Mosfet heater control

[AgilePeanut]

Hi all.

I am not very clued up with electronics. I am a mechanical engineer and did study some electrical engineering subjects at uni but that was awhile ago and I didn't do that well at the subjects back then.

Anyways, I am busy trying to get my masters degree and I have built an experimental setup which investigates heat transfer in water. I am not going to explain my project or setup in detail but I will try to give enough detail to my problem as I can.

I am using n type thermocouples to perform temperature measurements. These thermocouples are located very close to a 2500W / 230v AC heater element. These thermocouples cannot be shielded or moved away from the heater due to space limitations.

I have a problem with electromagnetic interference from the heater as one would expect. The readings from the above mentioned thermocouples are definitely influenced by the heater when it's switched on.

I am using an agilent data logger to measure voltages from several pressure transducers and thermocouples in my setup every 2.13s. These measurements are then sent to a pc running labview in pulses. I am using labview to post process my results but I am not currently using labview in any way for control.

My solution would be to create an arduino / Mosfet setup which would turn the heater off, take a measurement from the thermocouples, then turn the heater back on. This would happen relativly quickly and timing would need to be important hence the inclusion of the arduino for control. As the heater is AC, I will have to include a bridge rectifier.

So this is where I stand. I am afraid of blowing the arduino & pc if the circuit is not correctly designed as the current is fairly high in the heater. Is there any advice that you can think of? I have never designed my own electrical circuit before.

 

[Arouse1973]

Hello. What about using a large relay to control the heater. You could then switch the relay using the Arduino. This would also give you isolation from the mains and you could run this from a battery if you wanted to.
Adam

 

[AgilePeanut]

Hey.

I have thought of a relay but I am worried about how any spikes in current could have a negative effect on the heater and maybe damage it. With a Mosfet you can ramp the currrent with a gradient which I think would be less harmful. I could be wrong though.

 

[(*steve*)]

If you ramp the current up you may well evaporate the mosfet.

If you want to be kinder to the heater (assuming it is running from AC) switch it on and off at a zero crossing of the mains.

 

[AgilePeanut]

If you ramp the current up you may well evaporate the mosfet.

If you want to be kinder to the heater (assuming it is running from AC) switch it on and off at a zero crossing of the mains.

I've seen many examples of Mosfets ramping current and voltage. I have also seen examples of relays causing spikes and damaging circuits. I am assuming that you mean I must turn the power off manually. As I mentioned, timing would have to be controlled to get correct measurements so that isn't an option.

 

[Alec_t]

Can you just sense the mains zero-crossing point and take your measurment then? A measurement can surely be done within 1mS or so?

 

[Minder]

If it was switched on/off with a very brief period there would be no time for the heater to drop in resistance, also a zero crossing SSR could be used.
M.

 

[(*steve*)]

Damn, I'm sure I wrote a long and detailed response to this. Don't tell me I didn't hit "post" :-(

 

[duke37]

I wrote a response to AgilePreanut but it went in a different thread.

 

[AgilePeanut]

Damn, I'm sure I wrote a long and detailed response to this. Don't tell me I didn't hit "post" :-(

Ah darn.

 

[AgilePeanut]

If it was switched on/off with a very brief period there would be no time for the heater to drop in resistance, also a zero crossing SSR could be used.
M.

Hey. i've never heard of a zero crossing SSR before. thanks. Reading up on them now

 

[Minder]

Opto22 has a good site and listing.
M.