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Topic: Blowing mosfets on my nerf select fire board (Read 424 times) previous topic - next topic

muttstang

Jul 29, 2020, 02:46 pm Last Edit: Jul 30, 2020, 06:02 pm by ballscrewbob
So I designed a select fire board using an Atmega 328 AU and a CH340 chip.

I was running it fine with the PWM on the flywheels and using the pusher circuit to run a solenoid, but the solenoid wasn't strong enough so I went back to a standard rack/pinion pusher motor setup using a hall effect sensor to detect each rotation.  When I activate the pusher in the blaster it kills the mosfet, which fails as a short so the pusher motor just keeps on running.  

Here's the circuit for the flywheel and pusher mosfets:
https://imgur.com/a/CYo955Q

I'm using this diode for flyback protection:
https://www.digikey.com/product-detail/en/sanken/SJPB-L6VL/SJPB-L6VLCT-ND/4572598

This is my primary mosfet:
https://www.digikey.com/product-detail/en/nexperia-usa-inc/PSMN1R8-40YLC-115/1727-1052-1-ND/3885292

and this is my motor braking mosfet:
https://www.digikey.com/product-detail/en/vishay-siliconix/SQJ415EP-T1-GE3/SQJ415EP-T1-GE3CT-ND/8572093

Is there anything that stands out as a potential failure point?  I have a fairly large delay between shutting off the braking and turning on the pusher (and vice versa)

Thanks!



Dabbling in arduino

muttstang

Hmm, could it be the lack of a resistor across my p-channel mosfet on the motor brake side?  maybe that guy is staying active when I turn on the n-channel on the drive side of that circuit and then I'm getting blow through.
Dabbling in arduino

muttstang

Dabbling in arduino

larryd

#3
Jul 30, 2020, 07:02 pm Last Edit: Jul 30, 2020, 07:15 pm by larryd


What is the value of the battery voltage ?

Appears the diode across the motor/coil is backwards.

Redraw this to a more conventional schematic: left to right top to bottom.

The N channel MOSFET that D4 controls has no biasing on the drain (connected to the gate of the P MOSFET).

Why are you using both D9 and D4 to control this motor ?











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muttstang

#4
Jul 30, 2020, 08:14 pm Last Edit: Jul 30, 2020, 09:08 pm by muttstang
the p-channel side is for motor braking.  It is for a dart pusher in a nerf blaster.  Only one is supposed to be on at a time.  so the top portion of the circuit makes the motor go, and the bottom portion makes it stop quickly.

I'm very much a rookie at electrical schematics. (I'm sure that's already obvious)
Dabbling in arduino

larryd

#5
Jul 30, 2020, 08:27 pm Last Edit: Jul 31, 2020, 02:05 am by larryd


Quote
the p-channel side is for motor braking.  It is for a dart pusher in a nerf blaster.
That's all you are going to say  :o.


D4 and D9 are swapped




I see now off sheet you might have BATT+.

Your Sketch D4/D9 timing/sequencing will be critical !

R1 must be added to discharge the gate to source capacitance.

You may want to experiment with adding low resistance into your brake circuit.


A half bridge might be something you may want to investigate.









No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

MarkT

The standard circuits for MOSFET H-bridges are tried and tested, use one of them perhaps?
For instance checkout the HIP4081A datasheet.  Using a driver chip provides several protection
features (fewer chances for frying MOSFETs), and safety interlocks that prevent shoot-through.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

muttstang

much thanks.  I'm adding that resistor between the gate and source on the p-channel and changing some resistor values around.  I already have the board in hand so I'll see about getting that in order, if I end up doing a respin I'll look into the fet driver chip.
Dabbling in arduino

muttstang

The mosfets are replaced with an additional 10k resistor between gate/source on the p-channel.  I've also changed the pull downs on the n-channels from 10k to 1k for better performance on PWM.  So far it works fine.  further testing will come.. :)

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