12v dc Heat Control unit

[J Porter]

Hello,

I am in a senior engineering design class at my High School. My group is designing a heated shift knob for a manual car. We are using 18 gauge NiChrome wire to heat. I thought we could just throw some thermistors in the circuit, but then I figure out that thermistors don't usually handle that many amps. I now am thinking of going the thermostat hooked to a relay route. We want to have a low and high setting, and we already have three way switches for this. We are staying well bellow 10 amps at 12v dc.

So what I'm wondering is: what is the best way to control current via temperature? is there any way to fit all of the heat controlling components inside the shift knob or will we have to have an external control box? Is there any stores I can buy the electrical components I need in small quantities?

 

[ClydeCrashKop]

I think I would use a comparator like the LM393N to control an N channel MOSFET like this one IRLI540NPBF. The comparator can compare a thermistor, (temp gage sender) to potentiometer to turn the element on and off. All small enough to fit in the shift knob and run on 12 volts. Search for window comparator circuits.

 

[heydonms]

Do you actually need temp control or would "a bit warmer" and "much hotter" be sufficient?

I would have thought that 3-4 heating elements arranged in different combinations of series and parallel to give different resistances would work well enough.

 

[J Porter]

heydonms, that is a good point. I'll have to talk to my group tomorrow to see if they want to go the simple way or the sophisticated way.

ClydeCrashKop, It took me a little while to get my head wrapped around it but that is a really cool idea. One thing I am unsure of is that I saw on many diagrams that they have a resistor hooking the gate on the MOSFET to the ground. What is the purpose of this resistor and should it be any certain value?

 

[heydonms]

Probably just a pull-down so that the MOSFET tends towards a known state. Resistance should be low enough that it keeps the gate low regardless of any transients and high enough that it isn't drawing excessive power (since you are building a heater as long as you stay within the power limits of the comparator and resistor the "wasted" energy probably isn't an issue). I would say 1-2k would probably be a reasonable starting point.

 

[J Porter]

Would that MOSFET limit the current from the drain to source, or would I have to put a high watt resistor on the heating wire so it doesn't heat up too fast?

 

[tcmtech]

120 watts for a shift knob is way too much. Give the rough estimate on size 10 watts would be more than enough.

A simple cheat for that power level would be to bury a common tail light bulb inside the shifter knob.

 

[Joe G]

the resistor on the gate keeps the mosfet from "floating" ( flickering on and off) this way it know what its supposed to do until its told to do something.

 

[4pyros]

We are using 18 gauge NiChrome wire to heat.

Way to big you will burn someone's hand.

 

[J Porter]

Well I know we want about 2-3 Amps passing through the wire since it heats up at a reasonable rate at 2-3 Amps. If a car battery is supplying 12-14v. Then use V=IR, So 12v= 3 X R.

By that logic I want about 4-5 ohms total resistance on the system. Lets hypothetically say the final resistance of the wire is 1 ohm. So, Wouldn't I need a 4 ohm resistance added to the system? Would this have to be a bulky resistor to absorb the energy?

 

[heydonms]

Would that MOSFET limit the current from the drain to source

Only in the sense that it will let the smoke out and (hopefully) go open circuit if you overload it.

would I have to put a high watt resistor on the heating wire so it doesn't heat up too fast?

Your heating wire *IS* a high watt resistor. If it isn't drawing the current you want, alter the heating element to give a different resistance. Change the wire thickness, number of coils, etc.

 

[J Porter]

Only in the sense that it will let the smoke out and (hopefully) go open circuit if you overload it.



Your heating wire *IS* a high watt resistor. If it isn't drawing the current you want, alter the heating element to give a different resistance. Change the wire thickness, number of coils, etc.

Wow, Thanks for the simple solution.

 

[4pyros]

You need a thinner wire!! 18 gage is way to big for what you are doing.

 

[tcmtech]

Why do you feel you need to heat the shifter knob anyway?

If you are shifting that much it will be warm from your own hand. If the vehicle is that cold you have other bigger heating problems to deal with.

 

[J Porter]

We are designing a heated shifter knob for the same reason people have heated seats in their cars or heated handle bars on the atv's and snowmobiles. Its purely for comfort.

I think I can get the 18 gauge wire to work fine, but thanks for the heads up.

 

[chemelec]

Drive your Nichrome wire with a PWM Circuit
Such as in the attached Circuit.

It won't give a Specific Temperature, But it will allow you to adjust the Current from a Few Milliamps and up to Maximum.

Use a Mosfet with a Good High Current rating and put a HEATSINK in It.

 

[J Porter]

I think I would use a comparator like the LM393N to control an N channel MOSFET like this one IRLI540NPBF. The comparator can compare a thermistor, (temp gage sender) to potentiometer to turn the element on and off. All small enough to fit in the shift knob and run on 12 volts. Search for window comparator circuits.

Well, I sort of took this approach when setting it up. I planned on sending the voltage via the thermistor to the V+ of which comparitor (heat setting) I wanted. I used an 18 ohm resistor for the high setting and a 13 ohm resistor for the low setting, these would be compared to a 15 ohm thermistor.

So I tried to breadboard this and instead of using the thermistor I just used a jumper wire as it would always have the greater voltage of the resistor, to trick the comparitor high. Of course, like many of my breadboarding experiences, nothing happend. I didn't get any voltage readings from the MOSFET or even from the comparitor. Im thinking it might be my resistor values but I'm not sure. I attached a simplified diagram of how it is hooked together. Please let me know if you can help me out.

Thanks,
Porter

 

[ClydeCrashKop]

These are some of the readings from the temperature gage sender that I used.
You don't really need a "WINDOW" comparator. Just 1 will turn it on and off.
TEMP Resistance
F Ohms
30 6000
35 5200
40 4500
45 4000
50 3600
55 3200
60 2850
65 2500
70 2200
75 2000
80 1850
85 1650
90 1500
95 1350
100 1200
105 1080
110 944
115 870
120 794
125 720

Try this circuit. I think I got it right.

 

[J Porter]

Try this circuit. I think I got it right.

Well I spent my day today figuring this out. I designed a circuit with parts I know I can order.

So, basically there are always two separate comparisons running. One comparison for a high temperature, and the other for a little lower temperature. Then the three-way switch picks High, Low, or Off to send to the MOSFET. If you see any issues please let me know.

Thanks

 

[ClydeCrashKop]

It looks okay to me except that you should put the on / off switch on the main 12 volt input.
Because a MOSFET can turn its self on if the gate isn't intentionally held low to turn it off. Also, MOSFETs can fail in the shorted condition that would leave the heater on.