[rfengineers]

Originally Posted by engineer_Bill

I've heard of several people using a small wattage light either across the precharge resister, or as the resistor. When the light goes out the caps are charged.

Yes, that is a very simple and straight-forward way of monitoring precharge status.

It is not necessary to wait for the light to go out, that could take quite a while for some systems. Just let the light dim sufficiently to indicate that the voltage across the relay contacts is low enough to close without arcing.

[dr01allen]

Originally Posted by engineer_Bill

I've heard of several people using a small wattage light either across the precharge resister, or as the resistor. When the light goes out the caps are charged.

Would that work with an optical switch in series with the resistor? That way when the current stopped flowing, the switch would turn off.

How much current is involved? Would the voltage drop across a switch burn it out?

David

[rfengineers]

Originally Posted by dr01allen

Would that work with an optical switch in series with the resistor? That way when the current stopped flowing, the switch would turn off.

How much current is involved? Would the voltage drop across a switch burn it out?

David

David,

I would think not. The process involves charging a rather large capacitor on the input side of the motor controller, so the initial current will be large. Look at the attached RC charge curve, the steeper the slope the higher the current. From this you can see that when the precharge switch first closes the line is very steep.

[engineer_Bill]

You want an correctly rated opto to be in parallel with the precharge resister.Most opto switches are high impedance devices it would take all day to charge your caps. Better is to use your regular precharge resistor and have any sensing placed in parallel and rated for full pack voltage. They can be set to switch at anywhere from 50-30%, (caps are at 50-70%full).

[dr01allen]

Originally Posted by engineer_Bill

You want an correctly rated opto to be in parallel with the precharge resister.Most opto switches are high impedance devices it would take all day to charge your caps. Better is to use your regular precharge resistor and have any sensing placed in parallel and rated for full pack voltage. They can be set to switch at anywhere from 50-30%, (caps are at 50-70%full).

How about an incandescent bulb as you described earlier with a photocell attached to it and connected in series to a digital input? When the light was on, the cell would conduct and the input pulled low. As the light dimmed, the cell would stop conducting and the input would float high.

Could the same setup be used for a 12V relay?

David

[rankhornjp]

Wouldn't it be easier to use the "RUN" position on the ign switch to control the 'pre-charge' stage and the "START" position to close and latch the main contactor? Here is what I'm talking about using ladder logic.
The IGN switch would be place in 'run' until an "idiot light" turned off letting the driver know the caps are charged, then rotated to the 'start' position to turn off the 'pre-charge' and turn on the main contactor. (Think diesel engine with glow plugs)

[dr01allen]

Originally Posted by rankhornjp

Wouldn't it be easier to use the "RUN" position on the ign switch to control the 'pre-charge' stage and the "START" position to close and latch the main contactor? Here is what I'm talking about using ladder logic.
The IGN switch would be place in 'run' until an "idiot light" turned off letting the driver know the caps are charged, then rotated to the 'start' position to turn off the 'pre-charge' and turn on the main contactor. (Think diesel engine with glow plugs)

#What happens if the driver doesn't wait until the run light comes on?

[rfengineers]

Originally Posted by dr01allen

#What happens if the driver doesn't wait until the run light comes on?

Not a problem. Occasionally stressing the contacts by closing them with high voltage across the contacts should not significantly degrade them. It's doing it EVERY TIME that should be avoided.

Even with my step-start circuit you can force the contactor to close NOW if you wish.

[rfengineers]

Originally Posted by rankhornjp

Wouldn't it be easier to use the "RUN" position on the ign switch to control the 'pre-charge' stage and the "START" position to close and latch the main contactor? Here is what I'm talking about using ladder logic.
The IGN switch would be place in 'run' until an "idiot light" turned off letting the driver know the caps are charged, then rotated to the 'start' position to turn off the 'pre-charge' and turn on the main contactor. (Think diesel engine with glow plugs)

That is doable, but I would modify it to leave the precharge on after the contactor closes.

I prefer my step-start device since it allows me to turn on the ignition switch without applying precharge.

[rankhornjp]

You could still use the "ACC" position on the ign switch if you wanted to play the radio or something, without the "pre-charge" coming on. (If thats what youre talking about)

If you wanted to leave the pre-charge in the circuit you just take out the "normally closed" CR2 contact in the first rung.