Help using a PID temp controller with a 24V silicone heating pad

[cvriv]

Im planning to assemble myself a small electroplating lab in short. I want to use 24V silicone heating pads wrapped around beakers to heat the solutions. I would like to use a PID temp controller to regulate the temp of the solution, but the SSR output of the controllers is 12V at a pretty low current. I thought about using the SSR that comes with most PID temp controllers to merely turn on/ off a 24V power supply that will be used to power the 24 heating pad, but I'm not sure if that is safe or a good practice. I don't know how often the SSR switches its output on and off. Anyone think this is ok? I read that I could use some kind of MOSFET to supply the 24V to the heating pad from a supply? Thanks.

[BeBuLamar]

Most PID controller has a parameter called proportioning period which is the time for it to do an on//off cycle. Most is defaulted to 20 sec. I generally set for 1 sec. Using the SSR the fast switching frequency is of no problem. If you use an electro-mechanical relay or contactor then I would recommend to leave it at 20 sec.

[cvriv]

Thanks. I'm not worried about the SSR switching on and off regularly.  I'm wondering if regularly switching on and off a small 24v power supply is bad.

[ealex]

hello

the standard SSR's are designed for AC - they will turn off at each zero crossing.
if you use it for DC it might trigger once and then remained turned on until you remove the power.
might trigger: 24V might be too low for the triac inside, and for it's driver chip (if it has one), and usually the driver looks for zero crossing to actually turn on the triac.


if the SSR you have is specified for DC, then it should be fine as long as you're within it's operating specs.
get one that's at least 20-30% over your heating pad current - it might have higher current draw when cold.
even more if it's an ebay / aliexpress / amazon "special"


you can make your own from a mosfet and some other parts, but the premade ones have the advantage of a case / heatsink + you get it working right away

[coppercone2]

no those power supplies have inrush to deal with its probobly not the best idea to keep connecting and disconnecting them from mains (and its power inefficient probobly). it is mega bootleg

Get the right type of SSR to control the voltage on the output.

Just connect another SSR to it. The first SSR won't be doing much buts its way better practice then switching a supply on and off to save on buying a SSr.

[cvriv]

Yea I shortly after posting my last reply realized I should just search for a DC SSR. The heating pad I'm looking to use is a 24V 120W so the typical 25A SSR would be plenty and a for a few bucks more I can get the 40A version which is overkill but couldn't hurt. I know what I have to do now. Thanks.

[Jeroen3]

Can you use 24VAC? Transformers don't care about turning on of off.

[coppercone2]

magnetizing current

I don't think its good idea to switch transformer primary on and off

[Jeroen3]

You can proportionally control the primary of a 24VAC transformer with an SSR.
Eg: 10PCV2415

[BeBuLamar]

You can proportionally control the primary of a 24VAC transformer with an SSR.
Eg: 10PCV2415

Not the Crydom 10PCV2415. The datasheet said for resistive load only. I have tried it to replace the Celduc phase angle controller and it draws way too much current when I tried to reduce power. It seems that the output waveform at low power settings has a lot of DC component and thus the trandformer would draw very high current.

[Terry Bites]

Fine control will be difficult because of the high thermal mass. PID is overkill, in heated system it can't cool so theres's the "P"in disarray. "Bang-Bang" control will probably good enough.
Tune the control with simple hysteresis. Insulate the heater from the external environment. I had to make something similar to your project.
I used an LTC1041 with an external thermistor.

[MarkT]

Fine control will be difficult because of the high thermal mass. PID is overkill, in heated system it can't cool so theres's the "P"in disarray. "Bang-Bang" control will probably good enough.

PID is always worth trying as you can get more accuracy - systems always cool to ambient, extra cooling is not necessary unless you want to rapidly reduce temperature, or respond to other disturbances that add heat.

[Jeroen3]

PID with a on/off output will still be more accurate than a true bang/bang because of I and D action. If your system is slow enough the PWM period can be minutes. As for process ovens for example. However, be wary of using D action if there is a door!
Eg: Wachendorff Universalregler UR3274xA