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proportional thermostat

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SYNAPS

New Member
can someone help me and tell me if this is possible

i have a project that requires a constat temp with a tolerance of >1 deg f right know with the thermostat that i am using i am getting 4.6 deg f i was wondering if i used a rheostat and turned down the watts say25% and let my ceramic heat source run warm all the time would this help my tolerance and 2 can this be done by wiring the rheostat to the 2 post of the thermostat any help would be great thanx
 

MikeMl

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Most Helpful Member
Is your existing thermostat switching the heater directly?
Is it a bi-metal, snap-action thermostat?
What temperature?
Supply Volts to the heater?
How many Watts to the heater?

How much money are you willing to spend?
For example
 
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stevez

Active Member
Unless the load on the heater is constant you won't get the 1 deg tolerance. There is a difference in temperature, between the point where the switch goes on - then goes off. I think "differential" is the term used by some manufacturers. A mechanical thermostat is unlikely to have a differential small enough to give you the control you want.

If you were to reduce the power to the heater to a point where the heat input matched the load you would need no control at all. Unless you have one incredibly large rheostat you can't control your 1500 watt heater that way. Even if you reduce the power - if the thermostat has to cylce to maintain the temp then the swing in the room will still be significant.
 

stevez

Active Member
Take a look here: http://www.electro-tech-online.com/custompdfs/2009/06/z110-114.pdf They have some decent tutorials on temperature control.

Keep in mind that if you really do mean plus or minus 1 deg F that's very tight - if you mean that tolerance everywhere in the space and that the tolerance also includes the error in your measurement system.

With an electric heater and that kind of tolerance you will likely end up with some kind of proportional/integral control - or at least proportional with a PWM or burst-fire kind of output to the heater. These are readily available at relatively low cost - though I am not certain if they would within your budget.
 

MikeMl

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is there any one else availibile to help me on this matter
Does the controller need to have a display of what the actual temperature is?

Since your are building many of these, are you willing to adjust each one to 100degF individually, or are you expecting that if you build a batch, they will all be precisely at 100degF +-1degF without adjustment?

Building a proportional controller with an SCR or TRIAC phase-control using an inexpensive thermistor as the temperature sensor is doable, but will have a significant development curve, and I doubt that it can be done at a $20 unit cost.
 

tcmtech

Banned
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I have seen a homemade egg hatcher system some years ago. The guy used a simple thermistor to read the active heat level and then adjust the phase angle control of a triac type light dimmer that ran the small heater system.

There was one main pot that set the temperature and the rest was a closed loop feed back system.
His digital thermometer read a constant temp +- less than half a degree F of where he set it.

What do you need such a precise temperature control for?
 

Hero999

Banned
What's the power source?

If it's DC, just use a MOSFET and comparator with a very tight hysteresis band 0.05°C and the MOSFET with be PWMed at quite a high frequency keeping the temperature constant.
 

SYNAPS

New Member
What's the power source?

If it's DC, just use a MOSFET and comparator with a very tight hysteresis band 0.05°C and the MOSFET with be PWMed at quite a high frequency keeping the temperature constant.
sorry for the feather ruffling i am new at this and i must have hit the wrong button when i was trying to post it or trying to figure out how to post it i am ok now i think

any way no it is not dc it is 120 vac and the tollerance is very critical it is a oven for curring polymers and the polymerization piont has to be within 1 degc
 

MikeMl

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Look at this. It says it can be configured as a temperature controller
 

MikeMl

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thank you i am reading it now i am trying to educate myself please be pateint
Read up on SCR, TRIAC, Phase-Control, and PID controller
 
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Hero999

Banned
What about using a bridge rectifier to get unfiltered AC then PWMing it?

It's a heating element so it doesn't care whether it's run off AC or DC, it will still produce the same amount of heat.
 

tcmtech

Banned
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Phase angle control of the AC is basically the same as PWM. A heating element has a very slow heat up and cool time when compared to the speed of 50 or 60 Hz AC.
 
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