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
Eric in Burnley
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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.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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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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PostPost by: oatk » Thu Jul 02, 2020 12:00 am

Sorry if this is over-prolonging the thread, but I have a datapoint in defense of the smaller values.

My plus 2 has a very old heated rear screen - Mum thinks it was already fitted when my parents got the car, which would put it as pre-1974. When I switch it on with only the coil as other load, the car ammeter not quite doubles its deflection (and doesn't exceed the 10A mark.) Given a ~3 ohm coil, that would make about 3-ish amps drawn by the heated screen - maybe 35-40W.

I can confirm that the heated screen does work to demist the glass, after a fashion. It's nowhere near as good as that on a modern car, but fairly quickly produces clear areas around the elements through which you can see behind. All elements appear to be working.
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PostPost by: gwiz22 » Fri Jul 03, 2020 2:51 pm

Dear All,
I seem to have opened a Pandora’s box on this subject. Initially I enquired if anyone knew the current consumption of the heated rear window because I was doing some wiring upgrades in my Plus 2 and noticed that the HRW shared a fuse with other items serviced from the ignition circuit and it was not protected with a relay, so thought while the dash was out I would install one. Unfortunately my HRW doesn’t work so I couldn’t measure current consumption. Hence my original question. Also, trying to measure the resistance of the HRW was unsuccessful. It appears the conductive paint that makes up the element has lost its conductivity. I still wanted to install a relay as hopefully I can replace the HRW sometime in the future so wanted to know what power rating of relay to aquire.

As there has been some differences of opinion on the forum regarding the current consumption of the HRW, I thought I would do a little investigation in order to hopefully, put this matter to rest.
The HRW on the Plus2 has 10 rows of resistance material painted on the glass. Each row can be considered part of a group of resistors connected in parallel within an electronic circuit. It might seem obvious that to calculate the total resistance one would simply add each of the 10 resistance values together. However this is incorrect. The formula for calculating the overall total resistance of a number of resistors connected in parallel is:

1/Rtotal = (1/R1) + (1/R2) + (1/R3)+…………(1/Rn)

I have created a spreadsheet which calculates the overall HRW resistance using the formula above. However, this requires a known resistance of a single resistive row of a Plus2 HRW, which we don’t have. So my spreadsheet uses multiple sample resistances for each row, the resultant overall resistance and the resultant current consumption when 12volts is applied. I also created calculations for current consumption for the sample resistances for examples where only 4 and 1 of the rows are working. Here is a summary of the results:

All 10 Rows
Row Resistance Tot Res Current Consumed Power
30 ohms 3 ohms 4 amps 48 watts
20 ohms 2 ohms 6 amps 72 watts
16 ohms 1.6 ohms 7.5 amps 90 watts
10 ohms 1 ohm 12 amps 144 watts
856.12 ohms 85.61 ohms 0.14 amps 1.68 watts
400.34 ohms 40.03 ohms 0.3 amps 3.6 Watts


4 Rows
Sample Row Resistance Total Resistance Current Consumption Power Expanded
30 ohms 7.5 ohms 1.6 amps 19 watts
20 ohms 5 ohms 2.4 amps 29 watts
16 ohms 4 ohms 3.0 amps 36 watts
10 ohms 2.5 ohms 4.8 amps 58 watts
342.40 ohms 85.6 ohms 0.14 amps 1.68 watts
160.5 ohms 40.13 ohms 0.3 amps 3.59 watts


1 Row
Sample Row Resistance Total Resistance Current Consumption Power Expanded
30 ohms 30 ohms 0.4 amps 4.8 watts
20 ohms 20 ohms 0.6 amps 7.2 watts
16 ohms 16 ohms 0.75 amps 9 watts
10 ohms 10 ohms 1.2 amps 14.4 watts
85.4 ohms 85.4 ohms 0.14 amps 1.69 watts
40.02 ohms 40.02 ohms 0.3 amps 3.6 watts

The bottom two rows of each data array shows the resistance each row would have to be to achieve the two current consumption figures given by Persiflage, i.e. 0.14 and 0.3 Amps.

As we don’t have an accurate figure for the total resistance of a Plus2 HRW, I measured the resistance of the HRW in my wife’s mini as that seemed like a reasonable datum. It’s about the same width of the Plus2 and has 12 rows. A couple more than the Plus2 but within scope for this purpose. I could not measure the resistance of an individual row as they were all linked together by two vertical lines of resistive paint. The total resistance measures 1.6 Ohms. Looking at the 10 Rows data it would seem that a row resistance on Plus2 between 10 and 20 Ohms would result in a total resistance of 1.6 Ohms, and in fact my spreadsheet shows a screen with 10 rows, each with a row resistance of 16 Ohms confirms it. The current consumption would be 7.5Amps giving a power generated of 90Watts. Which I feel is in the right order of magnitude to generate enough heat to demist a screen.

Accordingly, it is difficult to see how either of the currents given by Persiflage can be correct, as even using the higher current figure of 0.3amps would only produce a power output at the screen of 3.6Watts; a little over half of what would be consumed by a sidelight bulb. This is of course assuming all 10 rows are working.

If I ever get a fully working HRW for my Plus 2, before I do anything else, I will measure the resistance of each row and the total, together with the current consumed when 12Volts is applied. Then we will know for certain.

If anyone wants a copy of my spreadsheet to check the figures or play around with readings from their own measurements, send me a PM.
Regards
Graham
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