SSR Behaviour

[Cracked]

#3591

I’m using an external Fotek 25A 240V SSR with my osPID to control a heating element. For a cooling cycle I step down from 225C to 20C.

When tracking the temperature using the front end, I see the osPID output fall to 0% and the temperature exhibiting exponential decay (as I would expect).
However, it tends towards 60C (not 20C) – which suggests to me that the input air is somewhere around 55-60C.

My original expectation was that the osPID would in effect open the circuit on the heating element, so no heater output, thus I should see cooling tending to ambient (~25C).

Is there some behaviour of SSRs or the osPID which would prevent heater output from going below a certain floor value (say, 25% output)?

I could just install a switch to open the heater circuit, but I’d rather not have to if I can avoid it.

[deejay1272]

#3989

Check to make sure that your PID output isn’t going below zero. If that happens, then the system gets confused and turns the heater on.

You might also check if you have insulation of some kind that is storing heat somewhere. Over a long enough period, your system should absolutely return to ambient temp (much shorter if you’re aspirating the system in some way).

[Cracked]

#3990

Cheers. On the dashboard it says the output is 0, and the LED on the relay card stops lighting up.

It could possibly be residual heat, as the outside of the unit gets warm, but not as hot as the roasting chamber. However, I suspect this is not the case.

I might test it by turning the unit on with a manual output at 0, and see what the temperature is.

[Brett]

#3991

Have you confirmed that this isn’t part of the inherent physics of the system? I would suggest physically disconnecting the SSR from the osPID at the step down phase, leaving everything else intact. if the behavior is different than when the osPID is connected, then we’ve got some work to do. if it’s about the same, you may need to look into changing how your system is configured.

[Cracked]

#3992

Cheers for the response Brett.

I will try as you suggest, and unplug the osPID from the ssr to see if there is any difference.

With regard to the inherent physics:

I have a small popcorn air popper that I am using to roast coffee. This consists of a nichrome wire element at 240V and a small electric fan. I have wired the ssr into the active wire to the nichrome element – the fan is in parallel.

The unit weights maybe 500g (~1 lb) and is mostly plastic, with a thin aluminium chamber in the centre (very light). The thermal mass is very low.

It’s possible that the temperature of the plastic housing gets to 70ºC, and that this is heating the intake air – I should be able to test this by testing without the osPID from a cold unit.

My next task will be to hook up a raspberry pi running linux, so that I can remotley monitor the progress…:)

[Cracked]

#3993

So, I have confirmed the problem is not the osPID. If I disconnect the 5v to the SSR, I still get some heat output. Is it likely to be leakage due to a cheap ssr?

[Cracked]

#3994

Sorted this problem. Turns out that the heater/fan arrangement in what I am controlling (a modified popcorn popper) uses a secondary heating circuit to step down from 240V to 24V for the fan. I had the SSR only switching the main heating circuit. I’ve now separated the fan power supply, and am switching both heating circuits. Working well!

[Author]

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