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# Ballast resistor info.

• 23-10-2014 2:08pm
Registered Users Posts: 3,953 ✭✭✭

I'm not too well up on auto elec technicalities so here's my problem.
I have a 24 volt DKW MUNGA 2 stroke jeep, but the ignition system is 12 volt, it has ballast resistor for each coil which are goosed.
Originals are no longer available, I have located ones with the same value on the internet (7ohms), my question is, is 7ohms suitable irrespective of voltage?.

• Registered Users Posts: 4,269 ✭✭✭

Well strictly speaking the Resistance remains the same, however as the Voltage is increased the current also increases. Therefore you would need to ensure that the Resistors you buy have the correct current rating to suit your requirements.

Its Ohms Law V=(IR)

12V=(Ix7 Ohms) therefore I (current) is 1.7 Amps

24V=(Ix7 Ohms) so current is 3.4 amps

You would need a 12V Resistor that is rated at 7 Ohms and 3.4 Amps or use one with double the Resistance - 14 Ohms and 1.7 Amps

• Moderators, Motoring & Transport Moderators Posts: 2,954 Mod ✭✭✭✭

How come they're goosed? (Age?)

Would take a fair bit to fry them as they look fairly chunk and able to dissipate a fair amount of heat (appropriate wattage)

It is this sub-assembly we're on about?

A quick Google says these 50W things are man enough:
http://www.wattbits.com/rb50-1-6r8-j.html

RB range specs:
http://www.ate-electronics.com/en/products/rb-series.html

Webpage seems down, but the Google cache is there:

• Moderators, Motoring & Transport Moderators Posts: 2,954 Mod ✭✭✭✭

Well strictly speaking the Resistance remains the same,

The first bit is fine. After that, I'd chose a resistor on 1) resitance in ohms, 2) tolerance (%) 3) max power rating (Watts).

• Registered Users Posts: 3,953 ✭✭✭

macplaxton wrote: »
How come they're goosed? (Age?)

Would take a fair bit to fry them as they look fairly chunk and able to dissipate a fair amount of heat (appropriate wattage)

It is this sub-assembly we're on about?

A quick Google says these 50W things are man enough:
http://www.wattbits.com/rb50-1-6r8-j.html

RB range specs:
http://www.ate-electronics.com/en/products/rb-series.html

Webpage seems down, but the Google cache is there:
Yes, that's exactly my setup, I have about 20 of them, some giving 24v, others just 5 or 6 v. it's probably age as most are at least 50 years old.
I'll get a few of those and hopefully it should sort it out.

• Moderators, Motoring & Transport Moderators Posts: 2,954 Mod ✭✭✭✭

Just bear in mind that those resistors are only rated at 50W if attached to a suitable heatsink. In free-air with no heatsink they are rated at 20W.

As you can see from the borrowed pic, the new ones are screwed onto an ally plate bent into a very wide and shallow U-shape.

• Registered Users Posts: 3,953 ✭✭✭

macplaxton wrote: »
Just bear in mind that those resistors are only rated at 50W if attached to a suitable heatsink. In free-air with no heatsink they are rated at 20W.

As you can see from the borrowed pic, the new ones are screwed onto an ally plate bent into a very wide and shallow U-shape.

By heatsink, are you referring to using the ally plate to dissipate the heat from the resistors.

• Moderators, Motoring & Transport Moderators Posts: 2,954 Mod ✭✭✭✭

Yes, like the bloke has done in the last picture.

In the video there appears to be not too much hassle with the triple relay and only seems to mention cleaning the contact points of the relay up if there is an issue with operation.

• Registered Users Posts: 3,953 ✭✭✭