# How To Calculate Seat Heater Resistance

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#### Jeffsg605

##### New Member
This is something that has stumped me for a long time. Say I am designing a seat heater for my car. I obviously have a voltage of 12 volts. That cannot change. Then I have a heating coil which has a 12-ohm resistance. Using ohms law, this will draw 1 amp.

Now say I switch the coil with a 24-ohm coil. According to Ohm's Law, it will now draw 0.5 amps.

It seems to me that this doesn't make sense. Wouldn't a higher load draw a higher current? Otherwise, how do variable temperature seat heaters work? Do they lower the resistance to increase the temperature?

I'm just having a hard time wrapping my brain around this so any help would be greatly appreciated. Thanks!

#### ronsimpson

##### Well-Known Member
Wouldn't a higher load draw a higher current?
12 ohms to 24 ohms is not a higher load. It is a lessor load.
Resistance is like opposition to. (To resist something you are trying to stop it from happening.) So resistance is trying to stop current. A wire has very low resistance so it will carry current easy.

There for 12 ohms causes 1A. (or should I say allows) If you double the resistance it will 1/2 the current. 24 ohms and 0.5A.

#### Jeffsg605

##### New Member
That's interesting and not what I thought. Let me ask you this then. The seat heaters in my car have a 30 amp fuse. This would mean they could only have a resistance of 0.4 Ohms. That is either a very short wire or a very thin one. The seats in my car get surprisingly hot. Is that really possible with just 12 volts? Or is the voltage bumped up before it gets to the seat heaters?

#### Diver300

##### Well-Known Member
Otherwise, how do variable temperature seat heaters work? Do they lower the resistance to increase the temperature?

Nearly all variable heaters, including stoves, room heaters etc., are controlled by turning them on an off fast enough that the heat appears steady.

With a seat heater, to be fast enough it would need to be more often than every 5 seconds or so. It might be much faster.

There may be temperature control, with a temperature sensor in the seat somewhere. Then the heating is on full until the temperature is at the correct temperature. Then the various settings are the different temperatures.

Without temperature control, the settings would vary the power by varying how much of the time the power is on. It might be something like where low is on for 2 seconds and off for 3, medium is on for 3.5 seconds and off for 1.5, and full is on all the time. (Those figures are guesses, just it illustrate the method)

If the control is much faster, maybe thousands of times a second, if you measure the voltage across the heater it will appear to be less than 12 V. So if the power is on for 0.001 second and off for 0.001 second, with a 12 V supply, that would show as 6 V on a meter.

#### MikeMl

##### Well-Known Member
How hot the seats get is purely a function of the Watts dissipated in the heating wire. P = E^2/R

The heated seats in our car has just a high, low, off setting; no pwm or timing. I'm guessing just two heating wires switched from parallel (high setting) to series (low setting).

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#### Diver300

##### Well-Known Member
That's interesting and not what I thought. Let me ask you this then. The seat heaters in my car have a 30 amp fuse. This would mean they could only have a resistance of 0.4 Ohms. That is either a very short wire or a very thin one. The seats in my car get surprisingly hot. Is that really possible with just 12 volts? Or is the voltage bumped up before it gets to the seat heaters?

You've misunderstood what makes a low resistance. It is a a short wire or a fat one. Thinner wire has higher resistance.

The 30 A fuse could feed more than one seat. Also, a circuit that is fused with a 30 A fuse will never be intended to take 30 A, as some margin will be designed in. It would be 20 - 25 A at a guess.

You can get a lot of heat with 12 V. 12 V and 20 A gives a power of 240 W, which is a bit more than an electric blanket uses, so the seat will get quite hot.

The voltage will not be increased for the seat heaters, as it is far easier to have the wires fatter so that they take the desired current. There are applications where the voltage is increased or decreased for heaters, where the heating element has to be particularly thin or particularly fat, but in the vast majority of heaters, the wire length and width is adjusted to get the power required. The heating elements are often made out of metals that have a higher resistance than copper.

#### Jeffsg605

##### New Member
Very informative responses guys! I really appreciate the input! I've had this all wrong in my head for so long so this discussion has really helped clean things up for me.

#### ronsimpson

##### Well-Known Member
One more thing.
12 volts is when the car is not running. 12 volt battery
When the car is running the battery is "charging" which is about 14.5 volts.

#### RODALCO

##### Well-Known Member
Or the seat is hot and the car battery flat in case the battery is old.
Ensure your battery is in good condition.

#### hyedenny

##### Active Member
And, although it's small in the case of seat heaters, the resistance of a conductor usually goes up as the temperature goes up (positive coefficient of resistance). For example, the current in an incandescent bulb is very high for an instant before the filament heats up, after which the current goes way down to tolerable levels!

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