h bridge current sensing

[aamir1]

hi guyz.........im using a relay based h bridge for wheel chair controlling i need to know how i can sense the current through the motor for overload conditions when i need to trip the circuit via microcontroller signal. ive also attached the circuit for your reference kindly help me

 

[dougy83]

If you're using SPDT/SPCO relays as you've shown in your schematic, you only need two relays to control the direction of the motor. To measure the current, you can put a sense resistor in the return path (the path to 0V) from the relays, and measure the voltage drop across the resistor to indicate the current. You may also wish to add some form of snubbing network or clamping diodes to reduce noise and spikes from the motor.

 

[audioguru]

You are using relays to apply full power to the motor.
Then how do you control the speed and steering of the wheel chair?

An electric motor draws maximum current when it starts running so if it trips the overload circuit then it will never start running.
The wheel chair motor will also draw a high current when it drives uphill. If it trips overload then it will stop.

 

[MrAl]

Hi,

Clamping spikes in a system that has to change polarity can be a little tricky, but i think you can handle this as if you had transistors instead of relays. Simply place a diode across each relay so it is normally reverse biased, then work at clamping the main DC supply line to some max value or just add more capacitors. That would make it very similar to a regular transistor bridge with reverse diodes. The diodes force slower spikes to dissipate back into the power supply capacitors. Faster spikes need local snubbing but with relays you may not need that...depends on how the relay contacts can put up with those very fast spikes over time.

 

[aamir1]

audio guru i will control the speed by changing voltage level of the motor through another relay and what should i do to do current sensing as u said that motor will never start because it takes a lot of current should i start sensing after the motor comes in running state ? and one more thing if i use a sense resistor in the return path which value will be suitable ?