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MG Midget and Sprite Technical - Amp rating electrics

Well the time has come to hurt the MK I again with some non original modifications. A couple of relays, thicker cable and a bunch of multiple fuse holders are going to be inserted into the car.

I am aware it is wise to put some relays in the headlight circuit. But I have some more. Please advise me what electrical parts are best to switch by relay: horn, wipermotor, blowermotor, rearlights, fuelpump, the 60 watts rear foglight to back off irritating people, blinkers, .....

Its a MK I so very much more isnt on the car. Please give me some advise on Amp ratings of the above parts. Ok I know the rear foglight does pull 5 amps. But does anyone has any ideas how much the rest draws?

Bas
Bas Timmermans

As a general rule I wouldn't bother with an indivitual fuse or relay for any circuit normally drawing under 10 amps. The one exception would be a radio or sound system where the possibility of grounding through a speaker or the case is higher. If you know the wattage of the device such as the lamp you mentioned you can calculate the amperage by the following formula P=I x E where P equals the wattage and I the amperage and E the voltage. For a car the votage would be 12v so for a 60 watt lamp the amperage would be 5 amps. Relays are usually not required for items drawing under 10 amps, but for frequently used circuits where the switches are known to be weak such as the brake lights I'd use a relay.
For most items on a LBC a 15amp fuse will be sufficient. If you have an electric fan then I'd use a 20 amp fuse in that circuit controlled through a relay.
Using a relay for the lights will help save the switch contacts but it's really the headlights that have the most wattage, so you could limit the load to that circuit. Not needed for the horn unless you have an air horn or motor driven unit. Save the relays for things that draw higher amps for longer periods of use such as the blower motor and wiper motor. Remember that adding a relay also adds a possible problem source to the car as well.
Bill Young

Agree with all of that except the horn on a frogeye was originally earth switched through a relay. The arrangement in the steering wheel boss is not terribly good for passing any apreciable current, hence the relay.
Graham P

But does anyone have any idea what amps the horn, blowermotor and wipermotor draws?

Bas

Besides the 2 relays I already used for the headlights I have 3 left so what devices have the highest wattage?

Besides from the relays I went from 2 to 12 fuses! Dont ask why:-( It has something to do with Spridget50, Heavy rain and an electrical coma)
Bas Timmermans

Don't know for sure, but I'd think all less than 5 amps each. All this stuff was run through only 4 15amp fuses originally, so the total has to be pretty low.
You can get a pretty good idea of the current requirements by looking at the wire size used, none of these is any heavier than the rest of the wiring used on the lights etc. If you had a 10 amp or higher load you'd have a heavier gauge wire used.
Bill Young

How are you operating the horn, steering wheel boss or flick switch? If via the steering wheel use a relay, that's one gone :)

Using relays for no good reason is counterproductive as you are introducing another thing in the circuit to go wrong.

The only original fitment that would benefit from non original relays are the headlights as it can cut down voltage drop to them, giving you slightly better output. Thats one or two more gone:)

Electric fan switching is probably better by relay as stats tend to be low current switches and cooling fan motors are relatively high current.So thats up to 4 gone :)

By using a relay on the reversing light you gain no benefit unless the relay is located nearer to the light than the switch, in fact if the switch is in the dash and the relay in the engine bay then you actually introduce a voltage drop due to the longer wiring run!

No other standard item really requires a relay, you could use one on the fuel pump and another on the coil feed to act as part of an imobiliser circuit, either from an alarm or from a hidden switch, but again you are introducing another layer of things to go wrong!

My frog also has a bank of 5 relays but most of them are just spares for future use.
Graham P

>in fact if the switch is in the dash and the relay in the engine bay then you actually introduce a voltage drop due to the longer wiring run!

not quite - what matters is the thickness of wire and the distance traveled, so as long as the wire from the battery is the same length then there should be little diffrence.

I might put a relay in the brake light circuit because those switchs are not great (and expensive)
Will Munns

Bas,

I used a relay in the lighting circuit of my 1500, switched on by the ignition switch. I fitted it not for any power/switch saving reasons but to give peace of mind that if the keys are out, then the lights are off. No more waking up at 2:00am wondering if my battery is draining away!

Allan
Allan ('76 Midget 1500)

Well, I have the horn switched by the steering wheel that should be one next to the two I use for the lights.

I have no reversing lights so 2 to go.

I imagine that driving in rainy consitions with the blower engine on and the wipers on that will draw some current across the switches so I might put a ralay into those 2 circuits. The rear lights, rear fog light and braking light wont draw that much current or will be orerated to short for real trouble.

If you look at the MK I wiring the only big wires are going to the horns.

I suppose the added electric fuel pump (AUF 214) wont draw that much so thats left without a relay but it is fused as it wasnt up til now.

Bas
Bas Timmermans

I've relay'd...

-Main & Dip headlights
-Spotlights
-Electric Fan
-General Accessories (essentially everything thats switched with ignition. E.g. Radio, Phone Charger, Accesorrie socket and a few others)
-Air Horn
Dean Smith ('73 RWA)

Will, I was assuming that the same gauge wire would be used in both cases, if you use a heavier gauge to feed the light then of course you are right. However IMO the relay is still an unecessary complication with no practical value.
Graham
Graham P

Graham,

If the loom contained much thicker wires and the switches were more robust I would agree but I myself find the wires very thin and the switches fragile.

I will bring a 6mm2 wire to the front to feed the lighting circuit and possibly also the horn. That way I Will be sure there is a virtually loss free feed from the battary to the consumers in front. The existing loom will be perfect to act as a switching loom for the relay and from there the new feed will do the job.

For a simple light I wont need a relay but for headlights, horn etc I feel more comfortable. The new feed and the separete feeds will be fused so if there is anything wrong it will show and it will be easy to correct.

Thats why I would like to know waht the Amp use is of the wiper and blower motor. Those are next on the list. Being 45 years old drag, old connections and fatigue will cause probably a higher need for electrical juice. Using a sound new lead and a relay would put some relief on the loom and switches I guess.
Bas Timmermans

I have the two relays I'm using, 1: on the cooling fan circuit and 2: on the brake light switch, 'cos I'm fed up burning out the modern things that look like switches

Headlights although an option will prolly not get one (or two!)
Bill

because I am using non-original switches

two for the wiper motor,
one for the horn (air)
one for the fan
one for the auxiliary circuit (radio, heated seats etc)
and a bank of 9 mosfets for
side Left, right
right dip,full
left dip,full
front fogs
rear fogs
dash lights

Also I have changed the brake light switch for one from a mini which looks more robust and the reverse light switch from a ford sierra

but to be honest there is very little one could gain about the original wiring circuits by looking at my setup!
Will Munns

Will

time for another electrickery lesson here please

So I can understand can you explain to me "MOSFETS for Dummies" and what part numbers you use from where

I'd love to experiment with stuff like that which I stopped playing with in the seventies when I learned I was no bl**dy good at it...

(mind you, the fuzz box I built for my brother worked first time out!)
Bill

Hmm

maybe not after all

I just looked at Wikipedia MOSFETS

My brayne hurts

P'raps I'll just stick to relays

I can cope with them

I just thought that solid state makes tidier wiring solutions, (naturally) but it seems mosfets require a different appoach and far deeper breadth of understanding.

Still I will have something new to ponder during those dark winter evenings if I take on looking in to it all


Now where did I leave that Wiki page?
Bill

Hi Bas, I've just done a little test for you with a clip on ammeter.
Heater fan 1.5 amps and wipers 6 amps. Please bear in mind that old, analogue clip on ammeters are not exactly precision instruments, but they do give a rough guide to the current being used.
Hope this helps.
Graham P

Graham,

Thanx! So heater fan isnt realy a device to put a relay on. The wipermotor could be a good one because if its old, connections are a little corroded etc the draw could be more then 6 Amps.

Sofar its lights in front (2 relays), Horn (1 relay) and the wipermotor (1 relay. I have one left on the bench but maybe a good idea to keep that as a spare.

Rest of the electrics of a MK I isnt realy heavy machinery. No reverselights, no foglights etc. Just an electric pump, rear foglight (single)rearlights with brakelights and a non functional period radio.

Bas
Bas Timmermans

Bill,

Mosfets are basicly voltage controlled dimmer switches. they have three terminals:
Source (+ve)
Drain (-ve)
and Gate which controls the resistance

for automotive use they need turning hard on or hard off. Hard on is about 7 Volts from Source or Drain depending on the type.

P type (IIRC) the gate must be pulled 7 volts below the source,
N type (IIRC) the gate must be pulled 7 volts above the drain

So in automotive use you can use:

an N type to control the -ve return path from a device pulling the gate to +ve to switch on

a P type to control the +ve line to a device, pulling the gate to -ve to switch it on

When the device is not in use the gate must be pulled to switch the device off, this is usually done by connecting a high resistance resistor between gate and Source on a P type or gate and drain on a N type.

Then there are various things you must do to protect the device, the gate and drain must neither be brought above the source, this is usually done with a diode so that spikes from motors etc bypass the device.

Care must be taken no to half turn on the device or it will get hot, and hotness needs to be vented somewhere with a heatsink or the device will go pop
Will Munns

Hmm, I guess I'll stick to my relays:-)

I got a new idea when looking at the wiring diagram.

What if I put a relay between the ignition switch and the fuse box? That way I Wont have all the current acros the iginition switch and all the devices behind the relay are switched by the relay (wich is a 40 Amps relay)

The wiring diagramm showed me the wipermotor, blowermotor, dashlights, fuelgause and tanksender are on the ignitionswitch. Normally the indicaters are too but there are now on the permanent current to accomodate a spade on the fusebox for the fuelpump wich isnt original for a MK I.

Bas
Bas Timmermans

This thread was discussed between 15/08/2008 and 16/08/2008

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