From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
CC: ApptechNZ <apptechnz@gmail.com>
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Date: Fri, 24 Jan 2020 07:42:51 -0800
Subject: Re: [EE] H-Bridge supply voltage spike only in "reverse" (not
 forward)
Thread-Topic: [EE] H-Bridge supply voltage spike only in "reverse" (not
 forward)
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The pwm alternates between "forward"/"backwards" and "braking". The lower
two fets are supposed to be conducting during the off period.

Is it correct to assume that the lower two fets being on should prevent
dumping into the supply rail?

If I get a chance I'll try bypassing D1 and S1.

No internal diode that I know off. The problem follows the H-bridge,
reversing motor leads has no effect.

Also of note is the ground Of the waveforms is the bottom side of the
bridge shunt resistor.

On Thursday, January 23, 2020, RussellMc <apptechnz@gmail.com> wrote:

> 1. Given the lack of supply capacitor, the question MAY be "Why doesn't t=
he
> motor pump up the supply on motor overrun when the PWM is off ?"
>
> The MOSFET body diodes allow regeneration into the bus.
> Voltage magnitude does not equal power - so a voltage similar to or above
> that seen during bridge-on does not equal that during bridge-off.
>
> Do D1 and S1 play any role?
> I assume that D1 is intended to prevent regenerated energy back feeding t=
he
> supply (in some cases magic smoke can happen (ask me how I know :-) )).
> BUT what then should regenerated energy do?
>
> 2. Can you short S1 temporarily (as it is an overvoltage protector).
> While it's not obvious why it would work in one motor direction and not t=
he
> other, what it DOES do is somewhat unknown.
>
> 3. Will the supply tolerate shorting D1?
>
> 4. I wondered if the motor may have an inbuilt parallel 'circulation' dio=
de
> - but, if so, it would get hardish to drive it backwards :-).
>
> Russell
>
>
>
> On Fri, 24 Jan 2020 at 08:09, Jason White <
> whitewaterssoftwareinfo@gmail.com>
> wrote:
>
> > Hello everyone,
> >
> > I am encountering the following. The supply voltage line to my
> > H-Bridge is rising only when the H-Bridge is running a motor in
> > "reverse." In forward the supply voltage is unaffected.
> >
> > Intuitively I am having trouble understanding why this should be. I
> > lack a decoupling capacitor on the supply line, but do have a zener.
> >
> > I have attached a schematic of the circuit.
> >
> > I will send the forward and reverse waveforms in two follow up emails
> > to avoid hitting the file size attachment limits.The waveforms will
> > show supply voltage and voltage across the H-Bridge.
> >
> > --
> > Jason White
> > --
> > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
> --
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>


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Jason White
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