From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 29 Jan 2020 01:30:54 -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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As it turns out it was an almost purely measurement induced phenomena. The
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 unregulated
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 1uf
capacitor across the bridge and shunt resistor to absorb a brief and small
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 grou=
nd
> > 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 usin=
g
> a
> >> > differential probe? If not, where is the probe GND connected?
>


--=20
Jason White
--=20
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