• Please review our updated Terms and Rules here

Oil filled dummy load for H777 PSU

intabits

Experienced Member
Joined
Jan 22, 2019
Messages
145
Location
Melbourne, Australia
This is a cheap and cheerful method to make a dummy load that will happily dissipate 150 Watts for 15 minutes, which is long enough for most types of testing.
It consists simply of PVC piping with end caps, immersed in mineral oil.

xkHyV5m.jpg



In this instance, it is used as a fixed load for the H777 power supply used in Digital Equipment Corporation PDP11/04 and PDP11/34 minicomputers.
It appears as a fixed load of about 95% of rated current for the four voltage supplies provided by the H777.

CBwYoYX.jpg


wM7ZaZW.jpg



But the method could easily be adapted for higher power dissipation, or longer operation:-
- Thicker tubing, and/or higher volume of oil.
- The thing could be weighted down in a bucket of water.
- The containing tube could be made of aluminium, with heatsinking and/or fan cooling.

Schematic used for this unit:-
58ouqgf.jpg


Video:-
https://youtu.be/0iOc-6LHQfk
 

bobaboba

Experienced Member
Joined
Sep 7, 2013
Messages
393
Location
Scotland
Reminds me too of a dummy load I used to use for my amateur transmitter. Non inductive practically speaking up to >30MHz which was all I was interested in and could dissipate several hundred watts for some minutes Nowadays I use thin film resistors on a large fan-cooled heatsink which can dissipate 1kW continuous and is good for up to a few hundred MHz but that’s a bit of overkill for a computer psu ;).
 

KC9UDX

Space Commander
Joined
Jan 27, 2014
Messages
7,468
Location
Lutenblag
Doesn't MFJ still make a substitute Cantenna?

I should know; I'm minutes from the HRO which yoostaby AES World Headquarters, but I don't.
 

glitch

Veteran Member
Joined
Feb 1, 2010
Messages
5,011
Location
Central VA
Nice work! I usually use a series of automotive tail lamps (6V and 12V) for dummy loads on supplies, but this one does look a lot nicer :) I wonder if you could get longer run-time using a section of copper pipe, or if the heat dissipation would be inadequate without heat fins/forced air cooling.

I guess it's just a DEC power system fixing time of year!
 

Al Kossow

Documentation Wizard
Joined
Sep 1, 2006
Messages
2,874
Location
Silicon Valley
Doesn't MFJ still make a substitute Cantenna?

I should know; I'm minutes from the HRO which yoostaby AES World Headquarters, but I don't.

The 'new' AES (Amateur Electronics Supply) which I've never been to
I worked at the old location fixing radios for a summer when Terry was still around (mid 70s)
and when the Milwaukee Heathkit store was nearby..
 

intabits

Experienced Member
Joined
Jan 22, 2019
Messages
145
Location
Melbourne, Australia
Nice work! I usually use a series of automotive tail lamps (6V and 12V) for dummy loads on supplies, but this one does look a lot nicer :)
Thanks. I had wild oscillation with light bulbs, due to their high PTC, which is the main reason why I made this.

I wonder if you could get longer run-time using a section of copper pipe, or if the heat dissipation would be inadequate without heat fins/forced air cooling.
As I mentioned, there are many options to extend operating time. I guess it all depends on how long you want to run it, but 15 minutes was plenty for me.
A copper tube might make it heavy enough to stay under water in a bucket?
When I say 160W for 15 minutes, that was based on the outside being too hot to touch. There was no evidence of softening, but I don't know how hot it needs to get for that. It may well last a lot longer than 15min before any sort of failure. I imagine the resistors and oil can withstand higher than the tube and hotter than boiling water.
The tube is only 90mm OD and mm thick, but the 112mmx4mm stuff would not only hold more oil, but possibly take a higher temperature.

I guess it's just a DEC power system fixing time of year!
Certainly is for me right now!
If this place was not so full of junk/stuff, I'd have room to also check out 3 other PDP computer PSUs...
(which I'd have made this suitable for as well)
 

KC9UDX

Space Commander
Joined
Jan 27, 2014
Messages
7,468
Location
Lutenblag
The 'new' AES (Amateur Electronics Supply) which I've never been to
I've not been there, either. I haven't needed any antennas nor feedline in a long time, and I don't have much desire for new radios.

I worked at the old location fixing radios for a summer when Terry was still around (mid 70s)
and when the Milwaukee Heathkit store was nearby..
Nice! I remember going to that store once or twice to buy tubes.
 
Top