H-bridge: BJT vs relays - Electronic Circuits Projects Diagrams Free

bananasiong · 18th June 2006, 09:52 AM

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

**Nigel Goodwin** · 18th June 2006, 09:58 AM

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 · 18th June 2006, 04:28 PM

Oic.. i think the relay is more suitable for me. Is the speed controlled by 555 timer? How does it work?

**Nigel Goodwin** · 18th June 2006, 04:54 PM

Quote:

Originally Posted by bananasiong

Oic.. i think the relay is more suitable for me. Is the speed controlled by 555 timer? How does it work?

With the relay example you gave above, there is NO speed control, only directional control.

bananasiong · 18th June 2006, 05:00 PM

Quote:

Originally Posted by Nigel Goodwin

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?

**Nigel Goodwin** · 18th June 2006, 05:14 PM

Quote:

Originally Posted by bananasiong

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.

bananasiong · 18th June 2006, 05:18 PM

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

**Nigel Goodwin** · 18th June 2006, 05:30 PM

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 · 18th June 2006, 05:39 PM

Quote:

you would need a high power transistor or FET to drive Vcc

how to drive Vcc with transistor???

**Nigel Goodwin** · 18th June 2006, 05:47 PM

Quote:

Originally Posted by bananasiong

how to drive Vcc with transistor???

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 · 18th June 2006, 05:55 PM

oh.. then the base to the microcontroller, then the speed is controlled by it. right?

**Nigel Goodwin** · 18th June 2006, 05:57 PM

Quote:

Originally Posted by bananasiong

oh.. then the base to the microcontroller, then the speed is controlled by it. right?

Yes, from the PWM output - you will probably require a driver transistor as well.

eagle1009 · 6th May 2009, 06:34 AM

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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18th June 2006, 09:52 AM	#1
bananasiong	H-bridge: BJT vs relays Hi, I need some comment for these circuit, their pros and cons. Which one is better? which one drives faster and etc.. thanks Attached Thumbnails

18th June 2006, 09:58 AM	#2
Nigel Goodwin	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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

18th June 2006, 04:28 PM	#3
bananasiong	Oic.. i think the relay is more suitable for me. Is the speed controlled by 555 timer? How does it work?

18th June 2006, 05:18 PM	#7
bananasiong	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

18th June 2006, 05:30 PM	#8
Nigel Goodwin	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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

18th June 2006, 05:55 PM	#11
bananasiong	oh.. then the base to the microcontroller, then the speed is controlled by it. right?

6th May 2009, 06:34 AM	#13
eagle1009	I am in really trouble can any one help me how to connect 555 timer to the H Bridge to control speed ???????