.

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Anyway, so curious and I don't understand what's happening when I use the single transformer to drive multiple "power modules" with different voltages. Works when I don't connect common GND on the output, doesn't when I do. Since I have two transformers now, though, and it appears to be working correctly with common GND with those, I plan to proceed with the new setup (two transformers, one driving the circuitry/motor and the other powering the LEDs).

[Steve Anderson]

Without going into it in depth, you generally can't use a common (single) winding into multiple bridge rectifiers. So if this is now performing as you wish with two transformers leave it as such.

Steve A.

[Andrew Davie]

Without going into it in depth, you generally can't use a common (single) winding into multiple bridge rectifiers. So if this is now performing as you wish with two transformers leave it as such.

Steve A.

Well I'm glad I've been through this process to learn that! My original televisor DID try to do exactly that, so the regulator doing the 12v for the main circuit was actually being fed 22v as a result. I'm also glad that my results are as expected, even if inconvenient. Thanks for your help and clarifications.

[Andrew Davie]

I am just about done with the power supply side.

I placed a 10K resistor across the output from each of my "power modules", which I understand is called a "bleeder resistor".
I wanted to drain the capacitors after power-off, because I use my "power modules" interchangeably and have occasionally been caught when one had a residual 22V when I was using it for something of much lower voltage. I chose 10K basically by observing the time different resistors took to lower the output voltage when board was not powered. It's something like 30 seconds now, which I'm happy with.

This also seems to have lowered the output voltage slightly, which is to be expected I suppose. When I get to using the boards in anger, I will probably remove/disable the resistor.

I did notice that with the transformers I have, the 9V AC converts to about 13.7VDC (so it's really about 9.7V AC) but in any case not under load.
I was looking at low dropoff 12V voltage regulators, and it seems you can get ones with dropoff 0.5 - 1V. I was wondering if this would be safe enough for me to continue to use the 13.7V DC as input to the regulator instead of going the next step up which is 12VAC --> about 17VDC and the consequent heat generated at the regulator.

I guess I will just try and see - power from the 9VAC and see how it works.

[Steve Anderson]

Yep, a low drop-out volt reg should do the trick, but they can be a bit fussy in other regards. Which device are you thinking of?

Steve A.

[Andrew Davie]

Yep, a low drop-out volt reg should do the trick, but they can be a bit fussy in other regards. Which device are you thinking of?

Steve A.

How about the L4940D2T12-TR?