DPRG mailing list archives: [DPRG] Question about "A Simple PWM Circuit Based on the 555 Timer"

Fri Feb 1 12:21:49 CST 2008

Mario,

The selection of components here is not critical. The 10K resistor is used to pull up the gate of the N channel MOSFET because the 555 output is an open collector configuration. (It only pulls down.) The gate of the MOSFET needs 4 volts or so to overcome the threshold voltage. Since it is a MOSFET, the gate really does not draw current (except on power on, but the capacitance is very low and the switching speed is not critical.) Therefore there is no need for a lower value resistor. 10K and 4.7K are just standard values I typically use for pullups. 

You could also use a P-Channel MOSFET here, although they typically have a higher on resistance than N-Channels.

As far as the MOSFET selection goes, this is just one that I like. It has very low on resistance compared with others and favorable characteristics for this application. (Plus I have a big drawer of them.)

There are many sources on the internet about the 555 device. I like "The 555 cookbook" which I have in my collection...

Good Luck,
Rick J. Bickle 

________________________________

From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf Of Mario Marín Richmond
Sent: Monday, January 28, 2008 9:52 AM
To: dprglist at dprg.org
Subject: [DPRG] Question about "A Simple PWM Circuit Based on the 555 Timer"

Hi, my name is Mario and I'm a Power Systems Engineering Student from Costa Rica University. I was searching in the web for any simple project to execute for an electronics course and I found your article on the PWM for DC motors. I would like to know a couple of details about it, such as the values of the external components and some theorical doubts. Specifically, I would like to know why the circuit uses the 10K resistance and the IRFZ46N?? I understand that it must be a N-channel MOSFET, but I must justify why I (in this case you) picked this component, and if there is any other suitable element that can be used instead of it. About the resistance, which is its function on the PWModulator. I would also like to know if there is any bibliographical resource that may help me create a Theorical and Conceptual Base, explaining the main concepts involved in PWM. Thanks for publishing the project and thanks in advance for any help given.

PD: sorry if my english wasn't very well. 

Mario Marin Richmond
Universidad de Costa Rica.

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