controlling a strip heater?

[Bill Kraus]

Hi -

I'd like to build a circuit that will control the temperature generated by a
high density strip heater. The goal is to use a temperature controller (with
a thermocouple) whose output is wired to the heater - the temperature range
is from 10 to 100 C.

The strip heaters I've seen are driven by either a 120 or 240 voltage
source. The temperature controller I'm looking at has an input voltage of
either 12 or 24 VAC (what is the advantage of each?) and an SPDT relay
output at 4 amps (26 VAC resistive). I assume that I can use a transformer
to go from the household outlet to the controller, but do I need a
transformer from the controller to the heater strip?

Any other advice/suggestions would be appreciated.

 

[John Popelish]

Bill Kraus wrote:

Hi -

I'd like to build a circuit that will control the temperature generated by a
high density strip heater. The goal is to use a temperature controller (with
a thermocouple) whose output is wired to the heater - the temperature range
is from 10 to 100 C.

The strip heaters I've seen are driven by either a 120 or 240 voltage
source. The temperature controller I'm looking at has an input voltage of
either 12 or 24 VAC (what is the advantage of each?) and an SPDT relay
output at 4 amps (26 VAC resistive). I assume that I can use a transformer
to go from the household outlet to the controller, but do I need a
transformer from the controller to the heater strip?

Any other advice/suggestions would be appreciated.

Controllers made to operate on low voltage are a move to miniaturize
and make these devices safer and cheaper. They are so small, now,
that it is getting dangerous to have line voltage wiring on the same
terminal strip as the thermocouple wiring. Also, the alarm outputs
are often wired to low voltage systems like 24 volt PLC systems. A
wall wart (small receptacle mounted transformer) is probably available
that will provide the power to operate your controller.

I would use an external relay to boost the contact rating for long
life (and easy replacement) rather than operate the heater directly
from the tiny internal relay. A solid state relay may be a good
choice for long term reliability, especially if your controller has a
high temperature alarm to handle the failed on state that may follow a
severe line surge. For redundant protection, wire a mechanical
normally closed relay operated by the high temperature alarm in series
with the control contact. That way, if the normal control contact
fails on, you will still have rough control accompanied by repeated
alarms.

--
John Popelish