H-Bridge short protection
H-Bridge short protection
(OP)
Hi,
I have 2 H-bridges being used to actuate 2 separate DC motors (forward and reverse on both) using a total of 8 electro-mechanical relays.
There are 2 outputs per H-Bridge/Motor combination.
The DC motors share a common so 3 outputs are connected to the 2 motors and a jumper is connected between the unused output and one of the other outputs.
During system startup all 8 relays close. If the power supply connected to the H-bridges is on and the jumper is in place a short is made and a fuse blows.
I am looking for a way to prevent this from happening, any suggestions? I was thinking about using 2 SSRs to exclusive OR two of the H-bridge relays that may be connected to jumpers but in this case I am worried about the SSRs being destroyed before the fuse blows in the case that a system operator shorts two wires while connecting motors...
Thanks,
Dave
I have 2 H-bridges being used to actuate 2 separate DC motors (forward and reverse on both) using a total of 8 electro-mechanical relays.
There are 2 outputs per H-Bridge/Motor combination.
The DC motors share a common so 3 outputs are connected to the 2 motors and a jumper is connected between the unused output and one of the other outputs.
During system startup all 8 relays close. If the power supply connected to the H-bridges is on and the jumper is in place a short is made and a fuse blows.
I am looking for a way to prevent this from happening, any suggestions? I was thinking about using 2 SSRs to exclusive OR two of the H-bridge relays that may be connected to jumpers but in this case I am worried about the SSRs being destroyed before the fuse blows in the case that a system operator shorts two wires while connecting motors...
Thanks,
Dave





RE: H-Bridge short protection
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: H-Bridge short protection
RE: H-Bridge short protection
On system power up all the relays are necessarily in the same(unpowered) states resulting in nasty shoot thru. (of course)
If you are going to do it this way David you really should have a relay that turns on the Motive power to the H bridges after control power has come on and the H bridge relays are in their correct states. You also have to design for the main power supply going away while the system is running.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: H-Bridge short protection
Then you'll need another relay (as mentioned by Mr. Smoked) to kill the power because the first relay only selects direction and can't make the system stop.
RE: H-Bridge short protection
The basic problem is the two motors sharing one common. If you want to operate the motors in all four possible combinations:
fwd/fwd
fwd/rev
rev/fwd
rev/rev
you need either to wire four cables or two separate power supplies. Otherwise one of the combinations will lead to a short circuit regardless of the swicthing devices used.
RE: H-Bridge short protection
An electro-mechanical H built with a DPDT relay can't short the supply.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: H-Bridge short protection
Why would you want that? Even if you didn't blow up the fuse, powering all the relays would certainly reduce the life, if nothing else.
TTFN
RE: H-Bridge short protection
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"It's the questions that drive us"
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RE: H-Bridge short protection
TTFN
RE: H-Bridge short protection
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Sometimes I only open my mouth to swap feet...
RE: H-Bridge short protection
RE: H-Bridge short protection
That's a very valid point. If the motors share a common then a full bridge becomes pointless. A half bridge run from dual polarity supplies would allow bidirectional control. A capacitor centre tap for the half bridge would be viable in a single DC rail is available. The OP seems to have lost interest anyway.
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Sometimes I only open my mouth to swap feet...
RE: H-Bridge short protection
It's good to see my explanation was understood, that did not seem to happen in the thread I posted in NI's forums.
Since the two motors being switched always share a common and only one power supply is available I am considering either an output card that does not initialize all relays to closed or a relay to switch the power supply on/off.
RE: H-Bridge short protection
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: H-Bridge short protection
That does seem like the best approach, otherwise!
RE: H-Bridge short protection
use two isolated power supplies
RE: H-Bridge short protection
Similar to what was just posted by 'electricuwe', use a garden variety split (+/-) power supply with the common connected to the motor common. Then you can connect either or both motors to either polarity and run the motors any which way you want in any combination.
You'll need one SPDT switch for each motor.
RE: H-Bridge short protection
Thanks all.