[MRCA] Dyno Starts Trip Breaker on PP-7333

[Doran Platt]

Christopher Bowne wrote:
> 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.
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The PP-7333, not unlike the PP-4763( ) will not tolerate a load greater than the rated current.  The breakers on both of these units will pop at the drop of a hat with an excess load. The ART-13, BC-375, and the GRC-19 (with dynamotors) can have start-up/in-rush currents in excess of 80-100 Amps, and in consideration of their individual condition. The quick drop out of the mentioned supplies won't hold the load. I have a rumor that if one substitutes with SB fuses, the supplies will tolerate the inrush current. Again, rumor only, although the rating of the components would seem to support that premise.  The PP-4763 does not have an overcurrent sense other than the input breakers.  I believe the 7333 is about the same?  The 7333 has an internal voltage adjustment pot.. I'll have to pop the case (yeah... after 30 minutes of unscrewing...hi!) on mine and get the R-number.  To mitigate the inrush, what I have been able to do with some success is the following:  I built an inrush limiter by using a high-current contactor and a .5 OHM, 350 watt resistor.  By using a "start-run" switching arrangement using an aircraft contactor, I can get the dynamotor started with the series resistor then short the resistor with the contactor.  I'd like to find a heater element resistor with a higher wattage rating but the Dale seems to hold the heat for a few seconds.  Maybe something from a heat-pump?  Also, I'd like to add time-constant activation of the contactor instead of the switch. BTW, I have had no success with boost batteries permanently connected across the above particular supplies.  The breakers still pop when the battery voltage sags on start-up. A had another USMC supply, I don't recall the nomenclature, that would hold up.

Oh yeah.... I was told that the PP-7333 was used for training and maintenance mock-ups???  The output connector is the same on the 4763 and the 7333.  de Jeep/K3HVG