[MRCA] Dyno Starts Trip Breaker on PP-7333

[WA5CAB at cs.com]

Chris,

The PP-7333 is too new for me to have anything on it, even to the correct 
nomenclature.  But if it was built to run 28 VDC vehicular or aircraft radio 
equipment and it is actually putting out 30 VDC, (a) it's seriously out of 
adjustment and (b) that's your problem.  The open circut voltage of a fully charged 
lead-acid cell is 2.1 volts.  So ignoring nominal 6 and 12 volt systems, the 
fully charged voltage of a 24 volt lead acid battery (12 cells) is 25.2 volts.  
The system buss voltage in 24 volt buss vehicles and aircraft is a trade-off 
between how fast you recharge the batteries between engine starts and battery 
life.  Depending upon contactor, the design figure varies between 26.5 and 28.0 
volts.  It should never exceed 28.0 volts.

This subject has come up several times recently and I wish someone would come 
back with the issue numbers of ER containing an article I co-wrote years ago 
so I could just refer to it.  But I can't remember.  The first thing you 
should do before hanging booster batteries (or capacitors) onto the output of an AC 
operated supply is to determine whether it can survive having full output 
voltage on its output terminals when it is turned off.  Some (70's & 80's vintage 
HP bench supplies, for example) can't.  It eats the pass elements and 
sometimes other components.  Since your PP-7333 has apparently already survived I'll 
assume it can.  What you need to do is to drop the output voltage to the point 
where the battery takes over the surge starting current before the output 
breaker trips but ideally isn't being discharged after the dynamotor starts.  The 
simplest way to do that is to try one PN drop.  If that doesn't work, reset 
the breaker and try two.  Etc.  Once you have it working, check and make sure 
that the battery is still being charged at least during key-up periods.

In a message dated 10/29/2007 7:40:26 PM Central Standard Time, 
aj1g at sbcglobal.net writes: 
> I finally got a chance to put the 60A 28VDC PP-7333 power supply I picked 
> up at Gilbert on service with my ART-13 and GRC-19.  I am floating my pair of 
> series connected 12V deep cycle batteries across the output of the power 
> supply to take up the starting surge of the dynos.  The setup works fine steady 
> state, with the PP-7333 carrying the load with no strain, but the unit's DC 
> output breaker keeps tripping on dyno startups, such as keying the mic on the 
> ART-13 in voice, even though the batteries have a full charge.  For now, I am 
> using a work around of starting the dynos directly off the batteries with the 
> DC output breaker on the power supply open then shutting it to take the 
> steady state load.
> 
> I am thinking that the trips are being caused by the output voltage on the 
> PP being slightly on the high side relative to the batteries, which I suppose 
> could also cause them to overcharge at some point.  Since the PP voltage is 
> on the high side, there is a lot of current draw from it, and pop goes the 
> breaker!
> 
> The previous supply I was using was designed to float a pair of batteries 
> and the paralleled load.  I would set the output voltage so that it was very 
> close to the battery voltage, hence little current draw at "idle" and most of 
> the starting surge came out of the batteries.  I never had a DC supply breaker 
> trip problem on dyno startup with that set up.
> 
> I do not yet have a manual for the PP-7333, just a schematic from the inside 
> of the unit.  Is there a way to slightly lower the output voltage to about 
> 27.6 V (2X13.8) which is about what the fully charged batteries voltage would 
> be?  Right now, I think I'm getting, accordng to the PP's voltmeter, about 
> 30V out. There is no ammeter on the PP7333.  
> 
> If the PP output is not adjustable, maybe a high current dropping diode or 
> two in series with the output?
> 

Robert Downs - Houston
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