Lotus Elan

Heated Rear Window Current Consumption

PostPost by: ericbushby » Sat Jun 20, 2020 5:28 pm

Hi, You have a 200 ohm scale on that meter so you can measure the resistance of the window heater.
Lets assume 10 Amps is needed to be useful for the heater, in fact if we say 14 Amps, then you need 100 times the current you are showing.
To keep it simple you need a resistance of about 1 ohm to produce 14 Amps. This is less than that meter will read. The meter lead resistance may be more than the window heater. So you would be looking for an apparent short circuit.
It would appear that the window heater is not working and requires further investigation.
Hope this helps
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PostPost by: Andy8421 » Sat Jun 20, 2020 5:30 pm

persiflage wrote: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:

0.14A at 12v is 1.68W, which is less than one of the dim bulbs that light up the instruments. With all due respect, whatever you are measuring, it isn't the current consumption of a functioning heated rear screen.

Can you explain how you have the meter connected?
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PostPost by: HCA » Sun Jun 21, 2020 9:29 am

persiflage wrote: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:


mmm. Be careful, as i suspect you are measuring in parallel that could blow the meter if it is not fitted with a fuse. either way, I assure you, a working HRW does not draw less than one amp 8)
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PostPost by: persiflage » Sun Jun 21, 2020 10:24 am

This discussion seems to have highjacked Grahams thread .... Apologies Graham. :mrgreen:

The meters are in series and is working correctly as confirmed by calculation and subsequent measurement of the current flow in the adjacent courtesy light.
I'll just have to wait for some suitable Autumn weather to roll the car out onto the drive and function test in real world conditions.
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PostPost by: HCA » Sun Jun 21, 2020 10:44 am

No, his question has been answered 8)

This additional part is to quell any ideas that an HRW runs on less than an amp! You must remember that these threads end up in an archive where someone could pick up your assertion that has not been corrected, think you are the expert and if so minded, wire up an HRW with 0.3mm2 cable and wonder why there is a smoke appearing from under the tunnel cover after switch on :lol:
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PostPost by: persiflage » Sun Jun 21, 2020 11:32 am

0.3mm2 cable and wonder why there is a smoke appearing from under the tunnel cover after switch on :lol:


:D Indeed. My original reason for fitting a relay all those years ago was because I wasn't happy with the thought of a heater cable running through the dash loom and I was surprised some years later to measure it as low as it is.
Being a simple supply one side and Earth the other it's difficult to understand what problems can arise except broken elements.
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PostPost by: mikealdren » Sun Jun 21, 2020 1:08 pm

If you're running your ammeter in series, i.e. breaking the circuit, can you test the resistance of the window. With such low current, it should be quite high, around 80Ω.
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PostPost by: Craven » Sun Jun 21, 2020 4:12 pm

Ah, zero resistance ammeters played with those in the past!!
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PostPost by: Baggy2 » Sun Jun 21, 2020 10:14 pm

That looks like it could be the current to the coil of a relay and the HRW current is going through the relay contacts?
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PostPost by: HCA » Mon Jun 22, 2020 8:17 am

I'd say you are spot on. Relay coils will generally pull 150-200mA.
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PostPost by: MarkDa » Mon Jun 22, 2020 9:17 pm

I agree that the biggest fault problem with HRW is broken elements.
I'd be tempted to say that nearly all the elements will be broken if the current draw is under 1A.
Just how effective is this demister in practice?
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PostPost by: mbell » Mon Jun 22, 2020 9:28 pm

Just removed my screen today to fit a new seal. On measuring the window I get an open circuit, so can't give a mesured value.

However, my screen has 10 wires, which are each basically a resistor. So you have 10 resistor in parallel. Now if you assume that persoflage screen is similarly damaged to mine but still has one functioning connection. You can calculate the resistance of one of the heaters/resistors. Then you can calculate the resistance if all then were functioning, which then gives you the current for a fully functioning heated screen.

So
Assuming 1 resistor/heater still functional:
0.15A and V=12, gives you 80 ohms per heater.

10 of these in parallel is 8 ohms. Which gives 1.5A at 12v and 18W for a fully functioning screen.
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PostPost by: HCA » Tue Jun 23, 2020 6:22 am

For heavens sake, please, 18W for a whole screen is less than an indicator light bulb!

Can someone not shut down this thread - it has gone beyond the ridiculous.
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