Thermocouple Temperature Controller

[curoi]

Multimeter

I don't think I can budget for a $100 dollar multimeter. I realize the TRMS is more accurate (I hope that's right) but I might not be able to afford one. I'll run it by my boss. As far as hooking up the multimeter to the heater pads, how do I go about doing this?

 

[KeepItSimpleStupid]

Assume the meter is for the lab.

1) you need the polarity of the DC adapter.
2) Eventually, you'll have to be able to determine if a heater is open.
3) You need to size the 12 V supply.

Without TRMS the relationship of P=VI won't be valid at the output of the dimmer, but you can use I of the input and V of the input with a guestimate efficiency factor. The freq response of the TRMS matters too.

Non TRMS meters measure the average of the rectified waveform and multipy it by a number that makes a sine wave input read in RMS. It basically makes P=VI valid for the same AC(RMS) and DC voltage.

I'd also recommend a precision resistor to be determined. e.g. if 1 A is expected, R=0.1 V/1 A. This will allow you to measure power easily. 100 mV is negligible compared to 12 V. P > 0.100/1 A

Also remember we think you need a bigger 12 V supply. You need

 

[curoi]

Can I do this without a TRMS multimeter? A hundred dollars is steep for something we will never use again.

 

[KeepItSimpleStupid]

Any meter is better than nothing and you need something.

If it measures frequency, you would have the ability to at least determine if the dimmer is working.

and you should buy at least one of these 0.1 ohm resistor. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RHRA-.10-ND

Why: To allow you to measure current without having to use the current scales of the multimeter. Power will also be more accurate.

Without a TRMS meter, you can only measure current and voltage at the input of the dimmer. With the addition of the resistor, you can measure input power TO the dimmer.

Right now, I'm assuming 1 AMP, but the resistor can safely handle more.

I would probably build it with resistors for all channels or even put an LED and resistor at the output of the dimmer. Not sure the usefullness of the latter. LED's are really cheap and resistors can cost < $0.10 each.