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C47 · 29th April 2008, 10:41 AM

arrgghhh!!!

the legs work separately but when i put the whole thing it doesnt work....i think that the timer cannot drive all four...

mvs sarma · 29th April 2008, 10:50 AM

Quote:

Originally Posted by C47

arrgghhh!!!

the legs work separately but when i put the whole thing it doesnt work....i think that the timer cannot drive all four...

legs drive 2 transistors only at any time
now worry. please take care of base current needs by managing the series base resistor.
555 has adequate capacity.

ericgibbs · 29th April 2008, 11:59 AM

Quote:

Originally Posted by C47

arrgghhh!!!

the legs work separately but when i put the whole thing it doesnt work....i think that the timer cannot drive all four...

hi,
The circuit you have built, NOT simulated, does it have the 10K resistor as the load or the motor and is it the last circuit you posted.?

If its a motor, whats it voltage and current rating.? The 20K/10K base resistor look too higher a value.

The 555 should be able to drive that bridge with no problem.

mneary · 29th April 2008, 06:57 PM

At extremely low currents, emitter and collector of many BJTs are almost interchangeable. This is one of the traps of simulation.

Lesson: Simulate with your intended load!

C47 · 1st May 2008, 05:32 AM

well its done....a 555 timer with an inverting buffer driving four NPN H-Bridge...it didnt work the first time coz of the base resistances...and also becoz the inverting buffer had popped (the second time)...so my 12VDC power supply showed a short circuit

it works.... finally

thankyou very much everybody

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29th April 2008, 10:41 AM	#31 (permalink)
C47 New Member Join Date: Apr 2008 Posts: 18	arrgghhh!!! the legs work separately but when i put the whole thing it doesnt work....i think that the timer cannot drive all four...

29th April 2008, 06:57 PM	#34 (permalink)
mneary Experienced Member Join Date: Feb 2007 Location: California Posts: 1,256	At extremely low currents, emitter and collector of many BJTs are almost interchangeable. This is one of the traps of simulation. Lesson: Simulate with your intended load!

1st May 2008, 05:32 AM	#35 (permalink)
C47 New Member Join Date: Apr 2008 Posts: 18	well its done....a 555 timer with an inverting buffer driving four NPN H-Bridge...it didnt work the first time coz of the base resistances...and also becoz the inverting buffer had popped (the second time)...so my 12VDC power supply showed a short circuit it works.... finally thankyou very much everybody