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H-bridge: BJT vs relays

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bananasiong

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Hi,
I need some comment for these circuit, their pros and cons.
Which one is better? which one drives faster and etc.. thanks
 

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Basically transistors allow you to add PWM speed control, relays don't. If you don't want speed control?, go with the relays - it's far simpler!.

You can also do a hybrid version, a single transistor for PWM speed control, and a single DPDT relay for forward/reverse.
 
Oic.. i think the relay is more suitable for me. Is the speed controlled by 555 timer? How does it work?
 
Nigel Goodwin said:
With the relay example you gave above, there is NO speed control, only directional control.
Yes, i know.. Then how to control speed if the bjt is used?
 
bananasiong said:
Yes, i know.. Then how to control speed if the bjt is used?

By using PWM on the inputs to the H-bridge, usually generated directly by the driving micro-controller - good reason for using a 16F876/7 as it has two hardware PWM channels.
 
The relay cannot be used to control the speed just because it switches not fast enough?

how if, the motor is drive too fast, i want to slow it down, by using a 555, the output of the 555 is connected to the Vcc of the H-bridge. Can this work?

Thanks
 
Yes, but NOT fed from a 555, you would need a high power transistor or FET to drive Vcc - and it's then fairly pointless using so many relays, a single DPDT one would do.
 
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