H-Bridge Contoller

Hello,

I am trying to cycle an h-bridge from forward to reverse at a frequency
of anywhere between 1 and 500Hz. I am using a function generator to
trigger the bridge, and I have separated a square wave output of the
bridge into a negative and positive component using diodes. The
H-bridge schematic that I am using is from Eugene Blanchard's website,
it uses MOSFET switching transistors. Anyway, I need to trigger the
bridge using the function generator, and I can't devise a way to
trigger one leg of the bridge using the negative f.g. output. Can
anybody tell me how to modify the bridge to accept this input? Or
invert the f.g signal to a positive value? Thanks in advance!

-- JJ

 

Are you using it as a pwm fwd/reverse speed control, or just as an
onfwd-off-onreverse switch/relay?

 

The H-Bridge is used as a relay to cycle a low resistance load from 8
to -8V, in a square wave type signal. During testing I am using a 0.5
and a 2 Ohm power resistors to simulate different amounts of load. I
can create two pulsed signals using the f.g. and diodes to trigger the
bridge, but one logical input signal has a negative sign, which is not
activiating one side of the bridge. The "off" time for the bridge
should be negligible, I am trying to switch directly from fwd to
reverse. I can supply a schematic of the bridge if necessary. thanks!

 

just as an switch/relay as you said, basically I am driving a low
resistance load that needs to be cycled from 8 to -8 V, with about 10
Amps of current. The load has negigible capacitance, and I am using a 2
Ohm and 0.5 Ohm power resistors. to simulate the load during testing. I
have a f.g. that can provide two logical input pulses via a square wave
and diodes, but the only problem is that one of the logical inputs is
negative, which is causing me problems. Also, I may have to vary the
driving voltage from 1 to 8V. do you need a schematic?Thanks for the
interest.

 

Schematic would help. If you have N chan mosfets on the top and the
bottom, the 'top' one needs to get its gate really pulled up to turn
on. Or you could have a 'complementary' rig where the top gate (P chan)
get pulled down to turn on, and the bottom is N chan and the gate gets
pulled up to turn on. What volts from the f.g. and what batt volts on
the bridge?

 

The schematic can be found here
http://www.geocities.com/fet_h_bridge/np-s.pdf. As far as the voltages
are concerned, I can put 0 to 8 DC volts on the bridge, and from 0 to
10 volts peak to peak on the f.g., which is used to activate inputs A
and B. C and D are connected as shown in note #4. Anyway, basically
what I need is to either activate B using a negative voltage, or invert
the sign of the voltage that is going to B. I hope this makes
everything clearer.

 

Looks like you put a pos pulse on A to put out pos volts, or you put
the pos pulse on B to put out neg volts. Need to have the jumpers in
the 2-3 position. So you need a spdt switch from the fg to A and B or
an inverter from A to B, so they are opposite from each other.

 

Exactly