car temp control (fuel heater + ventilation control)

Hello guys,

winter is here and I decided to put my Fuel heater which I have in my Toyota Avensis in use. I had to put electric water pump in to make it independend from engine.

My goal is to use remote starter output 12V/500mA to power circuit which will take care of temp control. This circuit will, when interior temperature is below 25C start ventilation+electrical pump, and if engine coolant is below 50C start fuel heater.

I've created circuit, which is combination of circuits taken from these sites:
http://www.solorb.com/elect/constemp/index.html
http://home.cogeco.ca/~rpaisley4/Comparators.html
and logical AND by using diodes from this site.

Here is what I have so far.


Fuel heater control and interior temp control have both positive feedback for hysteresis.
Fuel heater will start if interior temp needs to be increased and is below 50C and will continue until 75C. [Top opamp + top NPN circuit]
Output should be +Vcc when engine coolant is below 50-75C.

Interior temp control will heat interior up to 25C and will not engage until it drops below 22C. [bottom opamp + bottom PNP circuit]
Output should be +Vcc when interior is below 22-25C.

My questions are.
Which way do I compute values for resistor and choose NTC?
Would You change some of the parts to better suit this circuit?
Not sure about whether transistor will go off in this case once enabled.

Any help would be greatly appreciated.

Use an LM2903 comparator instead of a 741 opamp.

Your application calls for an ON/OFF type output rather than continuously variable. Opamps are not designed to be run in open-loop mode; comparators are.

Comparators generally have open-collector outputs; they require pull-up resistors in order to provide a current source. 3mA is a good target for the current source, or roughly 330 Ohms per volt of Vcc.

Furthermore, 741 opamps are not designed for automotive temperature ranges. LM2903's are specifically designed for automotive temperature ranges. Electrically, they are more or less equivalent to LM293/LM393's. If you are expecting temperatures to fall below -40°C/F, then you will need a mil-spec comparator like an LM193.

I've updated the circuit:


I've checked Rcoil of these Reed relays and its 1kΩ, which makes current at 14V (max) only 14mA.

I've computed resistors for the required values of NTC 1k at 20-25C and 50-75C.

Are the resistors R9 and R10 necessary? Since I have resistor at output of comparators...

I guess you didn't read what I wrote about the pullup resistors?
330 Ohms/volt would be 1650 Ohms. 1.6k is the closest standard value.

You're using some generic opamp symbol. This will cause you trouble later if you try to make a board.
I suggest that you delete them both, and replace them with LM393N's from linear.lbr.
If you drop both on the schematic at the same time, you will get the correct pinouts for an LM393/LM2903.

Your hysteresis resistors are far too low in value.

Yes, I've read it, but computed it for 12V instead of 5V. Thank You
Correct schematic should be this then.

I see you've removed the pots from the noninverting side.
That's the only means you have of adjusting the trigger levels.
I suggest you put them back in. 20K 10-turn trim pots should work OK.
Wiper (pin 2) goes to the noninverting (+) input. The "ends" go to +5v and ground.

Do you know what the characteristics of the NTC resistors are?

Yes, I've calculated values at 20-25C and 50-75C and converted it into Volts.
Then I used formula on comparator page and played a little in excel to get the proper values. This setup should work closely enough.

V+ 5 V

R1 10000 ohm

R2 4700 ohm

RL 1600 ohm

RH 6800 ohm

R1comb 22000 ohm

R2comb 2779.13 ohm

VrefON .8801 V 17.6 % VrefOFF 1.0874 V 21.75 % Iref 3.40E-004 A

Rntc 4700 ohm

NTC1k 20C 1228 ohm 1.0358 V NTC1k 25C 1000 ohm .8772 V




Intc 8.77E-004 A


V+ 5 V

R1 10000 ohm

R2 560 ohm

RL 1600 ohm

RH 22000 ohm

R1comb 22000 ohm

R2comb 546.1 ohm

VrefON .1241 V 2.48 % VrefOFF .2589 V 5.18 % Iref 4.73E-004 A

Rntc 6800 ohm

NTC1k 50C 387.4 ohm .2695 V NTC1k 70C 196 ohm .1401 V NTC1k 75C 167 ohm .1199 V Intc 7.15E-004 A



I will probably put the pots back, but I'm thinking of offsetting the pot with 15k on 5V side and putting 5k pot instead. I will need to test the circuit with resistors only as precise temperature control is not aim here. Fuel heater control can go from 45-55 -> 70-80 this is no problem.

Yes I have a table of values which tells me, whats the resistance for different temperatures. I'm using NTC 1k
http://www.gme.cz/_dokumentace/dokumenty/118/118-007/kgm.118-007.1.pdf

OK.

Your D1 and D2 are creating a logical OR, not a logical AND.
You need resistors on the bases of Q1 and Q2.

Yes You're correct. I've changed it to logical AND hopefully.

Not sure whether base resistor values, pull-up for Q2 and pull-down for Q1 are correct.

Using those two diodes, you still have an OR gate.

Guess I'm going to have to plug some of this stuff into a simulator to see how it'll work.

those diodes should be ok now.
With 5V on any comparator output will turn off Q1.

First comparator is 0V when engine coolant temperature is below 50-75C.
Second comparator is 0V when interior temperature is below 20-25C.

Only at these conditions fuel heater should start.

Can You please comment on the Base resistor values? I've computed with 120gain and base current 0.025mA where Rb came to 17.6k (176/10)

Question is, will T1 close?

IC1A output is High when temperature on NTC1 is initially below 50C.
IC1B output is High when temperature on NTC2 is initially below 20C.

Obviously I need a circuit, when IC1A and IC1B is High I want to close that reed relay1.
And when IC1B is High I want to close reed relay2.

Here is the circuit.