12VDC PWM Temperature Control - Frequency Question

Thread Starter

POS1000

Joined Nov 24, 2023
2
Hi all. Hope I get the detail correct

Putting together a 12VDC heater controller to replace two separate OEM types for gloves & jacket

Long story short - separate units are push button, cannot adjust on the go, too many wires etc
Combined 2 x dial OTS Components are not available from OEM due to upgrade in connections & controls.

I have Qty 2 separate original controllers - 12VDC
1. Gloves - push button - giving 0% 25%, 50%, 75% and 100%
2. Jacket - Push button - giving 0% 20%, 40%, 60%, 80% and 100%

Oscilloscope reading (Sample at approx 50%)
Original controller 1 and 2 : Square wave, Duty 50% M200ms 1.25kS/s sample, Frequency 'showing': 1.953 Hz
So I am (In my own head) happy to say I have some type of PWM.

Replacement PWM 'project' using 2 'small' test PWMs with onboard pot:
New Controllers 3 & 4: Square wave Duty 50% M2 ms 125kS/s, Frequency showing: approx 476 Hz

This is a heating circuit dependent on the duty cycle with no temp control. 12VDC at 100% being the limit.
for now, it seems to work well, but...
Is the frequency relevant, for resistive, does it average out regardless or should I have something closer to the original ?
'Finding it difficult to get a 'stated' frequency on any of these 'cheap / small' PWM's
As I don't fully understand, I am thinking the circuit(s) may not get enough time to cool before being pulsed again, but the same can be applied to not getting enough heat before the pulse is gone. is 12V at 50% ... 6 or 'Half on/off'
Any advice welcome.
 

crutschow

Joined Mar 14, 2008
34,047
I don't fully understand, I am thinking the circuit(s) may not get enough time to cool before being pulsed again, but the same can be applied to not getting enough heat before the pulse is gone. is 12V at 50% ... 6 or 'Half on/off'
And I don't understand your concern.
You don't want it to cool off between pulses, you want a nice steady average temperature.

For your purposes, the frequency of the PWM signal will have little effect on the heating of the resistive elements, since it's the average value of the PWM signal, based on its duty-cycle, that determines the amount of heat generated.
The resistors along with the clothes have a high thermal time-constant (temperature changes little with short periods of the resistor being on or off), so even a frequency of only a few Hz would likely work well.
 

Thread Starter

POS1000

Joined Nov 24, 2023
2
@ crutschow, thanks for the quick response. asked and answered, so sounds like my little project is complete.
I would be be more used to dealing with PID, bigger masses and a fixed temperature, so I'd often have to adjust for overshoot. my brain was just thinking that If I keep pulsing the elements quicker than the original setup that they may... overshoot ... each time and continue to rise.. and this isn't my area of expertise.
Your explanation clears that up, thanks a mill.
 
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