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Linear powers supply mystery...

M

marc.hull

Experienced Member
Joined
May 26, 2007
Messages
107
Hope someone can point the way for me here...

I have a linear power supply that is baffling me. Unconnected it runs 5x5 no issues. When connected to the Harness it is still good. When I plug it into the PCB the fuse does a slow burn and the 5V drops to less than 1 volt until the fuse blows

I can connect the PCB to a more modern switching power supply and my meter shows 5V every where it should be so the board doesn't have a short. I have Ohm'd the harness out and find no shorts in it. The only problem is when the original PS is under load.

The Caps Ohm'd out good both sides and before I start pulling other components I thought I'd ask. Thanks for your help.

MH
 
This is one that I'd do a wire at a time; i.e. connect just one ground and one +5 and see what happens. Also check the output of the supply for ripple. What reads +5 may in fact have a large AC component that will kick up all manner of awfulness with most stuff.

Most linear supplies are pretty simple-minded. An LM723 voltage regulator driving a darlington pass transistor mostly.

Have you tried the power supply with a dummy load? Automotive lighting (taillight bulbs or even headlight if you're drawing a lot of current) makes good loads.
 
Didn't occur to me to test under a dummy load. Time to do some math. This beast has no VR's just a couple of large transistors. I'll give it a test.....
 
Hi
Several things come to mind.
1. The supply has a over voltage SCR that is being tripped under load
because of ripple ( bad ESR in final capacitor ).
2. The supply is not rated for the load your using on it.
3. The wires are backwards when wiring the linear.
Dwight
 
I got tired of trying to chase bad components and in addition there are some that I can't or don't know how to test. I ended up cutting the 5V trace from the original PS and spliced in the 5V feed from another and connected the grounds. This rig seems to power my Burger Time cabinet just fine. Hope this isn't too bad an idea....
 
Well I agree to point anyway. I'll continue to live for now but eventually it will need to be fixed. I need a bit more education though before I can tackle it ;-)
 
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