Question for those Electrical Engineers

High Desert Ranger · Sep 28, 2007

I want to either use some sort of resitance pack or something to limit the power to a 200watt device. It is 12volt and I want to limit it to 110watts and do it rather cheaply. (Probably not possible for cheap)

martin · Sep 28, 2007

I = P/V so just stick a 9amp fuse in line.

MAKG · Sep 28, 2007

Then the fuse will blow constantly.

To size a resistor, you need to know the device's impedance. For a 200W device, Z = V**2/P = 0.72 Ohms. For 110W, you want 1.31 Ohms. So, you need a 0.59 Ohm resistor in series with the load.

But one that can take 110W. This isn't going to be something you buy at Radio Shack for 25 cents. Most of those are 1W or smaller resistors.

Note that you're not telling us what the device is. Resistors WILL change the phase for a reactive circuit.

Is this an amp? You'd do better to just buy a smaller amp.

Bob Ayers · Sep 28, 2007

MAKG said:
Then the fuse will blow constantly.

Resistors WILL change the phase for a reactive circuit.

.

No, I thiink the fuse is the best solution for limiting the power to 110W.

R does not change the phase in a reactive circuit, L and/or C does........

High Desert Ranger · Sep 29, 2007

MAKG said:
Then the fuse will blow constantly.

To size a resistor, you need to know the device's impedance. For a 200W device, Z = V**2/P = 0.72 Ohms. For 110W, you want 1.31 Ohms. So, you need a 0.59 Ohm resistor in series with the load.

But one that can take 110W. This isn't going to be something you buy at Radio Shack for 25 cents. Most of those are 1W or smaller resistors.

Note that you're not telling us what the device is. Resistors WILL change the phase for a reactive circuit.

Is this an amp? You'd do better to just buy a smaller amp.

No it's not an amp.

And I know that a fuse won't work

How about this?
http://www.radioshack.com/product/index.jsp?productId=2062289&cp=&sr=1&origkw=resistor&kw=resistor&parentPage=search

At 5w each, I'd need 22 of these in parallel (to each other) to achieve close to my 110w.

Since I am not an Electrical Engineer, What is a reactive circuit?

martin · Sep 29, 2007

there's a huge difference between an ac circuit and a dc circuit. If you could describe exactly what you are trying to achieve it would be easier to arrive at a possible solution.

Bob Ayers · Sep 29, 2007

I agree with Martin, we need to know exactly what you are trying to do. One other idea would be a power FET biased to go into saturation, creating a constant current source. Power FETs have an extremely low Rdson in the linear region. You could use a PFET in the +12V side, or a NFET in GROUND.

calypso93 · Sep 29, 2007

Here's an angle, for a steady state, non-reactive load. His 200W load draws 16.6A @ 12V. This implies a resistance of .72ohm. To have this load dissipate 110W, it has to have 12.4A flowing thru it. To limit the current to 12.4A, you'd need a .97ohm total resistance. .97-.72=.25ohms in series with the load in question. Check the results with either P=IsquaredR, or E=IR, and you'll get about 9V on the load, and 3v on the dropping resistor.

High Desert Ranger · Sep 30, 2007

calypso93 said:
Here's an angle, for a steady state, non-reactive load. His 200W load draws 16.6A @ 12V. This implies a resistance of .72ohm. To have this load dissipate 110W, it has to have 12.4A flowing thru it. To limit the current to 12.4A, you'd need a .97ohm total resistance. .97-.72=.25ohms in series with the load in question. Check the results with either P=IsquaredR, or E=IR, and you'll get about 9V on the load, and 3v on the dropping resistor.

Is there a Radio Shack Resistor that I could use for that?

if, like MAKG said, that the wattage would be too great for a single 1/4watt resistor, can i use a some resistors in parallel to do this.

calypso93 · Sep 30, 2007

Nah. the calculated wattage rating is near 40W. I'd use a 50W for a little leeway. If were talking all steady state, pure DC circuitry, it wouldn't matter what type of resistor construction was used. Just be sure it has lots of air circulation. It's gonna get hot.
(Edited for spelling. This keyboard made me miss a letter.)

Bob Ayers · Sep 30, 2007

calypso93 said:
Nah. the calculated wattage rating is near 40W. I'd use a 50W for a little leeway. If were talking all steady state, pure DC circuitry, it wouldn't matter what type of resistor construction was used. Just be sure it has lots of air circulation. It's gonna get hot.
(Edited for spelling. This keyboard made me miss a letter.)

I agree! Wirewound power resistors have a significant amount of inductance.

High Desert Ranger · Oct 1, 2007

calypso93 said:
Nah. the calculated wattage rating is near 40W. I'd use a 50W for a little leeway. If were talking all steady state, pure DC circuitry, it wouldn't matter what type of resistor construction was used. Just be sure it has lots of air circulation. It's gonna get hot.
(Edited for spelling. This keyboard made me miss a letter.)

50w and how many ohms resistance?

calypso93 · Oct 2, 2007

.25 ohms, (one quarter ohm).

Question for those Electrical Engineers

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