Improvised power resistor

[Robert]

Pure water is a poor conductor. All my connections to the power
resistor are bare..
I put it in a plastic bucket.
The only thing bad that happened was a little rust and there's steam.
D from BC

Ah. Didn't mention about the distilled Water.

Did you wash the Resistor first to clean any possible ion contamination off?

Robert

 

[D from BC]

Ah. Didn't mention about the distilled Water.

Did you wash the Resistor first to clean any possible ion contamination off?

Robert

Nah..It's just a test load.. If it decomposes,I just buy another..
I use tap water here..It's low enough in ions that it's no where near
the resistor conduction..
A little electrolysis is tolerable if it takes a long time to erode
the metals.

D from BC

 

[Jan Panteltje]

It's useful to put some of the story here on s.e.d.
First, we'll make the MOSFET's current follow Ohm's
law for a resistor, I = V/R, using a current-sense
resistor Rs, and a resistive divider, R1/(R1 + R2).

--+--------------+- V
| |
R2 |
| __ ||-'
+-----|+ \__||-,
| ,--|-_/ |
| |___________|
R1 |
| Rs
| |
--+--------------+---

If we select Rs so that R = Rs (1 + R2/R1), and also
R1>>Rs, we'll have made a nice active power resistor.
It's necessary to add an RC to the opamp output, to
prevent oscillation, as we've written about here many

If you place it in a diode bridge it should sort of work for AC too.
(if voltage is high enough).

 

[Rich Grise]

On Mon, 28 May 2007 06:48:35 +1000, Thomas Carlan


I liked all the suggests posted..wires, ribbons, ionic liquid, bulbs
..etc..
But I gotta add one obvious wacky suggestion. (IFAIK..not posted
yet..)
Using lots and lots and lots of 1/4W resistors together!!!

I once saw a construction project for an RF dummy load using a pile
of 1/2W or so carbon resistors and stiff wire. The end result was
about the size of a cantenna element, and of course the guy ran it
in oil.

The resistors were in groups that were arranged radially around the
center conductor, and there were wires along the length for the outer
conductor.

Cheers!
Rich

 

[Barry Lennox]

I know it is possible to make an improvised capacitor by using layers
of mylar sheet (or glass) and metal foil.
Is there any equivalent solution for a power resistor in the 20-50W
range?

It's a bit of a mission, but I know somebody who took a nice straight
stick, wrapped it in foil and left it in a fire for a while. The stick
became carbonised and ended up like the old "body-end-dot" carbon
resistors. I recall it being a bit hit and miss to get the correct
value.

Barry

 

[Robert Baer]

I've been using a 50W tubular resistor as a test load recently..
I got it dissipating 100Watts..so I keep it in a bucket of water...
D from BC

In air, those large hollow ceramic tubular resistors (with zig-zagged
"flat" wire) are good to where they glow orange, just before the ceramic
glaze melts...

 

[MooseFET]

I once saw a construction project for an RF dummy load using a pile
of 1/2W or so carbon resistors and stiff wire. The end result was
about the size of a cantenna element, and of course the guy ran it
in oil.

The resistors were in groups that were arranged radially around the
center conductor, and there were wires along the length for the outer
conductor.

I've seen the same sort of thing except the center conductor was also
a string of resistors. The ground was a cage of soldered #14ish
wire. It looked a little like a steel tower design. I suspect that
the design of the outer was more for mechanical reasons than anything
electrical.

 

[Rich Grise]

I've seen the same sort of thing except the center conductor was also
a string of resistors. The ground was a cage of soldered #14ish
wire. It looked a little like a steel tower design. I suspect that
the design of the outer was more for mechanical reasons than anything
electrical.

Actually, in the article I read - in QST or something, IIRC - the
outer shape actually was electrical - and the inner conductor also
had cones or frusta end-to-end - the cone-shaped parts are coaxial
impedance transformers. This apparently lets you use bags of surplus
resistors and tailor the physical design to what's available.

Cheers!
Rich

 

[joseph2k]

I know it is possible to make an improvised capacitor by using layers
of mylar sheet (or glass) and metal foil.

Is there any equivalent solution for a power resistor in the 20-50W
range?

Thomas Carlan

This runs along the active load lines, build a generalized immitance
converter with power operational amplifiers. The circuit is even
parametrically controllable for parasitic inductance and capacitance. The
choice of amplifiers will control the voltage, current, and power
capabilities.