My son’s rather upgraded Chinese Power Wheels (6V single seater, running a 16V lithium pack…) is prone to arcing up the switches something fierce. He likes to switch speeds while running, which is fine, but with the stiff lithium behind it, they arc badly (inductive loads with the motors), and it fries the switches in short order. They can be cleaned, but… still.
While it’s down for gearbox replacements, I’d like to ponder through arc mitigation on switches. Any suggestions?
I need to sit down and actually sketch out the wiring diagram… but it’s standard Power Wheels.
There are two DPDT switches. One selects between forward and reverse, one selects between low speed (motors in series) and high speed (motors in parallel).
The closest I ever got to Power Wheels was the things at the grocery stores that are powered by your parents rather than batteries. Is the throttle actually connected to a motor drive/inverter or is it also just an on/off switch?
I would think a full H bridge motor drive would be a good way to soup this up, and use low voltage I/O to engage reverse, and toggle switch that limits the motors to a low range. Are these brushed DC motors?
On the one in question, it’s just an on/off switch. The bigger one has a variable power drive unit, but that’s not the one arcing badly - it does use the switches as simple inputs to some larger relays, and other than dust intrusion, they seem to handle the loads fine.
Yeah, pretty simple.
I think this is probably the right term for what I need - thanks!
Use a hefty relay which the switch(es) engage? Would the relay avoid the arcing? Something like https://smile.amazon.com/Supco-90340-General-Switching-Contacts/dp/B004XS1ZT0 maybe, says it’ll handle up to 3/4 HP motor, although 24v coil. But since that’s low draw I imagine a cheapo boost from the 16v lithium could work.
I could go to relays, and I’m considering that, but DC relays have the same problem with arcing that switches do, and it makes no sense to put in some high amperage DC relays when $5 of caps and resistors, soldered in place, would do the same job. I just need to figure out what the voltage spike is and make sure I don’t blow the capacitors up.
Actually you don’t need to know the voltage of the inductor spike, since the snubber circuit operates more or less “seamlessly” with respect to the transient - it doesn’t see the high voltage since it’s snubbing the current to prevent that voltage spike from developing in the first place. Here’s one guide to sizing/choosing values: https://www.cde.com/resources/catalogs/igbtAPPguide.pdf though it’s a bit technical. I would start with a simple RC snubber, and then use a diode snubber if that’s not sufficient. But you’re dealing with relatively “slow” circuits, albeit with a decent amount of stored energy to deal with. You shouldn’t need particularly high voltage caps, but you will need high-current caps and maybe also a fairly beefy resistor.
Huh, good point. I hadn’t really thought through that the whole point was to prevent the spike in the first place.
Next step, figure out if I’ll be seeing any reverse voltage. I don’t think so. I’ll pencil out the schematic here (or find a normal Power Wheels one) for further analysis.
No, I’ve now ordered from 3 places that haven’t shipped (or haven’t shipped yet). I’m beginning to conclude that everyone is just drop shipping stuff from China, and that the retailer doesn’t have supply.
Well, placed another order from yet another place. I just don’t think these things exist, but if I do end up with extras, I can probably flip them as verified stock for an awful lot!
This is roughly what the system is. The switch in question is the Hi/Lo switch, switching between series and parallel wiring to the motors. This one arcs up badly during switching under load.
My initial impression is that a small resistor and capacitor in series, between both “end” contacts and the center, would solve the problem.
If I call the motors 5A loaded (I don’t know exactly what they are, but in the realm of 1A spinning freely):
C (in uF) = (I^2)/10 = 2.5uF, or a bit more if running current is higher. This is per motor, but it should be the same per contact (in parallel, each contact breaks one motor, in series both contacts are breaking the same current).
Resistance = V/(10I(1+(50/V))) for 16V ~= 0.075 ohm. o.O I’m not sure how much resistor heat dissipation I actually need here, because the capacitors will stop the current fairly quickly.
I briefly considered diodes, but given that the whole setup runs in reverse as well (both slow and fast - a 2 year old in high speed reverse is a dangerous thing), it seems like ceramic capacitors just solve my problem completely in terms of polarity.
Is it worth tossing a scope on his contacts, or does this sound sane/does my math pencil out?
The resistance seems really, really low to me. 16V / 0.075ohm = ~200A. Which I don’t think the switches will like on initial contact, but it’s only very briefly…
