18vDC brushed motor driven by a mosfet controlled by pwm for the intention of having speed control, shouldn't see more than 3amps
the direction is change by reversing polarity via 2 relays
now how and the heck do i add a flyback diode to a bidirectional circuit.
i know i need it whenever i shutoff the motor and with the way i believe mosfets work thats gonna be a lot
I've read online but anything I've found goes over my head, i did read somewhere that i can use a TDS i think is what they called it but essentially is specifically for creating fly-back protection for a bidirectional circuit, i couldn't understand how to go about choosing the correct one.
i also came across a few different diagrams usually replacing the standard flyback diode at the motor with 2 flyback diodes i believe pointed at each-other.
i would rather use diodes but again i don't know and haven't been able to understand the methodology of choosing the correct diodes.
i guess i could go on and on but i think i explained what i need and what i'm trying to do, as well as provided all necessary information needed i hope.
Thanks for any help you have to offer i really appreciate it.
Without seeing your circuit diagram (should have added that),
the solution could be four diodes, like a bridge rectifier, with the AC terminals connected to the motor, the + terminal to supply, and the - terminal to ground.
Or, if the reversing relays are in the + line, and the mosfet is in the ground line, a single diode from drain to + supply could work.
Should use Schottky diodes if PWM is involved.
Leo..
thank you for that, ive been trying different orientations and combinations of diodes, just hooking motor directly to battery, every time i have a good size arc, until i used the bridge rectifier, that seemed to reduce the ark easily by about 70%, so i thought it was working but i needed more advice, i didnt want to speculate and blow out a bunch of mosfets. not to mention it was the first time i saw a bridge rectifier being used for anything but basic AC applications in the common power supply circuits.
so if i essentially just put a bridge rectifier between power output and motor it will act as flyback protection?
I've found RC snubbers very effective (this sort of thing, but you could obviously make your own at much less expense!) Have saved many solid state relays using them (after I blew up the first £50 one!). Transils are also very good.
dlayman11:
thank you for that, ive been trying different orientations and combinations of diodes, just hooking motor directly to battery, every time i have a good size arc, until i used the bridge rectifier, that seemed to reduce the ark easily by about 70%, so i thought it was working but i needed more advice, i didnt want to speculate and blow out a bunch of mosfets. not to mention it was the first time i saw a bridge rectifier being used for anything but basic AC applications in the common power supply circuits.
so if i essentially just put a bridge rectifier between power output and motor it will act as flyback protection?
There's only one way to connect diodes in a relay H-bridge, just guessing sounds like a way
to blow up diodes!
For any H bridge there's a free-wheel diode across each switch, which functions when the partner of
that switch opens, thus:
So for instance if the left hand side relay NC contact opens, the top left diode carries
the motor current until the left hand side NO contact closes (assuming the current is flowing
to ground in the NC contact originally (ie the motor isn't acting as a generator at that instant).
