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Dummy Load for DEC PDP computer Power Supplies

intabits

Experienced Member
Joined
Jan 22, 2019
Messages
145
Location
Melbourne, Australia
A while ago I made this dummy load for testing the H777 power supplies in my PDP 11/04 and 11/34.
xkHyV5m.png



And I have three other PDPs that have power supplies that need to be tested. But I was too lazy to allow for them when I made the unit above. So now I've made a more versatile version.


The front, showing barrier strips where the dummy load resistors connect to the top row of screws. A cable customized for each type of power supply (the one shown is for the H777) will connect to the appropriate screw terminals on the bottom row.

First power supply to be tested is the H740 in the foreground (for PDP 8/f and 11/05). Behind that is the PDP 8/A power supply.
vUKlwfW.png



Rear view showing the banana jacks for monitoring the supply voltages under load.
6VtzTKE.png



The resistor banks that go into the two oil-filled cylinders.
Xgk2ndb.png



The wiring to the barrier strips.
8DetnxA.png



Schematic and wiring diagram.
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video
https://www.youtube.com/watch?v=QeyDxQv2YS4
 

pbirkel@gmail.com

Experienced Member
Joined
Apr 7, 2013
Messages
497
Location
Silver Spring, MD, USA
Very attractive :->! What are the dimensions of those tubes? While the oil fill (mineral oil I assume) will reduce hot-spots, in the end your heat dissipation capability is going to be limited by the cylinder surface area and material (plastic). How long can you realistically dissipate 200W per tube in your setup?
 

intabits

Experienced Member
Joined
Jan 22, 2019
Messages
145
Location
Melbourne, Australia
Thanks.
The tubes are 90 x 200mm and about 2mm thick, and yes, filled with mineral oil (baby oil actually - it's very cheap and smells nice!).

The original version could dissipate 240W comfortably for at least 15 minutes. The tube got very hot, but there was no sign of softening.
It really only needs to hold up for a short time to verify that the voltages are good at full load, 15 minutes is way more than enough.

The 90mm tubes are are for storm-water, but sewage grade (here is Aus) is 100mm and about 4mm thick, would have been even better.
 
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