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Posted by James on January 10, 2006, 2:01 pm
Hello,
I am trying to use an H-Bridge circuit as a fwd/rev relay to drive a
low (1 Ohm) resistive load. The set-up I am using is the revised bridge
from Eugene Blanchard's website. Anyway, I am activating the bridge
using a function generator set on a square wave. I am separating the
square wave into negative and positive components using diodes, and
then using these pulses to activate the logical inputs of the circuit.
However, the negative component of the f.g. output will not activate
one half of the bridge. Does anyone know how to invert this signal, or
perhaps modify the H-bridge to accept negative activation pulses? The
goal output here is a high current square wave across the resistive
load. Thanks in advance!
- JJ
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Posted by dan michaels on January 11, 2006, 10:50 am
James wrote:
> Hello,
> I am trying to use an H-Bridge circuit as a fwd/rev relay to drive a
> low (1 Ohm) resistive load. The set-up I am using is the revised bridge
> from Eugene Blanchard's website. Anyway, I am activating the bridge
> using a function generator set on a square wave. I am separating the
> square wave into negative and positive components using diodes, and
> then using these pulses to activate the logical inputs of the circuit.
> However, the negative component of the f.g. output will not activate
> one half of the bridge. Does anyone know how to invert this signal, or
> perhaps modify the H-bridge to accept negative activation pulses? The
> goal output here is a high current square wave across the resistive
> load. Thanks in advance!
> - JJ
When you say h-bridge circuit, this can mean just about anything, chip,
commerical device, or something you designed yourself. Hard to know
what is going on.
However, if you are using any avaialble off-the-shelf h-bridge chip for
this, it is doubtful it can handle a 1-ohm load. At 12v, a 1-ohm load
means 12-Amp, and few of the h-b chips can handle this.
- dan michaels
www.oricomtech.com
======================
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Posted by MetalHead on January 11, 2006, 11:58 am
James wrote:
> Hello,
>
> I am trying to use an H-Bridge circuit as a fwd/rev relay to drive a
> low (1 Ohm) resistive load. The set-up I am using is the revised bridge
> from Eugene Blanchard's website. Anyway, I am activating the bridge
> using a function generator set on a square wave. I am separating the
> square wave into negative and positive components using diodes, and
> then using these pulses to activate the logical inputs of the circuit.
> However, the negative component of the f.g. output will not activate
> one half of the bridge. Does anyone know how to invert this signal, or
> perhaps modify the H-bridge to accept negative activation pulses? The
> goal output here is a high current square wave across the resistive
> load. Thanks in advance!
James,
I am assuming that you want to re-create the bi-polar input signal
across your resistive load, so that when the input signal is above
ground, one side of the H bridge pulls high and when the input signal is
at ground or below ground, the other side of the H bridge pulls high.
From looking at Blanchard's H Bridge, jumper the low side inputs so
that they are driven off of the other leg of the bridge as Blanchard has
shown. Now you only have two high side control inputs to worry about.
One of the high side control inputs should be tied directly to the input
signal. The other leg of the H bridge gets it's control input inverted
with this circuit. If the Ascii art gets killed, it is just an NPN
transistor with it's emitter grounded and a 10K resistor from the
collector to +12V. A 4.7K resistor is used to tie the signal generator
output to the base of the transistor. The output signal is at the
junction of the 10K resistor and the collector of the transistor. The
resistor values I supplied may need some fooling with. No additional
diode stuff should be needed for separating the parts of the square wave.
+12V
-----
|
|
\
/ 10K or so
\
/
|------------------- To other leg high side
| input
/ C
|/
4.7K |
sig in --/\/\-----| NPN
B |
|\
\/ E
\
|
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GND
From looking at the drive circuits on that bridge, I don't think that
bridge will work very well, especially not at high frequencies. I would
expect problems to start showing up at frequencies above a few KHz.
Good Luck,
Bob
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Posted by James on January 11, 2006, 12:58 pm
Thanks for all the replies! For Dan, the schematic of the bridge can be
found here. http://www.geocities.com/fet_h_bridge/np-s.pdf . I made the
circuit using a printed circuit board and some standard parts, the
MOSFETS I am using are IRF4905, and IRF3205. For Bob, I'm going to try
the circuit you have suggested this afternoon, and hopefully it works.
I've been trying to replace the PNP transistor on the high side, which
didn't work. As far as problems when the frequencies are < 1KHz, I dont
expect to be using this circuit any higher than probably 100 Hz. I
think also I'll get rid of the diodes I have on there.
- JJ
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Posted by James on January 11, 2006, 2:42 pm
In the previous message, where I said PNP, I meant NPN
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> I am trying to use an H-Bridge circuit as a fwd/rev relay to drive a
> low (1 Ohm) resistive load. The set-up I am using is the revised bridge
> from Eugene Blanchard's website. Anyway, I am activating the bridge
> using a function generator set on a square wave. I am separating the
> square wave into negative and positive components using diodes, and
> then using these pulses to activate the logical inputs of the circuit.
> However, the negative component of the f.g. output will not activate
> one half of the bridge. Does anyone know how to invert this signal, or
> perhaps modify the H-bridge to accept negative activation pulses? The
> goal output here is a high current square wave across the resistive
> load. Thanks in advance!
> - JJ