Simple PWM Generator

[epineh]

I would like to know if anybody has (or can whip up) a simple schematic for a PWM generator using common components.

I want to use it to test my H-Bridges with servo's connected before I plug the rest of the system in "just in case"

I need to supply PWM and NOT PWM to the driver and I am using 5V logic.

Any help would be appreciated.

Russell.

[bborb]

This has been discussed extensively in the sci.electronics.basics newsgroup (usenet). Many use the famous 555 chip. The Velleman kit uses an SG3525 chip.
Googling either of these chips with PWM should get you started.

[epineh]

Forgot to mention 20kHz would be about right

[epineh]

That was my first port of call, but haven't come up with anything as yet, I was thinking that someone must have done this with a 555.

I will keep looking.

[epineh]

So this is what I have got so far, I can use the 555 in Astable operation, I have drawn up a quick schematic, all I then need to do is take the output through something like a 74hc14 to invert the output to give me the NOT PWM signal...

Period is 0.693(R1+2R2)C
So freq would be 1/Period

By changing R1 and R2 to a pot I should be able to change the duty.

Am I close or way off ?

[Mcgyver]

using a single 555, the frequency and duration will both change... must pwm 555 circuits you use two 555 together (a 556 is two 555 on one chip), one in astable, one in monostable. the astable sets the frequency of the pwm, the mono the % of the pulse that is on....use that pulse to turn a fet on and off. you only need a pot on R1 for the monostable (you're adjusting the time the pulse is on, not the frequency). can't seem to find the web page that taught me all this, but as suggested search pwm and 555 and you'll get lots.

one of the guys at homeshop machinist came up with this simple circuit and excellent tutorial - should get you started. voltages of the fet etc may need adjustment for your app
http://vts.bc.ca/workshop/pwm/pwm.htm

[2muchstuff]

Here is a link to a 556 PWM. http://www.m.case.btinternet.co.uk/h...enerators.html

[epineh]

Cheers guys, that is the ticket !

I like both sites, either will work for what I want, just have to rummage through the electronics pile to find which components I have and choose one to build.

Cheers.

Russell.

[NC Cams]

For a little bit more than the cost of a 555, you can buy an off the shelf PWM IC.

By using a dedicated power supply control IC, you can drive fets or fet drivers at duty cycles that are very, very low to nearly 99% while also adding shut down. over current and some other housekeeping functions via built in error amps and the like.

Look into the SG3524, SG3525 and SG3526. EIther these or their complements are available in PWM positive or negative logic. These IC's may be available with an LM prefix from National or some other industry number.

The LM3526 is one that I have a lot of experience with and it is somewhat easier to use over a wide frequency range than a 555 - the use of a fancier IC is also much harder to reverse engineer in a circuit if you scrub the numbers off of the IC when/if you ship the device.

The above IC"s have some well documented application notes publishes with/for them....

[epineh]

Hey Dennis, thanks for the tip, I was going to use components I had lying around, but at $1.00 Au for the SG3524 it seems a lot simpler to buy a dedicated chip.

The LM3526 came up as a Dual Port USB power switch, is that the right number ?

Cheers.

Russell.

[NC Cams]

The 3524/25's are stripped down versions of the 3526 - all are voltage controlled PWM's.

You set up a reference voltage and then compare it to/with the output voltage you create/integrate with the output side. The error amp keeps the output voltage constant by varying duty cycle - frequency stays fixed. You can daisy chain most of them and even do pulse by pulse current limiting and/or even soft starts.

I recall the 24/25 as being simple O/C outputs while the 26 is dual totem pole version. You can AND/OR the outputs as appropriate.

[OzarkCNC]

Hey Dennis, thanks for the tip, I was going to use components I had lying around, but at $1.00 Au for the SG3524 it seems a lot simpler to buy a dedicated chip.

The LM3526 came up as a Dual Port USB power switch, is that the right number ?

Cheers.

Russell.

Try SG3526 or UC3526 or IP3526 or LT3526

or go here http://www.alldatasheet.com/view.jsp?Searchword=3526

Since I found that site, I don't miss my TTL data book so much.