Help with PSU

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catatemycaps
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Help with PSU

Post by catatemycaps » Mon Mar 19, 2012 11:20 am

I just finished building the PSU and when I connect it to PA-20 I get 0V from the 15V terminals and neither one of the leds light up (+/- 15V indicators).

I can measure roughly 18V volts across the input terminals. Red led is lit up also. Wires are connected as instructed in the user manual.

I checked the resistors and those are ok. Polarity of diodes and electrolytes are ok. Same goes for leds. Other components are ok, too. I'll check them once again to be on the safe side.

I also tried looking for colds and solder bridges but could not spot any.

Something gives, but what?

Thanks.

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Re: Help with PSU

Post by wmonk » Mon Mar 19, 2012 3:28 pm

catatemycaps wrote: Something gives, but what?

Thanks.
Did you install the standby switch correctly? First think I would check when the AC led is on and the DC leds are off.
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thebot
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Post by thebot » Mon Mar 19, 2012 4:22 pm

Do you have any modules connected? If so check they're connected properly - I managed to have a module with the power connected incorrectly (one peg to the left or right, if you know what I mean) and it caused the issues you're describing.

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Post by catatemycaps » Tue Mar 20, 2012 1:29 am

Thanks for the tips guys!

I don't have any modules connected to it yet. Stand by switch could be the source of my problems in this case indeed. Wiring is ok, but I'll check the terminal and soldering on the switch...or the switch could be busted (highly unlikely though).

Thanks!

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Post by catatemycaps » Wed Mar 21, 2012 2:50 pm

Ok. It was the stand by switch. Wiring was ok but it just didn't work. I replaced it with two wire links and now it works. Go figure.

I noticed something else too. The current on +15 terminal starts to drop after a while. It starts at 620mA but keeps steadily dropping and the TIP35C starts to get quite hot even with a heat sink.

At the same time -15 terminal is showing a steady 760mA reading.

Seems a bit strange to me. Current limit resistors are 1R.

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catatemycaps
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Post by catatemycaps » Fri Mar 23, 2012 1:04 am

Any ideas what might be causing this current drop?

This only happens on the +15V and the TIP35C gets quite hot too.
TIP145 hardly gets warm and there is no current drop on the -15V rail.

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Post by Synthbuilder » Fri Mar 23, 2012 2:54 am

catatemycaps wrote:Any ideas what might be causing this current drop?
Are you shorting out the power rail when you are measuring current drop?

If so what you are seeing is the now rising temperature of the LM723 affecting the current limit. The current limit is set by the resistor and the Vbe drop of the current limit transistor inside the LM723. The higher the temperature the lower the Vbe which means it needs less current through the 1R resistor to set the limit. It'll probably keep dropping slowly until it bottoms out at around 0.6A.

The phenomenon will be more obvious in the positive rail since the current limit transistor is not rising in temperature in the negative rail. It's a different circuit and the current required to drive the big pass transistor has no warming affect on the discrete current limit transistor.

The LM723 on the other hand has to supply the TIP35 with a considerable bass drive current. That'll warm it up and with it the current limit transistor inside it.
catatemycaps wrote:This only happens on the +15V and the TIP35C gets quite hot too. TIP145 hardly gets warm and there is no current drop on the -15V rail.
They both will be roughly dissipating the same amount of power in a short condition - indeed the TIP147 will transfer more. So if one power transistor is getting hotter then this is probably related to the way that each are mounted to the heatsink.

Check that both are attached tightly to the heatsink but not too tight so that the body lifts up at the other end.

Tony

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catatemycaps
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Post by catatemycaps » Fri Mar 23, 2012 4:09 am

Hey, Tony.

Well, I set my multimeter to 10A and then stick the black probe into 0V terminal and the red one into 15V terminal.

I'll check the heat sink and see that it is tightly attached to the transistor.

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Post by Synthbuilder » Fri Mar 23, 2012 4:47 am

catatemycaps wrote:Well, I set my multimeter to 10A and then stick the black probe into 0V terminal and the red one into 15V terminal.
Yes, that's the shorting out test. It represents what would happen if the +15V was connected directly to ground (0V).

In practice this doesn't often happen and, if it does, it tends to only last a short while as the user will notice something is wrong. However, the aim is that even under these rather brutal conditions the power supply should not get too hot and should recover as soon as the short is removed.

In normal use the power supply would not have to dissipate that level of power.

Tony

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catatemycaps
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Post by catatemycaps » Fri Mar 23, 2012 7:51 am

Ah, I was not familiar with the 'short out test'. I thought it meant something else. Sorry. English is not my first language. :)

Ok. So it is pretty safe to say that it works then. Good.

Next up: MidiDAC.

Thanks for the help, Tony.

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