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Just to rehash some old discussions, and make sure I'm understanding what I'm going to be trying to do before I do it:

I want to set up full digital temp control for my heating blankets. I've got some solid state relays, 25 amp, they claim 240-380v (although the seller assured me it was 24 to 380v), and I'm looking at getting a PID and a thermocouple. Am I correct in assuming the voltage ratings on the SSR are maximum, and it should switch 110v or 220v power? If I've understood it all correctly, I wire up the PID to:

1) Mains power
2) Thermocouple (in bending sandwich) to thermocouple input
3) Control output (DC current, something like 12 volts) to the 'control' end of the SSR

As for the SSR: Iwire one of the two wires from the heat blanket through the relay, which is basically an interrput switch that opens and closes when the PID sends a signal saying so. So basically, wire it in series with one of the two power leads: AC current -> SSR in-line with one of the blanket wires -> blanket.

Does that sound right? Is the SSR I have suitable?

We have had a couple discussions on PID type controllers in the last year or so, I did mine with a Chinese made controller that I picked up on Ebay. The unit I got has an output current rating high enough for the blanket, but if not any relay solid state or magnetic will work. Get your controller and follow its documented wiring setup and you will be good. I did not use any of the PID program settings as simple on off control is more than enough accuracy for heat blanket use. Mine keeps the blanket temperature within 3 degrees F of set point once up to temperature.

Before retiring I was a controls Electrician in a auto plant paint shop and dealt with these types of temperature controllers on a daily basis and they are overkill for our use but I wanted the safety factor they provide. I also wired a timer into my setup again for safety, sometimes one gets distracted and I hate fire.

Check this thread http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=19892

Fred

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Fred Tellier
http://www.fetellierguitars.com
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Mattia,

Do you have a manufacturer/model number handy for the SSR? With that info Fred, myself, anyone else who's interested can help with the electrical controls sanity check. Since an SSR is an on/off device you'll need to find a controller with a discrete output (transistor output with appropriate voltage, or relay output) to connect to the SSR's control terminals; analog outputs wouldn't work with a relay.

I like Fred's thoughts on on/off control. While PID control can give the blanket more "oomph" when it's really cold & ease up on the heat when you get close to your final temperature, it can sometimes be a little tricky to set up -- if you're not careful it's easy to overshoot by as much as 25-50°C.


Fred,

Hi. I'm a PLC/HMI/controls guy. It's always nice to run into other controls people online... nice to meet you. I'll have to dig into those old discussions you linked to when I get a moment.

Mattia, I've also built one and I used a PID controller that has an internal relay with a current rating adequate for a blanket. All you have to connect to it is a thermocouple and 110 or 220 VAC.

If you're interested I can post a photo and get a part number for you.

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Steve Smith
"Music is what feelings sound like"

Steve: that would be great! The more idiot-proof, the better.

The SSR I have is a generic chinese unit, Mager, number's D4825

Mattia, I'll get that for you after I get home from work.

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Steve Smith
"Music is what feelings sound like"

I got the controller from Wolf Automation. The part number is TC3YT-B4R16 and they are about $65; the internal relay contacts are 250VAC 16A. I've got mine wired so that the power from the controller to the blanket goes through a 30 minute timer so I can't accidentally leave it on. The controller power is always on and it always reads the thermocouple (as you can see from my warm shop). The timer came from Home Depot.



I attached the thermocouple to the blanket with some silicon RTV. You can see the head of the thermocouple sticking out there. I'm not completely happy with this but it seems to work ok, the thermocouple is down by the end of the side during bending. I used the connector so I could also use the same controller with different blankets for repairs.

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Steve Smith
"Music is what feelings sound like"

Keep in mind that you want the relay to be on until the target temp is reached. That means you need your controller to be normally closed. I use a cheap mechanical relay on mine. I also recommend the addition of a timer. The normal failure mode for an SSR is to fail in the ON condition.

One other suggestion: keep the controller as a separate device. This allows you to use it for other tasks such as removing a fingerboard with a blanket. You can even make a cheap immersion cooker with the same controller. You can't over cook the fish or chicken if you vacuum bag it and have the water temp controlled at the max cooked temp and you can have it ready to serve hot when the rest of the meal is done like the high tech restaurants do.

Steve Brown