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'[PICLIST] H-Bridge Crowbar (was PIC reseting due '
2001\08\29@113151 by Lawrence Lile

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On the H-Bridge Crowbar effect:

I've got an H-bridge made up out of discretes driving an itty-bitty DC
motor.  I used discretes because of low cost and low power requirements.  I
have one side of the H-Bridge driven from one port pin, the other side of
the H Bridge driven from the other port pin.

Valid inputs to the H-Bridge are 1,0 (forward);   0,1 (Reverse) or 0,0 (Do
nothing).  If I input 1,1, the H bridge crowbars the power supply.  This
could be a handy feature if it was one of those self-destructing messages
they give to James Bond, but it's not.

I really hate having invalid states that are not locked out, either in
software or hardware.  I always like to check to see if variables are in
range, setpoints are not some crazy number, EEPROM contents are sane when
recalled, etc.

Now I'm careful to make my software never get even close to driving both
inputs to 1, with a nice delay of a few cycles, and a mirror byte so I can't
get hosed by the read-write-modify bug.  So far so good.  But what about
when a little glitch comes along and this thing begins to crowbar?  Maybe I
do want to destroy the device at that point... but let's assume I don't.

It would be simple to add a little logic circuit to prevent both inputs from
going high at the same time.  Digikey sells a 2-input Xor gate for US$0.35,
two of them would do the trick.  $0.70 might sound cheap to you rocket
scientists, but I build whole PC boards that cost less than that.   Can
anybody think of a simple, cheap way to implement this, say using diode
logic or a To-92 transistor?  Kind of a design challenge.

What I had originally wanted was an H-Bridge that would go forward for a 1
input, reverse for a 0 input, and do nothing for a high impedance input, but
I could not thinka one.

-- Lawrence Lile


{Original Message removed}

2001\08\29@124437 by Spehro Pefhany

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At 10:29 AM 8/29/01 -0500, you wrote:
>
>It would be simple to add a little logic circuit to prevent both inputs from
>going high at the same time.  Digikey sells a 2-input Xor gate for US$0.35,
>two of them would do the trick.  $0.70 might sound cheap to you rocket
>scientists, but I build whole PC boards that cost less than that.   Can
>anybody think of a simple, cheap way to implement this, say using diode
>logic or a To-92 transistor?  Kind of a design challenge.

In essence you want to force one low when the opposite one goes high.
Maybe you could use a transistor and base resistor.

Without more information on your circuit (especially the motor voltage and
current- I'd assume it is not the same as the PIC power supply), it's really
hard to guess what you're up to.

If you post your circuit, I'm sure we can make suggestions.

Best regards,


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'[EE]: H-Bridge Crowbar (was PIC reseting due to r'
2001\08\29@125105 by Lawrence Lile

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This time with the right tag.

----- Original Message -----
From: "Lawrence Lile" <llilespamKILLspamTOASTMASTER.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Wednesday, August 29, 2001 10:29 AM
Subject: H-Bridge Crowbar (was PIC reseting due to relays)


{Quote hidden}

can't
> get hosed by the read-write-modify bug.  So far so good.  But what about
> when a little glitch comes along and this thing begins to crowbar?  Maybe
I
> do want to destroy the device at that point... but let's assume I don't.
>
> It would be simple to add a little logic circuit to prevent both inputs
from
> going high at the same time.  Digikey sells a 2-input Xor gate for
US$0.35,
> two of them would do the trick.  $0.70 might sound cheap to you rocket
> scientists, but I build whole PC boards that cost less than that.   Can
> anybody think of a simple, cheap way to implement this, say using diode
> logic or a To-92 transistor?  Kind of a design challenge.
>
> What I had originally wanted was an H-Bridge that would go forward for a 1
> input, reverse for a 0 input, and do nothing for a high impedance input,
but
{Quote hidden}

frequency
> > on the choppers, which is poorly documented, and apparently none of
their
{Quote hidden}

here
> > in my signature line, but due to the inability of sysadmins at TELOCITY
to
{Quote hidden}

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2001\08\29@125119 by Lawrence Lile

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Bob Blick pointed me to a discrete H-Bridge on his site that might fill the
bill.  I'm going to look into it.

--Lawrence

{Original Message removed}

2001\08\30@141418 by Peter L. Peres

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The easiest way to foolproof a discrete bipolar H bridge is to add a
single 'digital' transistor in series with one of the control inputs and
commanded by the other. Choose one that runs emitter current into/from the
controller not into the bridge. These items were very common in
discrete-built servo drivers for RC 15-20 years ago (or more). There were
books on this and some featured some good bipolar bridge designs (from
tiny up to 5A or so).

Peter

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