Hey guys,

I'm a 2nd year engineering student, I'm working on my generally mechanical based project but I've come across a minor electrical problem that you guys may be able to help me with

Basically, I have heat blankets (850W) that plug straight into a wall outlet... I need to regulate its temp at 70°C. I also have a thermocouple.

I figure I can make a PID controller with a pot, op-amps and a few other minor components. My knowledge is pretty limited on designing electrical stuff, so I was wondering if you guys could help me implement http://www.ecircuitcenter.com/Circuits/op_pid/op_pid.htm to what I'm doing, or suggest easier ways to do this w/o spending very much money?

Thanks!

 

I'm trying to upload a scan to you, but this is the first time I've tried this. Here goes. If the file appears, I will elaborate.

 

I built this 30 years ago and it's still working! I changed a couple of resistors and it is keeping my bedroom at 70 degrees with a space heater attached.

The basic premise is that a 723 voltage regulator chip has a voltage reference and an op-amp. Add a 10 k thermistor and change the hysteresis resistor to a lower value so it snaps on and off, sharply. I expect you'll ask questions to clear up defects in the scanned image. I await your replay.

 

ps, the plastic box is not good enough. You must use a metal box and add a slab of aluminum to keep the triac cool. I changed the 7.5 k resistor to 12.4 k to get 70 degrees at "3" on the dial. (The dial is labeled 0 to 10.)

 

If you want cheap and simple, a bimetallic switch to turn the power on and off (like what's used in a kitchen oven) is usable. Once place to find one might be an old space heater (or check at the local building center or hardware store). If it can't switch the load directly, you could make it drive a solid state relay.

If you measure the temperature of an oven over time, it will usually be an asymmetric sawtooth wave. Thus, the temperature is never constant, but the mean temperature is nicely constant. If this doesn't work for you, then you might need the PID loop stuff. The solid state relay can be used for on/off control, but if you want proportional control, you'll have to look into using e.g. a triac or H-bridge.

It sounds like you'll be fooling with line voltages and a mistake can kill you. I'd recommend you check your circuits with your teacher before powering them up.

 

I built this 30 years ago and it's still working! I changed a couple of resistors and it is keeping my bedroom at 70 degrees with a space heater attached.

The basic premise is that a 723 voltage regulator chip has a voltage reference and an op-amp. Add a 10 k thermistor and change the hysteresis resistor to a lower value so it snaps on and off, sharply. I expect you'll ask questions to clear up defects in the scanned image. I await your replay.

Hey, thanks for the reply.

I talked to a prof and he recommended something similar but with a T2117 IC.

I'm going to order the parts in and just try it out. I'll come back if I have any issues.

Another question though..

I measured the voltage generated from the heat blanket and it's very small, what would you suggest I use to amplify the signal from the thermocouple?

 

I don't suggest you amplify the thermocouple. I suggest you get extravagant with your money and buy a thermistor. Mouser part number 871-B57891M103J 46 cents.

And while I'm here, the snubber on that triac is all wrong. (I was a lot stupider 30 years ago.) Besides, the snubber values depend on what your load is. Be sure to recalculate them.