Spec'ing current reqs on trafo secondaries

[scottjoplin]

I've got something wrong here. I put a 1ohm resistor in series between the positive of the second PSU cap and the PCB

Did you disconnect the PSU from the PCB?

 

[brucew268]

So, I've been ignoring the fact that the smoothing caps are parallel with the circuit, so lifting the 2nd cap's positive leg doesn't cut the circuit. Doh! I don't see any way to cut the circuit after the smoothing caps, because the traces branch immediately to 3 different places.

 

[scottjoplin]

No, you don't need to touch the capacitor. Is the PSU on a separate PCB to the volume control? Can you draw a sketch of what you have in front of you and what you've been doing? Pictures are needed now I think, or photographs

 

[brucew268]

No, everything is on one pcb. The traces spur off 3 ways on traces:
- to the microprocessor and support for the remote
- to the IR receiver
- to 2 relays between balanced and regular out circuits.

Being a two layer board, I'm not sure how photos would do it justice. But here is a quick and dirty drawing, all lines are continuous traces on one pcb.

An externally hosted image should be here but it was not working when we last tested it.

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[scottjoplin]

Not to worry. If I'm right you are saying that you haven't been able to break into the circuit to measure the current? Anyway it all seems to be working ok? It was just the high current draw when at idle that was a bit of a mystery?

 

[brucew268]

Yes, correct. I'm only able to break in at the AC portion. And trying to determine whether I'm spec'ing that winding correctly on the replacement transformer being built.

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[scottjoplin]

I see, the unknown is the consumption of the logic circuitry. You can either reckon by experimentation that the transformer you are temporarily using is adequate or you could rectify and smooth it then connect via your multimeter to the board's transformer connections

 

[brucew268]

Yeah, I may just go risk it and go with the measurements from post 10. Hoping that it's not over-spec'd and not leaving enough current for the signal circuits, 300mA remains for them.

 

[scottjoplin]

All the regulators are rated around 1.5A, so you could just go with that, it doesn't matter if the transformer is overrated, and for a transformer of that small size price variation will be minimal

 

[brucew268]

That's a lot of VA's! Not enough space in the enclosure for that many VA's. But I could probably go up to 20va from 15.

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[scottjoplin]

It's not impossible to measure to be sure, 4 diodes and a couple of capacitors.....

 

[AndrewT]

............ I measure AC voltage drop across the resistor and get only 12mV, .............

that is telling you there is a variation in the current flowing through the 1r0 resistor.
The biggest variation is possibly mains ripple voltage.
If the variation is a pure sinewave of single frequency, then you know you have a 12mAac current flowing. That would be read on a scope as ~34mVpp.
If the variation were random noise, the scope would show roughly double that ~60mVpp, but the pulses between the individual spikes would be relatively far apart.
The actual AC will be a mixture of sinewaves and half waves of lots of frequencies and noise, so you could end up with anywhere from 30mVpp to 60mVpp.
That AC current is superimposed on the constant DC current.
The mVdc reads the average DC current.
The mVac reads the average AC current.

 

[scottjoplin]

Any DC power source 9V to 12V able to supply the required current would suffice for your measurement. Connect it to the AC input terminals on the PCB, it doesn't matter which way round, the bridge rectifier on board will direct the positive and negative to the right place.

 

[brucew268]

Any DC power source 9V to 12V able to supply the required current would suffice for your measurement. Connect it to the AC input terminals on the PCB, it doesn't matter which way round, the bridge rectifier on board will direct the positive and negative to the right place.

Now that's a practical idea! First with ammeter, then calculate from with resistor voltage drop.

Ammeter: 123mA DC on idle, 158mA DC during volume motor use.
Resistor 4R7: 123mA on idle, 170mA DC during motor use.

So that's not too far off of the AC ammeter readings earlier in the thread (140mA & 180mA).

Here is another practical idea! Since my new transformer is bespoke, I could easily get 17VA or even 20VA and would have guaranteed at least the same current as the original 15VA trafo. I may do that anyway.

Thank you for all your help and ideas, bearing with me!

 

[scottjoplin]

You're welcome. You've proved the unknown current required for the volume control. Call it 200mA which equates to 1.8VA from the 9V winding