Ballast Resistors

[Chris Griffin]

Can anyone tell me what the ballast resistor does that sometimes comes with a battery master switch? We are close to finishing the prep on a GP1 Rover SD1 and I noticed that it has a ballast resistor fitted to the switch but one end has either been disconnected or has broken away.

[Tim Falce]

The other end goes to earth. I am not an electrician, but I think the wire earths the alternator to stop it producing power when the switch is turned off and the engine is running. The resistor is to stop it shorting out the fuses. But I am sure soemone with more knowledge will be along soon

[Chris Griffin]

Thanks falcemob that rings a bell. But why are they not always present? Different alternator types maybe?

[imull]

The ballast resistor is something to do with back EMF I think. When you flick the isolator there is a power spike of some sort that can do some damage, the ballast resistor cuts this out...

(I think)

[Tim Falce]

Chris the cheap cut out switches don't have the resistor, it is required on the FIA ones. The cheap ones just cut the power from the battery to the the car.
If the alternator is charging when the switch is turned off then it will still be putting power to the ignition so the engine will keep running. With the expensive ones you need to run one of the alternater wires via the switch.

imull could be right on the power spike thing. I have lost the wiring diagram for my switch but it all becomes a lot clearer when it is in black and white.

[GolddustMini]

i thought it had something to do with all the monkey boy racers going for the rally look on there nova's the resistor shorly allows enough current to keep the memory system alive on there top of the range kenwood stereo, so tey dont have to reprogram RNBrubbishFM and bangincrapmusicFM every time they turn the isoloator. although it could possibly be to do with some form of current spike thingie (i electroccuted a mechanic once he flew like a gud'un, he was faffing with the HT side of the ignition with the power on so i turned the power off at the master, it spiked, sent a big zap of power through him, most entertaining seeing as he had spent most of the day annoying the hell outa me!)

[Chris Griffin]

Goldust......NO, putting it mildly you are a tad wide of the mark

[Anuauto]

On a rally car or anything getting a lot of vibration the resistor needs some reinforcing of the connections and some cushioning where it is mounted to the shell There is a diagram of the switch wiring in a "Technical info" section of the Wessex Autograss web site (which is linked from the NASA Autograss national site - sorry dont have URL's to hand).

[thebear]

Originally posted by Chris Griffin
Can anyone tell me what the ballast resistor does that sometimes comes with a battery master switch? We are close to finishing the prep on a GP1 Rover SD1 and I noticed that it has a ballast resistor fitted to the switch but one end has either been disconnected or has broken away.

In "the old days" (cir. 1956) they were used on coil & condenser electrical systems where there is a large voltage drop when starting. :confused: A 6v coil was used on a 12v system, installed as follows: The resistor was placed across the "Start" and "Run" terminals on the ignition switch. The wire fom the "start" position on the switch went to the coil. This allowed sufficient voltage to generate a `lively' spark during cranking. When the key was released to the "run" position the voltage drop thru the resistor prevented destruction of the coil. It was not a fail-safe system as the resistors did fail, primarily by fracturing with the result that the car would start but quit as soon as the ingition switch was allowed to return to the `run' position.

[Tim Falce]

In "the old days" (cir. 1956) they were used on coil & condenser electrical systems where there is a large voltage drop when starting.

bear, they are still used on modern cars today.
Most coils on european cars are 8 to 10v so need a ballast resistor.
I assume Chris Griffin is talking about the resistor fitted to the FIA cutout switches which grounds the current from the alternator when the engine is turned off.

[Chris Griffin]

Originally posted by falcemob
I assume Chris Griffin is talking about the resistor fitted to the FIA cutout switches which grounds the current from the alternator when the engine is turned off.

Exactly falcemob. Sorry if this was not made clear at the start of the thread. I'm going to check the wiring again to see if I can tell where its come adrift from and then replace the whole lot including the cut-out switch.

[Chris Griffin]

BTW falcemob I've just noticed we were on identical "post" totals until I wrote this!!!

[Tim Falce]

Now we're the same again, I also see that we may be racing at some of the same meetings this year, if I ever get my car back together.

[RTH]

If you disconnect a main battery power lead when the engine is running it is very likely to destroy the alternator.
A correctly wired resistor will save this if a marshal tries to stop your engine after you have crashed

[zefarelly]

now I followed the diagram on the FiA cut out switch . . . .when I hooked the battery up, the rsistor got exceedingly hot and started smoking. If I crash that badly I'm happy to fry an alternator !

I've wired my coil through the cut out switch, and that goes with the battery a,d the rest of the power circuit . . . .presumably, if the alternator output also goes through the resistor its enough to avoid it cooking its diodes when its spinning with nothing to supply ?

[tangoed sparky]

Chris a cheap battery cut out switch will only have 2 terminals (m8/m10 stud & nuts) were as a better quality cut out switch (like autolec see demon tweeks) will have the two main battery connections plus 2 pairs of lucar/spade terminals one pair is used to cut the ignition circuit when turned off and the other pair is used to ground/earth the alternator via a resistor when turned off. If you go on google and search (how do I fit a battery master switch) the first site is a lotus seven site and it tells you how to fit the said item. It also gives you a good drawing of how to wire the thing (on a ford). If you are buying a new cut out then look for the ones with 2 pairs off spade terminals. If it is a good one it should have a wiring diagram with it.
What gold dust mini was refering to is that some times a small fuse can be fitted in parralel with the cut out (across the two main terminals). This is do so that if the battery cut out is turned off then there is still power for stereos ect but if a short circuit or to much load happens then the fuse blows. This is not very usfull on a race car as you want all the electrics off if your involved in an incident. Hope this is of help

[Chris Griffin]

Thanks Sparky. Great explanation.

Chris

[zefarelly]

Ive read that article, and I did exactly what it says on there, which correlates perfectly with the FiA switch instructions . . . .why therefore did my resistor try catching fire . . . . ???

[Anuauto]

Difficult to suggest why - other than obvious things like the wires out of the resistor being loose (I was recommended to smother them in araldite to reinforce against vibration - they are thin and not flexible). Assume earth is good? - and the connections from the thin resistor wires to what ever you use to run to switch terminal and to earth. I have had the main switch (the large terminals) get very hot when, with the main starter lead going to the m/switch, I had the starter working intermitently because of burnt armature. I later always wired the main starter cable direct from battery to starter in every car I built (OK so long as all other electrics from battery go through the master switch, and you can then use thinner cable to the m/switch - also allows for 24volt starting if you want to fit it).

[tangoed sparky]

Zefarelly it must be that you had it wired up wrong because the resistor should not have any power to it until you turn it off. It is possible that you had the 2 pairs of small conntacts the wrong way round and that when turned on your resistor is getting power to it. The resistor should only get power to it when the switch is turned off and the reason for this is to aloue any current that the alternator is still making after its been turned off to find a path to negative - via a load (resistor). If you have got it right then it could be a faulty swith.

[tangoed sparky]

My pleasure Chris what club/championship do you race in as I am a marshal and I will keep my eye out for your Rover. Being an SD1 I assume that it has a nice fire breathing V8 in it.

Anuauto your system is ok like you say but the only problem that you could have is if you were to have a crash (god forbid) the battery cable to the starter would still be live and if this were to be trapped eg; between engine and body then you could have a fire on your hands.
When I was 17 and thought that I could drive fast I had a 1275 GT mini (great car) and 1 day I was giving some welly and I understeered into a kerb (diesel on the road,well thats my excuse). The impact was enough to move the front sub frame and trap the main barrery cable. I could smell the cable burning but I could not due any thing about it and I wished that I had a battery cut at that time. Lucklly the battery was not very good and it did not burst into flames. I still had some serious welding and other work to do though.

[Anuauto]

Wouldn't do it on other than a fully comp-prepared car with batt cable inside and protected. Still some risk I know but allowed in most regs. Its effectively "works" rally system from 24v days and endorsed by Lucas comps dept.

[zefarelly]

thansk Tangoed . . .I'll double check . . .even I can admit to making mistakes from time to time !

[tangoed sparky]

Doh sorry Anauto I was not thinking obvsosly being a comp prepaired car then your cables would already be inside.