Anyone know what the spec is for the resistance of the heated mirrors? I think it's ~75 ohms, after checking one out, but the other is ~300 ohms. Which is right?

Thanks!

 

resistance = heat

 

thanks Brian... what an encyclopedia of knowledge you are!


Specs?

 

nothing? nobody has an extra set they can measure? all you need is a DMM and a door with the panel off. SOOOO easy to check

 

I probably have a spare set, but they are buried in my garage along with someones old white hood

 

tell you what, I may be getting into my doors this weekend, so I will post up if I do.
somebody else posts,

 

300 ohms is definitely wrong.

I'd say 75 ohms is incorrect as well.. that's only 1/6 of an amp at 12v, which isn't enough to do anything more than melt a single snowflake. I'd guess the heater puts out about 25W on each mirror, which means 2A of current. 12v/2A = 6 ohms.

 

if 75 ohms = 1/6 of an amp, how does 6 ohms = 2 amps?

 

75*1/6 =~= 6*2

 

V=I*R

12V = 2A * 6ohms

12V = 1/6A * 72ohms

 

Did anybody ever find this out for me? I'm sure SOMEBODY has their door panels off. Just run a DMM lead to each pin on the blue connector and check the resistance! Thanks all.

 

I will do it tomorrow. I am in pennsylvania atm, and as you can see from my profile, I am from OKC.
sorry, I thought you had gotten it covered from the other responses. I had just scanned them.
(will be about noonish for me when I get it up.)

 

sorry, traxtar, but I couldn't get to it today. I go the panel off, and went to find my mm, but the battery was dead. I have no other 9v stuff, so I couldn't swap the batt from anything. I will try to get it asap..

 

You're forgetting that heater circuits are like lightbulbs.. theyh change resistance based on temperature. they take more current when they're colder, and then less power as they warm up.

 

.... but to answer your question, DC resistance on the GXE mirror I have in the garage is 22.4ohms, slowly rising to 24ohms as I leave the meter's leads on it.

 

No, they don't - not much at least. The alloys they're made of usually intentionally made less temperature dependent and they don't reach the temperature of the bulb filament. This would make them headlights if they did .

 

That's funny. Just last night I witnessed it happen myself. DC resistance goes up by several ohms as the element heats up. no, not as dramatic as a light bulb filament, but it does in fact gain resistance as it heats.

I suggest you find someone else to argue with. I'd hate to have to embarrass you.

 

It is hard to embarras me by pointing out something I didn't know - keep it coming . OTOH according to your own measurements you got ~10% increase while bulb filament goes up at least 10 times (1000%). I don't think they can be compared and this was my point.

 

They surely can be compared- the ceramic elements just do it to a much lesser extent than a tungsten filament- which we both agree on. And we'll leave it at that to make us both happy.

 

Thanks all... can anyone with their door panels off confirm Blehm's findings? I want a second opinion since my readings are so far off of his.

 

I handed you the answer and now you're second guessing me? WTF? Why do I even bother?

 

Who would've guessed? I'm having my door off right now... really off, like 3 meters off the car . It is 20 ohm. Yours sound way too high. If you're sure you took a correct reading then your heaters need replacement. 100 ohm heater on 12V cannot heat anything, it's only 1.5 Watts approximately. 300 ohm one is even colder. I doubt you will notice them working.

 

Lol, sorry Matt. I should never question an electrical engineer's post. Thanks everyone!

I'll probably just add resistors to lower the resistance. I'll just put them in parallel to lower the resistance. Matt, what wattage resistors should I use? No second guessing this time, promise.

 

WTF are you trying to accomplish by doing that?

 

I agree, adding resistors to lower it is like adding them to injectors to make them work. wont happen.

 

Your problem are mirrors not heating up, not resistance being too high. This way you'd get bunch of hot resistors which won't help to heat mirrors.