Output pin voltages above battery voltage

[RallyRat]

What happens when voltage is applied to the output pins while power to the PDM is disconnected, or if an output voltage is higher than battery voltage?

Examples might include: Towing the car on an open trailer the cooling fans could spin, inducing a voltage.
Connecting the race car's tail lights and turn signals to a trailer connector for towing with a tow bar.
Using two cooling fans, a relay could be used to wire them in series or parallel to switch between high and low speed; when switching from high to low, the fans could briefly be at max RPM while they are connected in series, creating back EMF higher than battery voltage.
Similar to the case with the fans, switching any two speed motor from high to low speed (or simultaneously deactivating the output when the motor speed is switched).

Will this power the PDM on even if the battery is disconnected? Are there negative current limits to avoid damaging PDM outputs when an output is inactive? Can current freely flow backward through an output pin to the battery if that output is active?

Thanks,
Will

[adrian]

Supplying power to an output will turn the PDM on even if there is no supply voltage to the main battery stud. This won't have any detrimental effect on the PDM (as long as the voltages are within the range specified in the User Manual) but obviously depending on how the PDM is configured it is possible it will turn parts of the car on.

[RallyRat]

Is there any transient or steady state limit to how much current can flow backwards before damaging the PDM? Does it matter if the output is active or not?

[the_bluester]

Related to that, given you are talking about a race car assuming that you battery isolator has secondary contacts to isolate and/or ground ignition as an engine kill. Maybe the best thing to apply a ground to would be a switch input configured as the master shutdown with outputs configured to respect it.

I probably don't need to do that on my setup as the only thing that would be powered in the event a windmilling fan generated a high enough voltage to turn the PDM on would be the keypad. My master shutdown is currently only configured to kill the power to the ECU, which will shortly deal with everything else when the engine stops.

[RallyRat]

Some good points, Bluester. There will be a battery isolator wired up to a PDM input, but I don't want there to be a possibility of damaging the PDM if someone forgets to shut off the master switch after loading up the car.

Another possible problematic situation would be if the isolator switch were flipped with the car running, any DC motors that were running could briefly power any other load that was not turned off by the master shutdown.

The tricky thing is that there is very little documentation about the PDM's output characteristics beyond "voltage generated by the slow winding during fast [wiper] operation will cause braking of the motor and possible damage to the PDM." I think I'll have to put a diode on the outputs for the lights unless I can find some definitive information.

[the_bluester]

I would not reckon there should be any problem. Surely a fan windmilling on the trailer will not run up to anything like the RPM that the fan operates at in use, and they don't have any problems with them blowing up when fans are shut off after use.

As to the rest, how are you controlling the PDM, a keypad or physical inputs only? If a keypad I would expect your logic would revolve around it pretty heavily. Basically as I wrote above, if you backfed mine from an output, the only thing that will power on would be the keypad. You then need another input for any other output to be powered up, keypad buttons for accessory or ECU power, brake light switch etc.

Likewise if you are using physical switches you could easily arrange the logic so nothing was powered without a switch input being active, brake pedal switch for brake lights, wiper switch for wipers (Obviously) ignition switch for all other power, that sort of thing

I will leave how much current it can sink if powered from an output to the Motec guys, but you don't seem to hear about people blowing them up. One thing I would wonder about though is the fan scenario you mentioned, most fans I could think of will pull enough current that running two in parallel is probably not feasible anyway, you would probably need two outputs in which case you might as well run one normally and then both if conditions demand it.

[RallyRat]

I have a V mount radiator and intercooler with two fans pushing air into a common plenum. If I only connected one fan, most of its air would probably just flow backwards through the un-energized fan. Switching the two between series and parallel electrically is pretty simple: it only requires a single relay to switch the negative wire of fan 1 between ground and the positive wire of fan 2. Each fan has its own PDM output, and the relay coil is connected between the two positive wires on the fans. Turn on both fans at the PDM and both come on with 12v (high speed, about 35amps total). Turn on only fan 1 and the relay coil sees the voltage difference between fans 1 and 2, thus connects the negative wire from fan 1 to the positive wire of fan 2 and both fans come on at 6v (low speed, about 7amps). The 6 volt difference isn't always enough to turn a 12v relay on, but it is plenty of juice to keep in on.

I do have a keypad, and the only non-momentary switches will be the turn signal and wiper controls. It would be possible to turn those items off or some other shut down logic if output voltage were present on an un-active output. An alarm acknowledgment could bypass this in case something strange triggered the condition in error.

If the outputs are MOSFETs, they'll have a body diode which should be able to handle just as much current in reverse as forward. I assume this is the case, but it might not be good if that assumption turned out to be wrong!

[adrian]

The PDM will handle being reverse fed up to half the particular outputs rated current, so 10A reverse on a 20A output and 4A reverse on an 8A output. The important thing to note is that the current limit protection will not work if you are reverse feeding the output. If you exceed the above you will damage the PDM.

You will also have to take into account the fact that even if the PDM is isolated from the battery, in this situation the PDM will be turned on and anything connected to the main battery feed pin will also be powered. This could have undesired consequences depending on how the PDM is setup and if anything is wired to the PDM side of the isolator.

One of the guys here has some data showing about 5V being produced by an un-powered fan while doing 180km/h, I don't think you'll ever be able to spin the fan fast enough to cause an issue to the PDM. The wiper output is a special case because the slow winding of a wiper is spun much faster in high speed mode than normal operation so it can cause much higher voltages.

[RallyRat]

Thanks, Adrian!