Lotus Elan

Heated Rear Window Current Consumption

PostPost by: gwiz22 » Wed Jun 17, 2020 4:30 pm

Hi,
Does snyone know the current consumption of the HRW on a Plus2. I've looked in the manual but can't find any reference to it.
Thanks
Graham
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PostPost by: gwiz22 » Wed Jun 17, 2020 6:49 pm

Whoops. Should have read Does someone know............................etc.

Regards
Graham
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PostPost by: baileyman » Wed Jun 17, 2020 8:10 pm

Relatedly, my new screen from TTR is recommended to be relayed with a 25A fuse.

John
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PostPost by: mikealdren » Thu Jun 18, 2020 7:40 am

The fuse is to protect against short circuit and is usually much higher than the current actually drawn.

I would be amazed if the HRW uses anything like that Apart from anything else, the long cable wouldn't support such a high current and the 35A alternator would struggle too, let alone the cars with dynamos. My guess is that it's more like 10A.
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PostPost by: MarkDa » Thu Jun 18, 2020 8:03 am

You could measure its resistance to calculate a nominal draw.
In theory as the wire warms up it will reduce current but whether in practice it gets hot enough is debatable.
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PostPost by: HCA » Thu Jun 18, 2020 10:05 am

The generaic reply to this question is: 10-20A, depending on screen.

More precisely, for a +2 screen, I'd say the current draw is 15A when cold, reducing to 12A when hot. These are the figures I used when re-wiring my car. I used 2mm2 thinwall, ( but think I could easily have used 1mm2) with a 20A, 10 mins relay protected by a 25A fuse.
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PostPost by: gwiz22 » Fri Jun 19, 2020 10:34 am

HCA, I think you are about right with the figures you quoted. So I will fit a suitable relay to handle that level of current.

Thanks to all for your help and suggestions.
Graham
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PostPost by: HCA » Fri Jun 19, 2020 10:51 am

Here is the one I used, and tested a few times, works a treat!

https://www.autoelectricsupplies.co.uk/ ... tegory/210

Having said that, of course I am using new switches and for the HRW, I am using the modern method with a non latching switch. If you are using the standard Lotus switch, then a bog standard relay at three quid is more than adequate:

https://www.autoelectricsupplies.co.uk/ ... ategory/36
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PostPost by: persiflage » Fri Jun 19, 2020 6:34 pm

I've previously measured the current draw on my Sprint at 0.3 Amps.
Don't be fooled into thinking it's the same as one of those old electric fires where the coils glow red :lol:
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PostPost by: gav » Sat Jun 20, 2020 9:10 am

Persiflage

Is that draw across the switch at 0.3A?
One day I'll actually finish - completely - one day....
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PostPost by: mikealdren » Sat Jun 20, 2020 10:56 am

persiflage wrote:I've previously measured the current draw on my Sprint at 0.3 Amps.
Don't be fooled into thinking it's the same as one of those old electric fires where the coils glow red :lol:


This seems far too low, 4W wouldn't warm up much.
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PostPost by: MarkDa » Sat Jun 20, 2020 1:25 pm

Get more heat by blowing on it :) :D
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PostPost by: persiflage » Sat Jun 20, 2020 1:49 pm

Get more heat by blowing on it :) :D ...... I'm trying to remember :D

It's long time since it was used in anger and I would not describe it as being anything like the efficient modern hrw in my daily drive.
The original afterthought toggle switch located by the drivers knee was replaced by a relay and proper Sprint rocker switch in the dash years ago (before I measured current.)
Picture shows the current measured today at the window tag, following a recent rear harness remake. What was previously about 0.3 Amp has reduced somewhat.
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PostPost by: HCA » Sat Jun 20, 2020 4:05 pm

You have something wrong - the multimeter is shot, it is incorrectly connected or your HRW is shot. There is no way this item draws less than an amp! Physically impossible!

The catch-all for a 12V HRW is 120 - 240W dependant on size and efficiency. Maybe a small one 100W but not less. Look up any car spec.
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PostPost by: persiflage » Sat Jun 20, 2020 4:48 pm

As I say, I don't recall it being the most efficient demister, just that this is what my one draws.
2 different meters tried, both agree .... And they were connected correctly :roll:
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