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.
 
bananasiong said:
Oic.. i think the relay is more suitable for me. Is the speed controlled by 555 timer? How does it work?
Click to expand...

With the relay example you gave above, there is NO speed control, only directional control.
 
bananasiong said:
Yes, i know.. Then how to control speed if the bjt is used?
Click to expand...

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.
 
bananasiong said:
how to drive Vcc with transistor???
Click to expand...

With a BJT use a PNP one, emitter to Vcc and collector feeding the motor via the relay contacts, OR using an NPN in the ground side of the motor.
 
bananasiong said:
oh.. then the base to the microcontroller, then the speed is controlled by it. right?
Click to expand...

Yes, from the PWM output - you will probably require a driver transistor as well.
 
I am in really trouble can any one help me how to connect 555 timer to the H Bridge
to control speed ???????
 
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