Dual 555 Timer (with inverted signal) for H Bridge Problem

Wendy

Joined Mar 24, 2008
21,848
5V is low for a MOSFET. A regular 555 puts out 3.8V max (Darlington transistor high side drive) at 5VDC power supplies. Try a 7556 (CMOS) instead. That or use 12VDC power supply instead.
 

AnalogKid

Joined Aug 1, 2013
8,106
With Vcc = 5 V, the peak output of either 555 is only about 3.5 V. This is not enough to enhance a MOSFET fully. Also, it is not enough to guarantee turn-off of the p-channel FET. The bridge driver circuit works *only* unmodified. Adding external power transistors is more difficult than you think. If that is what you want to do we can start down that path. But even with logic-level MOSFETs, 5 V is not enough to run the circuit reliably.

ak

Wendy beat me by seconds ...
 

Wendy

Joined Mar 24, 2008
21,848
But the words, the pushed their way out of my finger. Rough being a fast touch typist then being forced to hunt and peck.
 

danadak

Joined Mar 10, 2018
3,577
555 timer not really a good gate driver for MOSFETs for quite a few reasons,
drive level current availability, voltage, control over dead band timing....

Also missing is small amount of R in series with MOSFET gate inputs to control
L effects, ringing, and all the issues associated with MOSFET being in active
region for excessive amount of time.

Some useful ref material -

http://www.ti.com/lit/ml/slua618a/slua618a.pdf

https://toshiba.semicon-storage.com/info/docget.jsp?did=59460

https://www.infineon.com/dgdl/an-937.pdf?fileId=5546d462533600a40153559ea1481181


Regards, Dana.
 

AnalogKid

Joined Aug 1, 2013
8,106
Without the additional transistors, it should work ok at higher operating voltages. There still might be a problem with the p-channel FET turning off completely.

The bipolar 555 is famous for a shoot-through current spike as the output transitions; another thing the CMOS version fixed.

ak
 

AnalogKid

Joined Aug 1, 2013
8,106
How can I make a potentiometer change the frequency while keeping the duty cycles at 50% for both pwm signals?
There is an alternate astable circuit that drives the timing resistor directly from the output. Varying that single resistor changes the freq while remaining 50/50. Actually, it's output duty cycle is around 52/48. For perfect 50/50, you have to run the 555 at twice the freq you want and run the output through a toggle flipflop (divide-by-two).

ak
 
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