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The one and only lobster

PWM issues

I have always wanted adjustable ROF for a rapidstrike.  I don't like being stuck at any ROF for any long period of time.  So I did some searching and found a 15USD PWM chip, and immediately wanted to buy it.  But I don't want to damage it by wiring dynamic braking for the pusher motor.  Does anyone have any knowledge about PWMs, and how I should wire it to have dynamic braking and not damage it?
SSGT

For the most part it shouldn't matter as long as the PWM module you're using is rated to handle the transient current draw of the motor and as long as you put a flyback diode in parallel with the motor to protect the FET(s) in the PWM module from voltage spikes caused when a motor circuit is suddenly opened and no path is provided for the current to dissipate (a FET's body diode will offer some protection but a flyback diode will do a better job especially with higher inductance motors - the body diode will only clip the voltage down to, for most TO220 FETs, 100V and the FET may still fail or have it's life significantly shortened if it has to dissipate too much energy repetitively).

If you wire it correctly the braking loop itself should be completely independent of the PWM circuit and so shouldn't have any effect on it. Alternatively if you program a microcontroller to provide the PWM signal to a FET (and potentially even control the motor) you could use also it to control the braking of the motor via a second FET.

What module do you intend to use and with what motor?
The one and only lobster

A pretty cheep one I found on amazon.  I have no experience with microcontrollers.


https://www.amazon.com/uniquegood...-Adjustable-without/dp/B018FZ3D6O

Edit- Also, should I put the switch after the PWM or before it in the circuit?  Thanks a lot.  This is a huge help.

Edit- Sorry if I am spamming your notifications box, but I am using a honeybadger pusher on a 3s, and I don't want to buy a new motor, but still have different ROF
OldNoob

EDIT

Please do not double or triple post. This isn't Reddit. If you have more to add, before someone else has replied,  simply edit your last post and put it in there. Re read the Code of Conduct.

SSGT

This should work:


I've followed toruk's convention (i.e. all switches as they would be in a RS at rest - rev and trigger switches are in normally closed positions and cycle control switch is held in normally open position by the pusher) so you can compare it to other RS wiring schemes presented here - the only thing I've added are diodes to protect against voltage spikes. Note that it is effectively the same as a standard "live centre" circuit the only difference being that the PWM module sits between the pusher motor circuit and the battery. Unlike toruk's live-centre diagram the motor and cycle control switch NO are connected to the Motor -ve terminal rather than directly to ground (this is because the PWM module switches the negative line from the motor to the battery on/off multiple times a second to drop the average voltage across the motor). As far as I can tell from the images of the PCB, Battery +ve and Motor +ve are effectively connected to each other although this is done through a fuse to protect the rest of the board. Note that whenever a pack is connected the board will be continuously drawing power so I'd recommend putting in a cut-off switch somewhere between the battery and the rest of the circuit (before even the feed to the rev switch) to make sure everything stays fully off until you need it (although you shouldn't be storing packs in blasters anyway). You may be able to connect power directly to the Motor +ve connection and replace the fuse with a switch to turn off just the board but I couldn't say for sure without taking a closer look at it - it would also remove the board's main source of protection.

Putting everything in between the motor outputs as shown means that the PWM module effectively won't "see" the motor braking and the resulting reversed current direction. I don't know whether or not you'll be able to use a live-centre scheme with a HoneyBadger on 3S without experiencing runaway firing but by effectively modifying toruk's diagram it should be fairly easy to see how you'd go about wiring a "dead-centre" or "two-switch" circuit to the PWM module instead.

That particular PWM module may be just within spec (probably fine in normal use but the fuse may melt if you get a stoppage) but if you can you might be better off looking for an otherwise similar 30A version.
OldNoob

I very much doubt a Honeybadger will pull 20A on a pusher.
SSGT

Should be just as likely as if it were powering a flywheel on 3S - it'll draw close to stall current at startup and will draw stall current at stall. The rotor isn't turning when you close the switch so the coils aren't moving through magnetic field lines which means that no back EMF is being generated and therefore full stall current will be demanded - it may not quite reach stall current at startup, due to coil and wire inductance slowing down the rate of change of current such that the rotor is already spinning when the current draw peaks, but the same applies to flywheel motors (granted they have to deal with the moment of inertia of the flywheel, which will increase the time taken to get up to speed, but a pusher motor has to get a load of spinning and oscillating plastic moving aswell). If you have a stoppage that prevents the pusher from moving that will stall the motor and it will draw full stall current.

Like I say though, in normal use it may not be an issue, especially if OP never runs the pusher at max duty cycle/voltage - it may not even blow the fuse if you react quickly enough to a stopped pusher (although with live-centre there's not a lot you can do unless you put a PTC in the "live-centre" connection).
The one and only lobster

Thank you so much.  I can't say how helpful this is.  I'll start looking for a 30A, then get to work.  Thank you again.  Also, the reason I went with 20A is honeybadgers draw 10A stall on 2s, so if I ran it on 3s (at least according to Ryan of MTB) it should pull 1.5 times what it does on 2s, or 15A.  Though if you think I should go 30A to be safe, I will.
SSGT

Honey Badgers are closer to 14A stall on 2S (11A at 6V and just over 20A at 3S). There's a full spreadsheet of motor datasheets here - data for the HoneyBadgers was taken from this datasheet from the manufacturer. Also the figures from the manufacturer are extrapolated from data at lower loads (and then further extrapolated to the higher nominal voltage) so in reality, along with individual motor variance, you may find your Honey Badger draws slightly more or less at stall than specced.

Like I say you may well be fine with a 20A PWM board but if you want to add a little overhead, and have the space to fit it, bumping it up to 25 or 30A will give you extra piece of mind.
The one and only lobster

I compeletly got honeybadger's current draw messed up.  I am totally looking for a 30A one now.  Also, (noob question alert) what diode should I look for.  Thanks again
SSGT

Any of the 1N5400 series would probably be more than adequate and you could quite possibly get away with something rated even lower than that. All it does is provide a path for the (mostly relatively low) current to dissipate rather than allow it to generate a voltage spike and arc switch contacts or damage FETs.
The one and only lobster

Ok.  And is the diode above on the flywheel motors necessary?  And the silver band on the diode goes on the positive, right? And would it be ok if I soldered the diode between the motor terminals to save space?

I found this 30A PWM.  Is this one better?

https://www.amazon.com/XINY-motor...4&sr=1-9&keywords=30A+PWM

Thank you for putting up with all my questions.  I am trying something completely new to me.

EDIT:

These diodes would work, right?

https://www.amazon.com/Ultrafast-...;sr=8-1&keywords=in5401+diode

EDIT

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Thank you.

SSGT

SSGT

Using a flyback diode there isn't strictly necessary but I like to include them as they keep switch arcing to a minimum and help prolong the life of the switchgear. Silver band to positive i.e. reverse biased is correct - that way they don't conduct current under normal use which would otherwise short the pack. Placing them between the motor terminals is perfectly acceptable.

I can't find any decent images of the underside of the PCB for that PWM board but it should be suitable.

Yes, those diodes should do fine.

And don't worry about asking questions - if you don't ask you don't learn.
The one and only lobster

I am going to get right to work.  I appreciate your help a ton!

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