POT sizing?

[PolyPill]

Sorry for the elementary question, it's been a while since I messed with any electronics.

I have a 12v 30 amp power source I want to put a POT on, what size ohm should I be looking for? I want to make sure I get the right one because if it burns up due to too many amps it'll cost me a lot of money.

[Dan Baldwin]

What do you want to use it for? Are you trying to set voltage with the pot, or limit current? Generally speaking, it's not a good idea to do either with just a pot, and especially not if you're going to use the full 30 amps. It can be done, but you would have to use a huge wirewound pot.

Dan

[PolyPill]

Good thing I asked.

This is driving an electric motor, the documentation says peak is 30 amps, I'm sure it doesn't his that often, but I don't want everything to crap out if it does.

I want to adjust the speed and right now it runs full speed off of the 12 volt 30 amp line.

[PolyPill]

What about those POTs for dimming lights?

[Dan Baldwin]

What about those POTs for dimming lights?

Light dimmers are designed for AC operation. They won't work with 12 VDC. You need a DC speed controller. The easiest would probably be to use a 30 amp RC ESC (electronic speed controller) and a servo driver, but you could also build your own with an NE555 chip, a few passive components, and a couple of FETS.

Dan

[tino_j]

Apart from problems with cooling and power losses, a resistor in series with a motor is a very bad idea, at least if the power varies. When you use more power from the motor the current rises, as do the voltage drop across the resistor, and adjusted for the right idle speed the motor will slow down and maybe stop even with a slight increase of torque.

[PolyPill]

After more thought about this I think I can make due with 2 speeds, a fast and a slow (maybe 50%-75%). So I was thinking I'd just use a relay to turn between the 2 speeds.

So resistors are a bad idea? Even those big high watt ones? I didn't think it would be this complicated, maybe I could just put some large heatsinks on the resistors?

[slipstick]

If you're dropping say 6V at 30A that's 180W....you'll need a REALLY big heatsink. Not to mention the size of that 30A relay. They'll also probably cost you more than a cheap 30A ESC and a simple servo tester circuit.

An ESC is the right tool for the job.

Steve

[Miami Mike]

Tell us more about your 12v 30 amp power source. Is it a regulated or unregulated power supply? Is there any information about it on the web?

[MatC]

You'll need a big heatsink, but I'd guess the 30A is only briefly at power on at 12v. At 6v that is probably more like 15A, so you only need to dissipate around 90W.
If it's only power-on that needs that much, you could get away with a lot less, and use a slow-blow fuse (or a thermal fuse) to protect everything if the motor is jammed.

You will need a big heatsink (as mentioned) AND lots of power resistors. Sensible ones are normally 10W each, so you need around 10 of them, and they will need very good thermal attachments to the heatsink.

Speed controllers are usually switched, so they won't dissipate nearly so much power. As mentioned, this might be your easiest solution.

Use your own judgement, take care, hire a professional, I take no responsibility, etc. Careful not to burn your fingers on those resistors!

[TMorita]

If you're dropping say 6V at 30A that's 180W....you'll need a REALLY big heatsink. Not to mention the size of that 30A relay. They'll also probably cost you more than a cheap 30A ESC and a simple servo tester circuit.

An ESC is the right tool for the job.

Steve

A 30 amp relay won't be that much. You can get an automotive relay that will handle that amperage for less than ten bucks.

The heatsink will be huge, yep. I think I was using a 5" x 5" (25 square inches) finned heatsink to dissipate about 9 watts on a linear regulator. So if you scale this up proportionally, you would need about 500 square inches, which would be a heatsink roughly 2 ft by 2 ft....but I am not a heatsink expert.

Toshi

[tino_j]

No matter what, you'll get a low torque motor. A solution with a resistor may be good for f.x. a fan, where load barely varies, but unregualated (no rpm feedback series regulation) is not a good idea in most other cases. I agree, I'd go with an ESC these days, as they're cheap....

[Dan Baldwin]

Polypill;

What's the application?

Dan

[dj2u]

just to the first question:
if you get a huge enough pot (as sayd above not the best idea) you have to tke into account that the power rating is always for the whole length of the resistive material, so if your pot is set to half-way, the max power dissipation of the pot mustn't rise above half the indicated wattage (or was it a quarter?) anyhow best is to calculate a max current out of the max power.

hope I managed to confuse you a bit

ciao diego

[vintage1]

Here is a 5 amp rated 'pot' like what we used to have in physics experiments.

Just about enough to control a speed 280.

Its about a foot long, and weighs about 3lb from memory.

Now what was the question again? :D
'Why can't I use a pot instead of a speed controller?'

:D :D