Ballast resistors

Paul Marx

Paul Marx

Hero member
Location
France
I've finished reassembling my wife's 500. The ballast resistors gave me some aggro because the metal supports needed replacing.

I know those resistors can be done away with.
1) What exactly is their function? Something to do with keeping maximum volts for ignition ?
2) If I junk them, what about the wiring? Do I need to join the 2 loose wires formerly feeding the ballasts?

Paul
 
The reason I know of with other bikes, is to reduce the voltage to the coils to about 10v. The coils are designed to run best at 10v because that’s about all they’ll get while the starter motor is running. So the resistor is normally in circuit and then bridged (shorted) out of circuit when the starter motor runs.
I presume it’s the same for you? But of course if the coils have changed etc… maybe not helpful now!
 
Andy is about right .. with a decent coil you can do away with them (and bridge connector) but keep them if you can if the coil is older
 
Andy said:
The reason I know of with other bikes, is to reduce the voltage to the coils to about 10v. The coils are designed to run best at 10v because that’s about all they’ll get while the starter motor is running. So the resistor is normally in circuit and then bridged (shorted) out of circuit when the starter motor runs.
I presume it’s the same for you? But of course if the coils have changed etc… maybe not helpful now!
Click to expand...
Pretty much how they're supposed to work, and how they do work in most cars. So that answers Paul's first question.

To answer your second question Paul: Yes, if you junk them, you do need to join the 2 loose wires formerly feeding the ballasts. But you may need to think a bit more before doing that.

Laverda wiring doesn't bypass the resistors during engine starting, so they're in the coil circuit all the time including when the battery voltage is depressed while cranking the starter motor.

I presume ignition coils of the day were all designed to run on lower than normal system voltage, assuming a ballast resistor shunting system would be employed. Running them continuously at full available voltage (up to 14V) could overheat the coils. That's why they were installed with resistors by the factory in the first place.

So, further to your question about getting rid of them. There are situations when junking them is recommended and others when it is not.

1. If you have updated coils designed to operate on 12-14V, they'll be OK without ballast resistors.

2. If you have the original coils but a modern electronic ignition upgrade (as if Paul would have such a thing ;)), the old coils will be OK without ballast resistors because electronic ignitions control the dwell time better than points.

3. If you have original coils with contact breaker points you might over-cook the coils without the resistors, so leave them in the circuit. But to improve starting, you can add the starting bypass that they were always meant to have. Just run a wire from the switched side of the starter relay to the positive side of the coils (between coil and resistor). That will shunt the full battery voltage to the coils for as long as you have your thumb on the starter button.
 
Dellortoman said:
Pretty much how they're supposed to work, and how they do work in most cars. So that answers Paul's first question.

To answer your second question Paul: Yes, if you junk them, you do need to join the 2 loose wires formerly feeding the ballasts. But you may need to think a bit more before doing that.

Laverda wiring doesn't bypass the resistors during engine starting, so they're in the coil circuit all the time including when the battery voltage is depressed while cranking the starter motor.

I presume ignition coils of the day were all designed to run on lower than normal system voltage, assuming a ballast resistor shunting system would be employed. Running them continuously at full available voltage (up to 14V) could overheat the coils. That's why they were installed with resistors by the factory in the first place.

So, further to your question about getting rid of them. There are situations when junking them is recommended and others when it is not.

1. If you have updated coils designed to operate on 12-14V, they'll be OK without ballast resistors.

2. If you have the original coils but a modern electronic ignition upgrade (as if Paul would have such a thing ;)), the old coils will be OK without ballast resistors because electronic ignitions control the dwell time better than points.

3. If you have original coils with contact breaker points you might over-cook the coils without the resistors, so leave them in the circuit. But to improve starting, you can add the starting bypass that they were always meant to have. Just run a wire from the switched side of the starter relay to the positive side of the coils (between coil and resistor). That will shunt the full battery voltage to the coils for as long as you have your thumb on the starter button.
Click to expand...
The history (as I understand it 😉) is that older bikes had 12v coils, but the advent of electric start and the resultant voltage drop (and therefore weak spark) at that critical time made everyone use 10v coils with a resistor to reduce the voltage when the starter wasn’t running. I suppose that had already been figured out on cars.
Case in point: I had an old Yamaha 650 which almost never started on the button, although it turned over fine. But I’d still use the starter just to get some fuel through then it always started first kick 👍
With all this in mind I don’t understand why Laverda wouldn’t have the bypass wired? Unless the coils were good for a wider voltage range?
 
Andy said:
The history (as I understand it 😉) is that older bikes had 12v coils, but the advent of electric start and the resultant voltage drop (and therefore weak spark) at that critical time made everyone use 10v coils with a resistor to reduce the voltage when the starter wasn’t running. I suppose that had already been figured out on cars.
Case in point: I had an old Yamaha 650 which almost never started on the button, although it turned over fine. But I’d still use the starter just to get some fuel through then it always started first kick 👍
With all this in mind I don’t understand why Laverda wouldn’t have the bypass wired? Unless the coils were good for a wider voltage range?
Click to expand...
They didn’t fit bypass wire because when the bike was new, the ignition switch/ kill switch/ loom & connectors etc where not burnt or corroded and you got 12v at the coils- reduction of volts to voltage sensing wire at rectifier also causes failure.Ducati/ Cagiva suffer in same way - Italians also use cheap earth/ starter motor leads that get worse over time. My Cagiva Elefant has 3 foot of starter lead - I replaced it with aero spec lead and the motor spins over much faster and fire immediately .
Your XS 650 didn’t fire on the button because it used an excited stator- the generator saps battery voltage whilst supplying the carbon brush’s to Magnetise the stator ( like old BMWs) the starter eats what’s left.
 
Exmoor beast said:
Your XS 650 didn’t fire on the button because it used an excited stator- the generator saps battery voltage whilst supplying the carbon brush’s to Magnetise the stator ( like old BMWs) the starter eats what’s left.
Click to expand...
So it would have paid to fit a relay to those bikes to prevent the stator stealing current? My mates R75 BM would start on the button for the first few days with a new battery, after that it was kick only. Always wondered wtf.
 
you can easily fit a car type starter (proper) solenoid (replacing the Bosch 80amp relay that is working at the top end of its capability) this will give you a full voltage output but only whilst cranking, a 12v outlet whilst ignition is on, and another at all times on
very easy to fit under the battery tray on a 180, and then wire the 12v whilst cranking wire to the out put side of the ballast resistors, which if I remember correctly drops the currant to 9 volts, the other terminals make great positions for after market devices like a sat nav or USB charger point., much cheaper by a country mile than a Bosch original type unit at around £15.00 and my first lasted 20 years, and was replaced about ten years ago, like for like, no loosened and jamming contact ends either, a proper moving lump of iron to connect the high amp cables for the starter.

CLEM
 
Exmoor beast said:
Similar set up- huge amp Draw on starting, minimal battery size, crap earth/ starter leads/ ignition switch etc . I would compare battery voltage at terminals to battery voltage at coils ( and also rectifier) in situ to see how much is being lost en route .. that’s French!
Click to expand...
Quod est demonstrandum. That's latin.

Paul
 
Paul Marx said:
I've finished reassembling my wife's 500. The ballast resistors gave me some aggro because the metal supports needed replacing.

I know those resistors can be done away with.
1) What exactly is their function? Something to do with keeping maximum volts for ignition ?
Click to expand...
Hi Paul,

FWIW.

Others including Rob and John Wilson have suggested that the ballast resistors had a different function. According to them, the ballast resistors were introduced by Bosch to protect their ignition models. I quote this from Rob:

"The ballast resistors were specified by Bosch to avoid burning out the ignition box transistor in the event that its coil had too low an impedance or developed a short. Or if you left the ignition on without engine running. So not required unless you're a paranoid German application designer!"

John wrote something similar, most of which went straight over my head. But see below* .

Now, I don't know my Arse from my Elbow when it comes to this kinda stuff. And I don't want to start a flame war. But there you go, it's said! :eek:
-------------------------------------------------------
* As in:
" NPN silicon power transistors as used in the Bosch ignition modules have some hideous characteristics compared to today’s power devices. One such characteristic is the voltage drop across the transistor between the emitter and collector when fully conducting, known as Vc-e(sat). If you multiply this Vc-e(sat) by the collector current you get the Pd or the ‘Power dissipated’ by the power device.

It does not take a lot of current coupled with a high Vc-e(sat) to broach the Safe Operating Area (SOA) of a power device - in this case, the ignition module.

This is my interpretation of why Bosch slung an extra resistor of 0.9 Ohms in series with the DC resistance and impedance of the ignition coils of the day."

 
Ventodue said:
Hi Paul,

FWIW.

Others including Rob and John Wilson have suggested that the ballast resistors had a different function. According to them, the ballast resistors were introduced by Bosch to protect their ignition models. I quote this from Rob:

"The ballast resistors were specified by Bosch to avoid burning out the ignition box transistor in the event that its coil had too low an impedance or developed a short. Or if you left the ignition on without engine running. So not required unless you're a paranoid German application designer!"

John wrote something similar, most of which went straight over my head. But see below* .

Now, I don't know my Arse from my Elbow when it comes to this kinda stuff. And I don't want to start a flame war. But there you go, it's said! :eek:
-------------------------------------------------------
* As in:
" NPN silicon power transistors as used in the Bosch ignition modules have some hideous characteristics compared to today’s power devices. One such characteristic is the voltage drop across the transistor between the emitter and collector when fully conducting, known as Vc-e(sat). If you multiply this Vc-e(sat) by the collector current you get the Pd or the ‘Power dissipated’ by the power device.

It does not take a lot of current coupled with a high Vc-e(sat) to broach the Safe Operating Area (SOA) of a power device - in this case, the ignition module.

This is my interpretation of why Bosch slung an extra resistor of 0.9 Ohms in series with the DC resistance and impedance of the ignition coils of the day."

Click to expand...
Me voilà bien avancé!

Paul
 
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