H-Bridge Variation --Full Disconnect using Relay or Extra Transistors
July 2, 2011
This is a continuation or update of the earlier page on making an H-Bridge for motor control without allowing a full short of the power supply under certain conditions as with other H-Bridge drawings I see all over the web.
The original 4-transistor design was bread-boarded and tests showed it worked as planned.
The following designs have not been bread-boarded (as yet), but I believe they should also work as designed. (See truth table on second drawing.)
It was brought up that it might be usefull to have a condition where the motor can be fully disconnected from the rest of the circuitry for a motor 'coast' condition, so I came up with a couple possible ways this could be done without increasing the circuit cost by much.
The first example would use a double pole relay. (One of the small reed relay units would work fine.)
Second, I decided that 2 additional transistors could be introduced to disconnect the motor under the condition of both inputs low.