Re: [CAD_CAM_EDM_DRO] Re: H bridge progress

First, I want to thank everyone who has offered advice on the H bridge
circuit. What I have at the moment is the FET's and 2104's on a
little milled PCB, with the L297 on a breadboard. Next revision is
going to put it all on the PCB, but I want to be more sure of the
design first.

The gate resistors in question are shown in the IR2104 example circuit
between the 2104 and the gates.

> > 1. The gate resistor is there to prevent oscillations. Keeping the
> > driving circuit VERY close to the FET helps to reduce this, as do
> > ferrite beads on the gate line placed very close the FET.

It sounds like maybe I should go for a 10 ohm or so resistor, possibly
with a ferrite bead on one of its leads? Another idea, from the ARRL
handbook, would be to use a larger value resistor as a physical
carrier over which to wind a small coil of wire (shunting the
resistance). The main issue I need to decide now though is if I plan
for a component like this on the PCB, which does increase the distance
between the 2104 and FET's, but also provides a convenient way to
cross over a power trace (at right angles, so coupling should be
minimal).

> > 3. The L297 turns both sides of the bridge high when turning off a
> > winding as this is the fastest way to get the energy out of the winding.
>
> No, that actually allows current to recirculate (through the reverse diode
> on one of the transistors) indefinitely, reduced mostly by the IR drop of
> the coil itself.

Fortunately, switching modes on the 297 is a matter of chaning one
jumper, so I can try both ways.

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
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