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Huge voltage drop on PSU
Huge voltage drop on PSU
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Old 27th January 2013, 11:48 AM   #21
Mooly is offline Mooly  United Kingdom
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Huge voltage drop on PSU
OK. So the 25 volts is confirmed as good.

That means that you must be seeing around 34 volts DC across the caps. If that voltage is climbing slowly during the time the 25 volts is good then you have to look to the rectifiers and caps. If something was "pulling the voltage down" on the cap side then there is no doubt whatever that the diodes would literally be on fire.

It doesn't add up what is happening. You have to look at the rectifiers.

A definitive test is to scope across the DC rail and see what is happening. That would show excess ripple due to a cap problem. If there is no ripple then the rectifiers aren't passing the current (and as such they have to be faulty).

I suppose there could be a wiring error or construction error but I'm assuming not. Something like the amp circuitry "in series" with the supply somehow but that would be a bizzarre error.
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Old 27th January 2013, 11:49 AM   #22
Mooly is offline Mooly  United Kingdom
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Huge voltage drop on PSU
What gets warm ?
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Old 27th January 2013, 11:51 AM   #23
SjoerdSmits is offline SjoerdSmits  Netherlands
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Huge voltage drop on PSU
There is nothing connected there... only the resistor 1K7W.....

Could that be it?
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Old 27th January 2013, 11:54 AM   #24
KatieandDad is online now KatieandDad  United Kingdom
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That's your problem. You need to connect an external resistor of say 0R47 or just for testing a loop of wire.
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Old 27th January 2013, 11:54 AM   #25
SjoerdSmits is offline SjoerdSmits  Netherlands
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Huge voltage drop on PSU
Checked again and nothing gets more than say warmet than the hand.....
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Old 27th January 2013, 11:55 AM   #26
Mooly is offline Mooly  United Kingdom
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Huge voltage drop on PSU
I see what K and D is getting at.

You should have the full voltage immediately on the caps to the left of this point.
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Old 27th January 2013, 11:56 AM   #27
SjoerdSmits is offline SjoerdSmits  Netherlands
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Huge voltage drop on PSU
Hi KatieandDad,

Can I make it a 0.47 Ohm 5W wire resistor?
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Old 27th January 2013, 12:01 PM   #28
KatieandDad is online now KatieandDad  United Kingdom
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It depends on how much current you are expecting to pull. 0R47 5W should do for starters for up to about 3A.

0R22 5W for ~4A.
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Old 27th January 2013, 12:10 PM   #29
SjoerdSmits is offline SjoerdSmits  Netherlands
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Huge voltage drop on PSU
You refer to the AC power or the secondary DC power? Sorry, I'm new at this :-)

And what will happen if I just put a wire in.....?

Thanks!
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Old 27th January 2013, 12:13 PM   #30
KatieandDad is online now KatieandDad  United Kingdom
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All the resistor across X3 and X4 is doing is acting as part of a CRC filter to reduce ripple slightly.

For testing you can just add a shorting link in place of X3 and another in place of X4.

Once you are happy it is working you can consider what value of resistor you would like to try in those positions.

The current I am referring to is the DC current drawn by the amplifiers from the secondary side of the supply.
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