470 Ohm resistor purchase

[JohnnieB]

Originally Posted by KLA

Looking at the schematic of a PDS cart in the stickies, it looks like that resistor's function is to limit the inrush current into the capacitors of the controller when the batteries are first connected. It also supplies the controller with current even when the contactor is off.

Close, but no cigar.

The statement would be closer if the blue highlighted phrase was changed to: when the contactor connects the battery pack to the controller.

The purpose of the resistor across the solenoid's (contactor's) contacts is to reduce the arcing on those contacts when they close. However, they accomplish that task by keeping the filter capacitors charged so there will not be an inrush of current when the contacts close.



The resistor does supply current to the controller when the contactor is off, but only to the filter capacitors. The power that actually runs a PDS controller's electronics and control circuitry is supplied via J1 Pins 4 & 5. Other type controllers get power for their electronics and controls via different jack and pin numbers, but none receive power via the resistor for anything except keeping the filter capacitors charged.

[KLA]

Originally Posted by JohnnieB

Close, but no cigar.

The statement would be closer if the blue highlighted phrase was changed to: when the contactor connects the battery pack to the controller.

The purpose of the resistor across the solenoid's (contactor's) contacts is to reduce the arcing on those contacts when they close. However, they accomplish that task by keeping the filter capacitors charged so there will not be an inrush of current when the contacts close.


..

Thanks JohnnyB. However, that means that the wattage rating of the resistor can be much smaller than 10W. One can also increase the resistance quite a bit. If the resistor is only there to keep the capacitors charged, it has to supply only the leakage current of the capacitors, power transistors and freewheel diodes. That should be < 1 mA. Which means the resistor could be made quite a bit bigger, like 2.2 kOhm, which means it has to worst case only dissipate 1.5W, and that only if the controller is broken (B+ to B- short). In normal operation only 0.75W (average) during the time when the capacitors charge when the battery is connected.
As resistors can be pulse loaded for up to 1 second with up to 10 times their rated power, a regular 1W 2.2K resistor might do the job.

[yurtle]

Only thing to consider is how discharged the caps are, vs. how soon you punch it after being discharged. To charge them overnight is a different load than letting off the gas then seconds later punching it again.

[JohnnieB]

Originally Posted by KLA

Thanks JohnnyB. However, that means that the wattage rating of the resistor can be much smaller than 10W. One can also increase the resistance quite a bit. If the resistor is only there to keep the capacitors charged, it has to supply only the leakage current of the capacitors, power transistors and freewheel diodes. That should be < 1 mA. Which means the resistor could be made quite a bit bigger, like 2.2 kOhm, which means it has to worst case only dissipate 1.5W, and that only if the controller is broken (B+ to B- short). In normal operation only 0.75W (average) during the time when the capacitors charge when the battery is connected.
As resistors can be pulse loaded for up to 1 second with up to 10 times their rated power, a regular 1W 2.2K resistor might do the job.

If you wish to find out what the current flow is for your cart, measure the voltage drop across the resister while the solenoid is de-energized. For most 36V carts the drop is in the 3-5 volt range, so the current flow is in the 11 to 19 ma range.

The reason they use a 10W (or so) is to prevent/reduce the shrapnel when the solenoid contacts fail to make contact and the controller attempts to pass amps the the motor.

The control element in the controller is a dozen or so MOSFETs in parallel and they tend to fail open rather than shorted, or at least they burn open a few milliseconds later if the do fail shorted. Anyway, if the did fail shorted, the short would be between B- and M-, which may be labeled A1 ob a PDS controller.

[KLA]

Sounds like the perfect application for a self-resetting fuse like this:
http://www.littelfuse.com/products/r..._datasheet.pdf
Actually, that's what these things are used for in similar applications. These fuses are really PTC resistors that become a high value resistor when they heat up and are a low value resistor when they stay cold.
The 60R010 one, with a hold current of 0.1 A, should do the job in this application.

[KLA]

Originally Posted by yurtle

Only thing to consider is how discharged the caps are, vs. how soon you punch it after being discharged. To charge them overnight is a different load than letting off the gas then seconds later punching it again.

It depends on the total capacitance the resistor has to charge. I doubt very much that there is more than 1000uF of capacitance in the controller. With 1000uF and 470 Ohm, the capacitors can be fully charged in 2.35 seconds from completely empty (5 tau). However, because the resistor keeps the capacitors charged permanently as long as the batteries are connected, the full charge is reached much faster even if you only switch between idle (contactor off) and "punching" it.

[JohnnieB]

Originally Posted by KLA

Sounds like the perfect application for a self-resetting fuse like this:
http://www.littelfuse.com/products/r..._datasheet.pdf
Actually, that's what these things are used for in similar applications. These fuses are really PTC resistors that become a high value resistor when they heat up and are a low value resistor when they stay cold.
The 60R010 one, with a hold current of 0.1 A, should do the job in this application.

Likely to be okay in a 36V cart, but iffy for a 48V system since the PPTC device is exposed the the battery pack's On-Charge voltage, which can exceed the device's 60VDC rating.

If the 470 Ohm resistor I use in my 42V system ever fails, I might give one a try.

[JohnnieB]

Originally Posted by KLA

It depends on the total capacitance the resistor has to charge. I doubt very much that there is more than 1000uF of capacitance in the controller. With 1000uF and 470 Ohm, the capacitors can be fully charged in 2.35 seconds from completely empty (5 tau). However, because the resistor keeps the capacitors charged permanently as long as the batteries are connected, the full charge is reached much faster even if you only switch between idle (contactor off) and "punching" it.

The voltage on the filter capacitors will track the pack voltage while the solenoid contacts are closed, and the pack voltage varies inversely to the amps being drawn from it, so the peak current flow through the resistor will be the instant the solenoid contacts are opened and will be elevated somewhat until the pack voltage fully recovers after the load is removed.

I don't know how many µF the individual filter capacitors are, but there's a bunch of them.
Here is a picture of the power board for a Curtis 1206 (36V series controller) posted by Roady89 over in the Extreme DC forum.

[KLA]

Originally Posted by JohnnieB

Likely to be okay in a 36V cart, but iffy for a 48V system since the PPTC device is exposed the the battery pack's On-Charge voltage, which can exceed the device's 60VDC rating...

No, if the ON charge voltage exceeds 60VDC, the only way to exceed the 60VDC rating of the device would be if you disconnect the controller, let all caps discharge, and then connect the controller again while the charger remains connected to the pack. You would have to work hard to exceed the 60VDC rating with a 48V system.
The (instantaneous) voltage across the resistor is the difference between pack voltage and last capacitor charge voltage, not the pack voltage itself.

Originally Posted by JohnnieB

The voltage on the filter capacitors will track the pack voltage while the solenoid contacts are closed, and the pack voltage varies inversely to the amps being drawn from it, so the peak current flow through the resistor will be the instant the solenoid contacts are opened and will be elevated somewhat until the pack voltage fully recovers after the load is removed....

However, as I understand, the controller drives the solenoid coil. I am pretty sure the controller will open the solenoid only AFTER it switched the motor current off as it would be very bad practice to open the contacts under high inductive (motor) current load. That would be a sure way to burn them up quickly.
Because the recovery time constant of the batteries is pretty long compared to the capacitor charge time constant, the current through the resistor to track the recovering battery voltage is small. With capacitors in series with a resistor the charge current is proportional to the rate of change of the pack voltage.

[JohnnieB]

Originally Posted by KLA

1. No, if the ON charge voltage exceeds 60VDC, the only way to exceed the 60VDC rating of the device would be if you disconnect the controller, let all caps discharge, and then connect the controller again while the charger remains connected to the pack. You would have to work hard to exceed the 60VDC rating with a 48V system.
The (instantaneous) voltage across the resistor is the difference between pack voltage and last capacitor charge voltage, not the pack voltage itself.



2. However, as I understand, the controller drives the solenoid coil. I am pretty sure the controller will open the solenoid only AFTER it switched the motor current off as it would be very bad practice to open the contacts under high inductive (motor) current load. That would be a sure way to burn them up quickly.
Because the recovery time constant of the batteries is pretty long compared to the capacitor charge time constant, the current through the resistor to track the recovering battery voltage is small. With capacitors in series with a resistor the charge current is proportional to the rate of change of the pack voltage.

1. I stand corrected.
Depending on the battery brand and charger type, the ON-Charge voltage of a 48V pack approaches or exceeds 60.0V on each charge cycle, however the voltage across the open solenoid contacts is the pack voltage less the charge on the filter capacitors. Therefore exceeding 60V across a PPTC isn't very likely to happen without human intervention.

2. Depends on the type controller. A PDS controller supplies both B+ and B- to the coil, a DCS controller only supplies B- and a Series controller doesn't supply either.

However, the EZGO engineers took care of that problem. The throttle input telling the controller to pass amps to the motor is removed before the pedal microswitch causes the solenoid coil to de-energize, so the PWM duty cycle is Zero when the contacts open.

The linkage connected to the accelerator pedal allows the NC contacts in the pedal microswitch to close (energizing the solenoid coil) mechanically sooner than the throttle device (Pot-Box or ITS sensor) begins increasing the throttle signal to the controller telling it to increase the duty cycle of the PWM DC sent to the motor. The sequence is the opposite when the foot is lifted off the pedal.

Due to the contacts being closed before the PWM duty cycle is increased above 0% when the pedal is pushed and not opening until after the duty cycle has returned to 0% when the pedal is released, there is no current flow through the contacts to/from the motor while they make or break.

Of course, without the resistor (or other device) installed, the inrush current to the filter capacitors would occur while the contacts were still bouncing on close.