From: Sean Breheny <shb7@cornell.edu>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 29 Jan 2020 01:50:52 -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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Thanks for the update. Glad it worked out!

On Wed, Jan 29, 2020 at 4:35 AM Jason White <
whitewaterssoftwareinfo@gmail.com> wrote:

> As it turns out it was an almost purely measurement induced phenomena. Th=
e
> scope earth ground was interacting with the isolated bench power supply
> earth ground. Switching to an isolated probe fixed the problem - made the
> H-bridge behave as expected with nice square output and a flat unregulate=
d
> supply rail (except for the 50ns or so of shoot through that I expect to
> address in a later revision.) Although I should add that I did insert a 1=
uf
> capacitor across the bridge and shunt resistor to absorb a brief and smal=
l
> spike in rail voltage observed every pwm cycle - perhaps due to power
> supply lead inductance on my test setup.
>
> Thanks to everyone for all the help. Things get pretty tricky when
> measurements influence circuit operation.
>
> -Jason White
>
> On Tuesday, January 28, 2020, Sean Breheny <shb7@cornell.edu> wrote:
>
> > Any update? I'm really curious.
> >
> > On Sat, Jan 25, 2020, 10:32 PM Sean Breheny <shb7@cornell.edu wrote:
> >
> > > I agree - beware that power supplies which are isolated from earth
> ground
> > > may still have substantial capacitance to ground which can affect
> > circuits
> > > when you ground one of the nodes in the circuit.
> > >
> > > On Sat, Jan 25, 2020 at 5:58 PM Jason White <
> > > whitewaterssoftwareinfo@gmail.com> wrote:
> > >
> > >> Isolated differential probe for Vunregulated and "directly connected=
"
> > >> for PWM pulses out of H-Bridge. Power supply is isolated from earth
> > >> ground.
> > >>
> > >> I suppose I should try just the differential probe.
> > >>
> > >> On Sat, Jan 25, 2020 at 5:22 PM Sean Breheny <shb7@cornell.edu>
> wrote:
> > >> >
> > >> > How are you measuring  the voltage across the H-bridge? Are you
> using
> > a
> > >> > differential probe? If not, where is the probe GND connected?
> >
>
>
> --
> Jason White
> --
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