# How do I determine the range of wire wound adjustable resistor?

I'm looking at buying a 300W tubular adjustable resistor. Like this: https://www.digikey.com/product-detail/en/vishay-dale/RBEA03001R000KFB00/541-10193-ND/9765057

Digikey has four part numbers in that RBEA series, 1Ω 25Ω, 50Ω, 100Ω. All of them are 300W, For any of these, how do I know the adjustable range for a given part number?

UPDATE: For background, I want to build a dump load and I would like it to be a little bit flexible, if I can. I'll end up wanting somewhere around 0.5Ω to about 2.0Ω per resistor (I'll probably have four of these in a 2S2P array). It doesn't need to be quick to adjust, and the resistance doesn't have to be dead on. I'll be dumping a max of about 600W and have a plan for a fan and nothing flammable around.

Figure 1. The resistors in question.

simulate this circuit – Schematic created using CircuitLab

Figure 2. The symbol and normal method of connection.

The adjustable range is from 0 to the specified resistance. Figure 1b shows the normal way of wiring these so that if the wiper fails to connect for any reason then the circuit will see the maximum resistance rather than an open circuit. This is preferable in most situations.

Note that the 300 W rating relates to the full length of the resistor being used. If only using half of the resistor (wiper in the centre) then the rating will be reduced to 150 W. This is because the resistance wire will be rated by power per unit length. Other settings will have the same proportional relationship.

Also, be aware that 300 W will be the power you need to dissipate in your enclosure or space where the resistor is used. The derating curve in the datasheet shows it can work (at low power) up to 400°C so make sure you test this, allow adequate cooling and clearance between flammable components.

• Ahh, thanks for the point about power rating. So, then it seems like the 1 Ohm part will resistor have more "resolution". Than the 25 Ohm part. That is, it might be easier to position the wiper to find 0.25 Ohm on the 1Ohm part than on the others. – kbyrd Jan 5 at 23:43
• Yes, but don't treat them as precision parts. Make sure you understand the temperature coefficient. Also note that there are about 50 turns on the unit in the photo so resolution is about 2% unless you rotate the contact around the tube as well as along it - if the element is exposed on the rear. – Transistor Jan 5 at 23:54
• I agree, I'm not looking for precision. Just things like "about 2Ohm", "about 1Ohm", about "0.5"Ohm. I'm planning on either a 24V load or a 12V, with occasionally something in between. To set it up, I'd unhook load and measure the resistance with a known good DMM. I'll error on the side of more resistance than less. – kbyrd Jan 5 at 23:58

Normally, I would say "read the datasheet" but there's not a lot there.

The resistance will be adjustable down to approximately zero, you can figure on < 10% of the full resistance for sure.

Also when the resistance is set to less than the maximum you must limit the current to less than $$\I_{MAX} = \sqrt\frac{{P}}{R}\$$, so a 300W 100 ohm (full) resistor can only handle 1.73A regardless of the resistance setting.

Here is a similar part, though the maximum current setting formula is a bit ambiguous.

The setting resolution will be quite poor. Maybe you can count on 5% of full scale. Typically higher resistance resistors will have better resolution as a % of full scale, but not in ohms.

• "Normally, I would say "read the datasheet" but there's not a lot there." <--- This. I did try to figure this out on my own before posting! – kbyrd Jan 5 at 23:53

From looking at the geometric design of the helicoil, I suggest you can adjust from 5% to 95% range . The very small increments sliding the sleeve around the helix depends on how sensitive/loose the clamp is, as you adjust it, then tighten the clamp to minimize the contact resistance.

Yet overall length 10% tolerance is given.

simulate this circuit – Schematic created using CircuitLab

The sleeve or shunted resistance between the wipers above is not to scale, but gives you an idea how it works. You ought to be able to calibrate it to 1%. But then you have to factor temperature and inductive effects.