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13,8VDC Power Supply won't support a load.

 
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spytekon
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PostPosted: Mon Aug 27, 2007 2:33 pm    Post subject: 13,8VDC Power Supply won't support a load. Reply with quote

Purchased two used 13.8VDC Astron Power Supplies at the Harvard-MIT Flea Market and both are defective. The output reads 13.8 VDC , but when a load is connected, a 50 Watt auto bulb, the bulb does not light. and the voltage drops to Zero Volts. (One of the two 13.8 VDC power supplies is the LS-11, 7-11Amps, and the other is the RS-7A, 5-7Amps.) The capacitors are OK, the Power Transistors seem OK when tested for continuity (not removed from unit), and the LM723 control DIP seems OK (swithed with a new one and the defect not corrected.) My experience in electronics is limited, if that was not already obvious, and any help which comes from practice will be much appreciated.
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Fri Aug 31, 2007 9:17 am    Post subject: Reply with quote

A light bulb has very low resistance when it is cold, and will draw a huge startup current (up to ten times its rated current). So it is very possible that the startup current triggers the short circuit protection of the power supply.

Try to load the power supplies by resistive load instead, and see if they work better then.
Also, try to find the shunt resistor used for the short circuit protection (probably fitted between the emitter of the series pass transistor and the positive output terminal of the power supply) and make sure that it has the correct value. If there are several resistors in parallel, disconnect them and check them out individually.
If the power supply has several paralleled series pass transistors with individual emitter resistors, make sure that each of them takes its part of the load current. First check the resistance of all emitter resistors and then, with a light load on the power supply (say, 1-2 A), check that you have the same voltage drop across all those resistors.
Study the data sheet and application notes for the '723 regulator to find out details about its operation that can give you clues for continued troubleshooting. Links to those documents can be found here: http://www.national.com/mpf/LM/LM723.html
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Fri Aug 31, 2007 2:43 pm    Post subject: Reply with quote

I have worked on some Astron's before. They are rather similiar in design & have both Amps/volts protection circuitry which can malfunction & prevent an output. The measured 13.8, is simply the charge across the output capacitor
and it sounds like both of your units are in "protection" mode. Need to have a schematic if there is nothing obvious.

I know some of the schematics are posted on the net , do a google search
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