Model 152 PSU dummy loads - Re: NeXT Cube - powers on briefly then off again

[Paul Koning]

> On Oct 26, 2015, at 10:45 AM, Jerry Weiss <jsw at ieee.org> wrote:
> 
> Water in the dummy load?      Water cooled - sure.  Water immersed?  Even though I see it posted on the web, i have may doubts it would be a stable method.

How stable do you need?
> 
> Oil  is the usual medium here. 

Transformer oil is a nice dielectric, so for high voltage transformers that's what you want.  

But for low voltage dummy loads, or for medium power ham transmitters, the voltages involved are not that high.  Plain tap water is slightly conductive, but nowhere near as much as the resistors you're using.  And water is a MUCH better coolant than oil.  This is why machine tools typically use water as the coolant, or more precisely, water with a small amount of "soluble oil" mixed into it.

It's true that resistors are not designed to sit in water indefinitely.  But that means that, after a year or so, they may have been damaged.  Hours or days of immersion doesn't bother them at all.

If you want a good UHF dummy load, or one with 1% accuracy, this is probably not the best answer.  If it needs to dissipate multiple kW, or last for a decade, ditto.  But for a basic solution at near-zero cost, it works great.  If you're not confident the load is still good 6 months from now, just spend 50 cents making another.   Or lift the resistors out of the water-filled jar when done with the day's testing.

	paul, ni1d