Heated front Screen - Wiring Calculations

[GaryL]

There is a lot of chat on here about Heated Front Screens - Have people fitted them themselves, or use a professional service?
I have purchased a screen from ESM and have been working on the relay circuit and cabling.

I measured the resistance of the screen element, and it came out at 4.1 Ohms, which means that it would be drawing 3.6A.

Even when you take into account that on the coldest of winter mornings it could be 20 degrees C less than it was today, the resistance (calculated) would drop to about 3.55 Ohms and the current would still only rise to about 4.2A.

For those who have designed their own circuitry, does that sound right?

[simmitc]

Congratulations on a good choice of upgrade, heated front screens are fantastic things. I fitted mine about 25 years ago and can't recall the measurements or calculations, but know that I wired it with a 15amp rated illuminated switch and cable (no, it doesn't draw that much), no relay, just switch it off when the screen is clear, only a couple of minutes.

[GaryL]

Great - Thank you

If you can run it through a 15A switch, then my calculations are probably OK then.

The car is my wife's daily driver and (No disrespect to her) she won't remember to turn it off!
I am going to wire it in through a 10A timed relay and adjust the on time so that it stays on for as little time as required. I'll go with 14A or 16.5A cable, and protect the relay with a 10A fuse

[oliver90owner]

A well positioned,large/bright warning lamp might suffice to remind your wife?

[Biggles1957]

On my previous Traveller I fitted front and rear heated screens wired through relays and an illuminated two way centre off toggle switch. I did that so that you couldn't put both on at once and hence draw a lot of current, and anyway the front screen was so good it rapidly defrosted and then I didn't need it again.

[kevin s]

We used a timed relay like the oem's do on front screens, I think it was a Ford part, used a momentry toggle switch which closes the relay then it stays closed for about 5 minutes.

[GaryL]

According to my measurements and calculations, my screen will draw about 4.2A.
I am going to design the circuit around a 10A rated adjustable timer relay I have found for £32.
https://www.altecautomotive.co.uk/timer ... 1614-p.asp

My next question is - Can any of those who have heated screens fitted, post pictures of how the wires are routed away from the screen?

The cables come from the bottom edge of the glass and so either exit up between the window rubber and the glass, or down through a hole that would need to be created in the rubber.
I was worried about getting the hole wrong, and having the cable rub against the metal bodywork ridge, so I have got it coming upwards between the glass and the rubber.
I am planning on drilling a hole in the dash panel, and routing the cable through a nicely sized grommet.

I am interested in seeing pictures of other installations before I actually drill the holes.

[simmitc]

Here you go, sounds similar to what you are aiming to do. Example 1 was a "temporary" connector about 20 years ago, example 2 was done with the connection hidden properly behind the dash. Both are working cars, so a little dusty. The mark on the rubber is polish - they do get cleaned occasionally. In both cases, hole drilled on inside edge of rubber. Neither has leaked, and both of similar age.

HSW1.jpg (37.27 KiB) Viewed 35510 times

HSW2.jpg (40.06 KiB) Viewed 35510 times