Morris Minor Owners Club

Can anyone tell me the fuse size for a front heated screen and a rear one.
Thanks

I imagine the inrush on a cold screen could be as high as 25/30 amps!

bmcecosse wrote:I imagine the inrush on a cold screen could be as high as 25/30 amps!

is that each, or both together? if so what's the split. No instructions came with them. If people have them What fuse do they have. I'm putting them both on the same relay so could do with knowing if it will be ok I have the 50 amp relay protected by a 35 amp fuse and from the relay each screen is fused. Both on a 30 amp fuse for test I suppose I can keep reducing the fuse till it blows or I can try to calculate it with V/I X R or ask on the forum.
Thanks
Paul

Paul presumably you meant V=IR

billlobban wrote:Paul presumably you meant V=IR

Thanks for that from what I can remember, isn't V/I R the triangle. However way you transpose it 35 ampa X any resistance is never going to equal 12V?
What do you think

To find Voltage (V)
[V = I x R] V (volts) = I (amps) x R (Ω)

To find Current (I)
[I = V ÷ R] I (amps) = V (volts) ÷ R (Ω)

To find Resistance (R)
[R = V ÷ I] R (Ω) = V (volts) ÷ I (amps)

I usually take it that HRW elements are a shocking 500W - thats 500w/13.5v generator voltage say = 37 amps. Measure the resistance of the front one in Ohms: V ÷ R = Amps that will flow.

Hope thats right!

The 'triangle' describes that the ratio of the Voltage (E) applied across a resistance to the current (I) flowing through that resistance is a constant. The ratio is equal to the value of the resistance in Ohms.

Voltage (E)
_________ = a constant = Resistance (R)

Current (I)

Good job we don't usually need the 'opposite' of this, whereby Conductance is being considered - guess what, it is measured in units, logically named Mhos ! (Ohms backwards!)

Hello Paul J,

in answer to your question, the resistance in that instance is just over 1\3 ohm.

Bearing in mind that as the element heats up it's resistance will increase so the cold reading will only give a switch on current, as BMCE mentioned in one of the first posts.
I'd be very surprised if it is 500 watt, my guess would be a running current of between 6 to 10 amps?

Alec

I've got a feeling the resistance will vary with the temperature of the screen.
As it was designed to be used on a car I'd go with the empirical method of upping the fuse until it doesn't blow.

Part of the fun will be seeing if it will start next day .

I'm sure 500W would be in the order of a maximum figure, however memory from a long time ago was that around 30A draw could be involved. The best ways to check are put a good ampere meter in the line temporarily and measure it directly. They do get warm very quickly though over a large area of glass usually..

Hello Mike,

I'm referring to heated screens that are fitted to Minors, not moderns.

I doubt that there are many people on here who have a DC ammeter that can measure 30 amps? Most multimeters have a 10 amp range and few have shunts. Even rarer are DC clip on meters.

Alec

Hmm, I was thinking of the stick-on silvery ones we used to buy as an accessory. Ammeter, no, you are right about multi-meter types, mine caters for 10A. Short of making a shunt for it, so could bypass an estimated 20A say. A spare dash-mount hot-wire ammeter of 30 - 0 - 30 might give an indication.. Other than that, accurate mu-Ohm resistance test..

From Birmingham MM the rear one will draw about 8 amps and the front 13.5 so I will add a bit and go with 10 and 15 from the relay and 30 amps to the relay. There must be many people with these heated screens so why the confusion I just don't understand. Going through the forum, there is all sorts of figures for fuse rating

Hello Paul,

quite a significant draw if you have both. I think it would be wise to copy modern installations and incorporate a timer to switch off one or both after a specific time and have a warning light when they are on.

Alec

I suggest you don't ever use both together or you will flatten the battery, especially as you are likely to have lights/wipers/heater running at the same time.

kittyfell wrote:I suggest you don't ever use both together or you will flatten the battery, especially as you are likely to have lights/wipers/heater running at the same time.

lets just hope the front and back screens freeze on different days then