# Dimmer or router speed control as resistance heater level control

1. Nov 10, 2007

### drcrash

I'm trying to figure out how to safely use a dimmer or router speed controller as an inexpensive heat level control for a (nichrome coil) resistance heater.

I've read that you can use a dimmer switch as a heat control if you don't run it at more than 80 percent of its watt rating continuously. (I'm not sure what the issues are there... why would it be rated 25% higher than its continuous power handling ability?)

I've also seen people using a router speed control as a heat control for small heaters. My understanding is that a router speed control is similar to a dimmer, turning the power off at a certain phase in each AC half-cycle to vary the average power. (But maybe chopping the falling edge of the half cycle rather than the rising edge, or something like that...?)

Does it make sense to use a router speed control as an average voltage control for a simple resistive load? I would guess it's designed with an inductive load in mind, but basically a switched inductive load (brushed motor rather than inductive), and I don't know whether a straight resistive load would be a problem.

BTW, the heaters in question do have a small startup surge, with the resistance going up 10 or 15 percent in the first second or so. (But less of a surge than for a light bulb; the filament of a light bulb burns much hotter.)

I'm interested in using a router speed control, because Harbor Freight sells a 15-amp model that goes on sale every couple of months for $10-$15; a dimmer that could handle the same amps would be much more expensive. (The usual wall dimmers are 600- or 1000-watts, and even the 1000-watt models are several times as expensive as the 600-watt model, and more expensive than the router speed control.) I'd like to be able to run at least 6 amps @ 120V through whatever I use, maybe 12 amps. (Two 720 watt coils in parallel.)

Would that work, and would it be safe?

2. Nov 10, 2007

### dlgoff

3. Nov 10, 2007

### drcrash

Thanks. I think those are way out of my price range and not easy to apply to what I'm doing---heating sheet plastic with mostly IR for a very cheap DIY vacuum former.

In general you want to control the heating rate rather than the oven temperature, and then use exposure time to control how hot the plastic actually gets given that rate of heating. For a simple system, it's easy to do a pretty good job if you get the heat level about right and then wait until the plastic sags enough to show that it's sufficiently hot and stretchy. If you actually measure temperature, you need more sophisticated sensors that can measure the temperature of the surface of the plastic itself, rather than the oven air.

That ends up costing way more than a $15 router speed control, and would be much more complicated to set up, so if a$15 "15 amp" router speed control can actually handle a 12.5 amp resistive load, I'll be really happy. If I could safely use it up to 15 amps, that'd be even better.

4. Nov 10, 2007

### dlgoff

"If you actually measure temperature, you need more sophisticated sensors that can measure the temperature of the surface of the plastic itself, rather than the oven air."

In applications like this one can measure the temp at an array of points (temp. map) then select one of the points as your reference; where you put the controllers probe (thermocouple).

9. Dec 2, 2009

### drcrash

oops... that should have said "There are variations [...] from batch to batch [...] so that the ideal forming temperature is variable."

10. Dec 2, 2009

### NH_EE

I see... Looked at a blog that mentioned RC enthusiasts use the Harbour Freight product (43060) on the primary side of a stepdown transformer to control a stainless steel wire heater for cutting styrofoam. The device might be a lot less sophisticated than I expected. If you do use it, you might want to try putting a light bulb across the load to give you instant feedback on the power change as you adjust the control, just in case the control's response is highly nonlinear.

11. Dec 3, 2009

### drcrash

NH EE,

That post about the plug-and-play hot wire cutter power supply (using a big wall wart plugged into that Harbor Freight item) was probably from me... that ~\$20 setup is about ten feet behind me as I type this. :-)

I've never needed to crank the power way up on the hot wire cutter, and have been a bit worried about using it for the vacuum former heater---can it do near its rated amps under a purely resistive load?---which is why I came here to ask actual EE's. (I also wanted to know if I'm giving good advice to people making hot wire cutters---am I just lucky my articular speed controller hasn't burned out?)

Your lightbulb suggestion is a good and timely one. While I was logging in here, I was thinking what a drag it is that I don't have a voltmeter I trust to measure the voltage of a funny-shaped AC waveform. Then I saw your comment.

A lightbulb would do the absolute-value and integration thing trivially. Visual response to light inttensity is logarithmic, so it wouldn't be very precise, but it'd be lots better than nothing.

Thanks.

12. Dec 3, 2009

### NH_EE

What I got from the hot-wire and transformer post is that the controller is not putting out pulsating DC. That's one technique that has been used for monitoring the actual motor speed, by using the remnant field flux to use the motor for a tachometer between current pulses. Thus my comment that it may not be as sophisticated as I expected.

As for supplying current to a purely resistive load, the only concern I'd have is that the current risetime when the thyristor triggers (turns on) is faster with a resistive load. Thyristors have a di/dt rating as well as an on-state current rating. Without knowing the actual component used in the speed controller, I can't say whether that's a problem or not. If the thyristor fails, it will most likely short out and give you full power regardless of control setting.

Gut feeling is I wouldn't hesitate to try it at 6 amps. When you add the second heater, will you want a single control for both, or dual controls?

re: light bulb - don't forget the color changes too as the RMS voltage varies, so you wouldn't be relying only on perceived brightness. Best thing about using a bulb is probably that you don't have to take your eyes off the work. If you wanted something repeatable, an average-responding voltmeter would still be useful even though it's not giving the true RMS reading. You could preset the control to give a particular meter reading based on experience. I'd use a mechanical meter rather than a digital one for this. Easier to read at a glance and quicker response to changes.