20 amp variable speed control

[Tex]

I was showing a friend my different roasters (Sono, UFO/CO, and air popper) and he said I should use something like this 20 amp variable speed control for the heater. He uses one for his router to keep big cutters from burning the wood.

Thoughts?

[Warrior372]

Is that like a variac? If so I have also seen a lot of people using them to control drum speed, fan speed and heating element temp on roasters.  That just limits the electrical flow to whatever is plugged into it based on how 'open' or 'closed' the control knob is, right? If those are the same things, that is an awesome price!

[ecc]

The heater is a purely resistive load, I think it would work.  Worst case, the controller wouldn't handle it and heat up, your heater shouldn't be hurt.  The heater may be too responsive to the rapid drop off in voltage, which wouldn't give you much of a control band between full on and full off, but the price is right.

The routers it was likely designed for are probably DC motors with brushes, and use current for torque, voltage for rpm. 

The controller is likely an inexpensive rheostat, (variable tap resistor) in contrast to an expensive triac based dimmer, which manages power by time slicing the AC signal, or variacs, which are variable current to voltage transformers, and can raise or lower voltage.

[Warrior372]

OKay, the price difference makes sense now! Thank You

[ButtWhiskers]

I have a couple of these from Harbor Freight Tools, they go on sale for under $10 on occasion.

[Tex]

I have a couple of these from Harbor Freight Tools, they go on sale for under $10 on occasion.

Thanks, I'll drop by HF this week and see what they've got. Are these just rheostats? It's hard to find any specs on these. Are you using them to control the heating element or fan?

[ButtWhiskers]

I have a couple of these from Harbor Freight Tools, they go on sale for under $10 on occasion.

Thanks, I'll drop by HF this week and see what they've got. Are these just rheostats? It's hard to find any specs on these. Are you using them to control the heating element or fan?

Pretty sure they are only 15A, but they work great for the heating element.  I have used them on modified turbo's and poppers.  They retail for about $20, but they have gone on sale for as low as $7.99 before.  They are basically just heavy duty potentiometers.

You can get by with a cheap dimmer switch for a fan. 

[Tex]

I have a couple of these from Harbor Freight Tools, they go on sale for under $10 on occasion.

Thanks, I'll drop by HF this week and see what they've got. Are these just rheostats? It's hard to find any specs on these. Are you using them to control the heating element or fan?

Pretty sure they are only 15A, but they work great for the heating element.  I have used them on modified turbo's and poppers.  They retail for about $20, but they have gone on sale for as low as $7.99 before.  They are basically just heavy duty potentiometers.

You can get by with a cheap dimmer switch for a fan. 

I'm still trying to figure out which is better, a pots or PID? IIRC, a pots works by reducing the power to the element, basically flattening the sine-wave? A PID works by varying the on cycle, but it's outputting full power when it's on.

Seems like the pots is the better tool for temp control for this app?

[milowebailey]

Tex, for a heating element it doesn't matter.  If you get to regulating fan/motor speed then the SCR type controls are the only way to go.

[Tex]

Tex, for a heating element it doesn't matter.  If you get to regulating fan/motor speed then the SCR type controls are the only way to go.

Hmmm...

Now I'm confused again (SNAFU)? Isn't the SCR firing so fast that it would work perfectly on a heating element - IOW, it'd be firing so fast the element would remain at a constant reduced temp?

[milowebailey]

Hmmm...

Now I'm confused again (SNAFU)?

ok, I'll type slower for you  

A rheostat is fine for resistive loads because by changing the total resistance you change the current through the resistive load V=IR where V is 120 volts and when R goes up... I goes down proportionately.

With and SCR you trigger off the zero crossing of the sine wave and clip the voltage waveform.  The Area under the curve is available power....


BTW, most newer wall dimmers are SCR.

[Tex]

Hmmm...

Now I'm confused again (SNAFU)?

ok, I'll type slower for you  

A rheostat is fine for resistive loads because by changing the total resistance you change the current through the resistive load V=IR where V is 120 volts and when R goes up... I goes down proportionately.

With and SCR you trigger off the zero crossing of the sine wave and clip the voltage waveform.  The Area under the curve is available power....


BTW, most newer wall dimmers are SCR.

OK, it's coming through the fog, and your really badly drawn pics helped. Thanks Larry.

[milowebailey]

Hmmm...

Now I'm confused again (SNAFU)?

ok, I'll type slower for you  

A rheostat is fine for resistive loads because by changing the total resistance you change the current through the resistive load V=IR where V is 120 volts and when R goes up... I goes down proportionately.

With and SCR you trigger off the zero crossing of the sine wave and clip the voltage waveform.  The Area under the curve is available power....


BTW, most newer wall dimmers are SCR.

OK, it's coming through the fog, and your really badly drawn pics helped. Thanks Larry.

Not my drawing, but shows my point.  Here is a tutorial...

[rasqual]

LOL!

Awesome, it's a real explanation! Depletion zones and all.

That's just good stuff.

[gt]

If you get to regulating fan/motor speed then the SCR type controls are the only way to go.

This depends on the type of motor you're controlling, for a P1 a variac does a very nice job of controlling the fan speed.