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Temperature switch for 5A load

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bighand

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I'm trying to put something together to control the heated seat in my car, and have come up with this:

Image1.gif

The idea is to keep the seat at a reasonably constant temperature by switching the heating element off once a preset temperature is reached, and back on once the temp drops slightly.

R1 = NTC thermistor attached to the seat (5-15KΩ in the temperature range I'm interested in.)
R2 = Pot for setting the cutoff point
LOAD = The heating element - a resistive load of approx 2.6Ω @ 12V nominal (automotive, so really anything from 12-15V)

The circuit I based the design on used a relay to switch the load on/off, but I wanted to avoid this as a relay clicking every few seconds would be annoying, so I've replaced it with a MOSFET. This is cobbled together from a variety of sources, so the big question is: Will it work?

I know that the 741 isn't an ideal choice so I looked at using a comparator instead, as suggested by a few people elsewhere, but I don't know how to drive the MOSFET from an open-collector output.
 
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MikeMl

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Several things:
The reason not to use the 741 is because its output will not swing very close to ground or 12V. Use an LM358 instead.

I would configure the opamp with positive feedback (Schmitt Trigger) so that its output snaps between two states, fully high and fully low, so that the FET is either fully on or fully off, with minimum time spent in the transition. Otherwise, the FET will dissipate a large amount of power while the thermistor heats or cools, and you will have to mount it on a large heatsink.

Making an opamp Schmitt Trigger is accomplished by connecting R6 to the + input instead of the - input. The value or R6 should be more like 2meg which will provide just enough hysteresis for the output to snap high/low without shifting the trip point much.

You can leave out R7 and R8. The gate voltage of the FET can be driven directly from the opamp output.
 
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Boncuk

New Member
Hi MikeMI,

R8 makes sense to me. It helps discharging the MosFet's gate when the OpAmp output swings low.

Boncuk
 

MikeMl

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Most Helpful Member
Hi MikeMI,

R8 makes sense to me. It helps discharging the MosFet's gate when the OpAmp output swings low.

Boncuk
Since the LM358 can sink 10mA at Vo=1v, and 0.1mA at Vo=100mV, I figure it can discharge the gate capacitance of the big FET without adding a 10K resistor (which can only sink 0.1mA at 1V) ;)
 
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Boncuk

New Member
Since the LM358 can sink 10mA at Vo=1v, and 0.1mA at Vo=100mV, I figure it can discharge the gate capacitance of the big FET without adding a 10K resistor (which can only sink 0.1mA at 1V) ;)
That's why I normally use a 1K resistor. :D
 

MikeMl

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That's why I normally use a 1K resistor. :D
But then the LM358 must source 12mA when it is driving the NFET gate high. All that does is waste power, and slow the risetime on the gate. Since the threshold for most of these NFETs is about 3-5V, suffice it to say that the 358 can turn the FET hard on, and it can pull the gate low enough to completely turn off the FET without any resistors.
 

Willbe

New Member
The R2/R3 voltage accuracy depends on the 12v accuracy. Auto voltage varies from 11v to 15.5v, plus it has a load dump transient so you might want to use a voltage reference/regulator chip somewhere in this circuit.
 
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bighand

New Member
The entire circuit is powered from the same "12V" source, so won't the outputs from the R1/2/3 and R4/5 voltage dividers both stay the same in relation to each other even if the actual supply voltage changes?
 

MikeMl

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Most Helpful Member
The entire circuit is powered from the same "12V" source, so won't the outputs from the R1/2/3 and R4/5 voltage dividers both stay the same in relation to each other even if the actual supply voltage changes?
Yes, the circuit utilizes a balanced four-arm bridge, whose balance is not effected by the supply voltage. Your circuit will work just fine without a regulator. Load Dumps are just as likely to kill a three-terminal regulator as an LM358. If you are really concerned, put a series 100Ω resistor between the power input and the VCC pin on the 358. Feed the bridge from the downstream side. Feed the seat heater from the upstream side of the 100Ω resistor. Bypass the downsteam side of the 100Ω with a 47uF, 50V capacitor. The RC time constant will take care of most spikes.
 

Willbe

New Member
The entire circuit is powered from the same "12V" source, so won't the outputs from the R1/2/3 and R4/5 voltage dividers both stay the same in relation to each other even if the actual supply voltage changes?
I used to know this stuff!:(
 

MikeMl

Well-Known Member
Most Helpful Member
Like this?

Note the hysteresis on node "p" caused by the current through R4. If that is too much, make R4 even bigger. Note the tricky use of a voltage controlled resistor R2 controlled by V2 to simulate the thermistor heating and cooling.:)
 

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