Slot car controller for PWM

[Northbanu]

While trying to wrap my head around controlling a heating element w/ PWM, I noticed this little tidbit on #FK804 DC Motor Speed Control kit :

Perfect for making a hand held throttle for model railroad switch where extremly slow motion is required. With a little modification, it will fit into the FB02 box and make a great little hand held throttle.

Does this mean I may be able to use a slot car controller as a PWM?

Just a thought, and please know that I'm just starting to get my head wrapped around what it will take to control a heating element....

 

[Brumateur]

It won't work without modification. You need a PWM with 0.5-1 Hz frequency. FK804 has a 265 Hz .

 

[muhteeus]

PWM pulses ac voltage on and off. Think of it as a light switch that is cutting power to the element. You essentially have a on-off-on-off-on-off pattern where the ONs and OFFs are at some set time interval. For simplicity, let's say that ON-OFF should take 1s, or a 1Hz period. The output % is the percent of the ON pulse that is dedicated to actually being on. So if you are running at 50%, your ON period is 50% of the cycle or .5s ON, .5s off. If you have 15% output that is 0.15s ON, 0.85s OFF. If you have 100%, that is 1s ON and 0s OFF.

The way the PID controls is a bit more complicated, with automatic adjustment of the output based on feedback from the heating element. Error is calculated by set temp-actual temp = error (SP-PV = error). Then the error is used to calculate adjustments based on 3 different "bands"; proportional (P), integral (I), and derivative (D). There are books written about the function and programming of these things, but IMHO they are SUPER robust and work well for most controls

An SSVR essentially takes some resistance signal and converts it to PWM. So you would turn the dial on your potentiometer up for MORE heat or down for LESS heat. Very similar to how the electric stove works.

What you need is a PID or SSVR.

 

[Brumateur]

Plus this is DC, we aren't controlling DC.

Actually DC PWM is OK since it does not control heating element directly. PWM output is connected to SSR and SSR requires DC input.

 

[Northbanu]

Thanks. I guess that trx racing stuff in the basement will stay in the basement. Holy crap, could I be more of a cheap ass?

 

[muhteeus]

Actually DC PWM is OK since it does not control heating element directly. PWM output is connected to SSR and SSR requires DC input.

I am dumb again, this is exactly what an SCR does. Some VFDs control based on this method too.

Would work were it in the realm of a reasonable frequency, which it ain't.