I couldn't reply to your email, so I'll post it here.

Gord,

This is very early in the design stage so I'm really
only trying to line up how I will approach the
problem. The temperature I am looking for is 30-33
degrees C so I might be way off on the wattage
estimate. I haven't even looked into the possible
wattage values and I have little experience with
thermal systems. I know it will be a low power system
though. The control voltage and current have not yet
been determined.

Since this is a personal project (for my hobby) cost
is a concern. My approach is:

1. Find out if it can be done.
2. How much will the major components cost
(controller, TC, various circuits)
3. Spend time determining exact requirements
(wattage, etc...).
4. Physically design the system (CAD modeling etc...)
5. Order the parts.
6. Fabricate it.

I'd suppose I'm currently at stage 1 or 2. I have
heard of Peltier elements being used for systems like
this. Do you know anything about these?

Thanks,
Bob



"GS" wrote in message
...
Howdy Robert.....Based on my own experience designing a similar situation
I'd want to know what the thermal mass is that you're trying to heat to
get a handle on the stability issue. Based on my own experience, if you
can get away with something simple like a relay to switch your heater, use
that (could be a solid state relay etc). Depending on the anticipated
repairability of this circuit, you could design your own switching based
on everything from bipolar, Power FET to the SCR's or TRIAC's as others
have alluded to.

Because you mentioned stability, the amount of times you'll have to switch
on versus heat loss will be the basic issue assuming you have sufficient
heat input to overwhelm a quickly cooling load. If the heat load is
small and the heater has a lot of output in relation, then you'll be
playing with overshoot a lot more.


Gord