Mirror Heat 24V on 12V?

rv_safetyman · October 10, 2006, 06:39:58 AM

Well, I am getting ready for the trip to Acrcdia starting out in late Nov.

I would like to have mirror heat, but can't afford the really expensive after-market units. I have found some Prevost take-offs. One fellow put them on his Eagle and they looked pretty darn good.

However, I am concerned about the amount of heat that I will get. They are probably better than my unheated mirrors, but if I spend a couple of hundred, I want to make sure they will work.

Has anyone had any experience in this area? Are there other "inexpensive options for an Eagle?

plyonsMC9 · October 12, 2006, 08:10:37 PM

Hi Jim,

Any luck with your heated mirrors? Folks, any help for Jim with the mirrors? My idea of holding a hair dryer outside the driver's window only works on that mirror, and until frostbite sets in.

Hope this helps, Phil

Hi yo silver · October 12, 2006, 08:31:26 PM

Jim,
I'm a little confused reg. the subject line. Are you saying the voltage requirement for the mirrors is different than you have on your bus? Are the mirrors rated for 12 volts or 24. Which voltage is available on your bus?
Dennis
Hi Yo Silver!

rv_safetyman · October 13, 2006, 06:10:06 AM

The mirrors are take-offs from a Prevost (24V) and I am thinking of installing them on an Eagle (12V).

DrivingMissLazy · October 13, 2006, 06:13:38 AM

Then you are going to get very little heat. By reducing the input voltage you will decrease the heating by at least 50% and very possibly more, depending on the design of the heating elements.
Richard

Quote from: rv_safetyman on October 13, 2006, 06:10:06 AM
The mirrors are take-offs from a Prevost (24V) and I am thinking of installing them on an Eagle (12V).

JLL · October 13, 2006, 09:29:08 AM

A friend gave me a set of take offs from his 1987 Newell. I don't know the voltage on the Newel but they work good on my 12 volt 4106. I don't know how fast they are supposed to heat up but they seem to do the job.
JLL

Sean · October 15, 2006, 10:31:13 AM

Jim,

Actually, the heat you will get on 12V will be only 25% of the rated output at 24V -- remember the power formula, P=V^2/R (Power is the square of the voltage over the resistance, and mirror heaters are nothing more than resistance elements). So halving the voltage means one quarter the power.

I recommend you get yourself a little transformer to step 120VAC down to 24VAC, and run the heaters on that. The heater doesn't care about AC or DC, and these transformers are readily available in hardware stores to power home heating thermostats and irrigation valves, both of which are nominally 24VAC. Should be less than ten bucks. Check their power ratings, though -- you may need one for each mirror.

Run the transformer(s) off your inverter, and set up your mirror heat switch to supply 120VAC to the transformer, so that it will not be a phantom load when the mirror heaters are not in use. If your mirror heat switch is rated 12VDC, use a relay to supply the 120VAC to the transformer.

Just my $0.02.

-Sean
(From Mexico's Copper Canyon -- http://ourodyssey.blogspot.com)

Hi yo silver · October 16, 2006, 07:37:40 PM

Yep, the transformer is the simplest approach. Good luck!
Dennis
Hi Yo Silver!

ndbusnut · October 17, 2006, 05:23:23 AM

my older trucks had a 12volt tail light bulb in each mirror worked great

DrivingMissLazy · October 17, 2006, 05:40:27 AM

That would probably work OK if the mirrors are only heated. However, a lot of heated mirrors are also remotely adjusted, like the ones I had on my Eagle, and they would not like the AC being applied to the DC motors. In that case I would find a 120 volt AC to 24 volt DC device to operate everything.
I would also incorporate a latching relay device that could only be turned on when the ignition switch is turned on and would latch. If the switch is turned off, it would un-latch to prevent the device from remaining on.
Richard

Quote from: Sean on October 15, 2006, 10:31:13 AM
Jim,

Actually, the heat you will get on 12V will be only 25% of the rated output at 24V -- remember the power formula, P=V^2/R (Power is the square of the voltage over the resistance, and mirror heaters are nothing more than resistance elements). So halving the voltage means one quarter the power.

I recommend you get yourself a little transformer to step 120VAC down to 24VAC, and run the heaters on that. The heater doesn't care about AC or DC, and these transformers are readily available in hardware stores to power home heating thermostats and irrigation valves, both of which are nominally 24VAC. Should be less than ten bucks. Check their power ratings, though -- you may need one for each mirror.

Run the transformer(s) off your inverter, and set up your mirror heat switch to supply 120VAC to the transformer, so that it will not be a phantom load when the mirror heaters are not in use. If your mirror heat switch is rated 12VDC, use a relay to supply the 120VAC to the transformer.

Just my $0.02.

-Sean
(From Mexico's Copper Canyon -- http://ourodyssey.blogspot.com)

rv_safetyman · October 17, 2006, 06:47:58 AM

Thanks for all the input guys. I just did not feel comfortable with this combination.

A fellow bus nut contacted me off line, and it seems like he has a neat set of mirrors that will work much better.