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PWM Control IC Problem (SG3525)

A

alphamnemonic

Jan 1, 1970
0
Hi Everybody

This chip normally puts out square pulses. In the power supply I am
working on (http://www.laserfaq.org/sam/150rcsch.pdf) it is putting
out a sawtooth wave. I am testing it in 'standby mode', in which Q4 is
off and R33 is in control of the duty cycle. Replacing the IC1 has no
effect. Does anyone have any ideas?
 
Hi Everybody

This chip normally puts out square pulses. In the power supply I am
working on (http://www.laserfaq.org/sam/150rcsch.pdf) it is putting
out a sawtooth wave. I am testing it in 'standby mode', in which Q4 is
off and R33 is in control of the duty cycle. Replacing the IC1 has no
effect. Does anyone have any ideas?

Where are you seeing sawtooths? Pin 13?

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header
above is
ignored unless my full name AND either lasers or electronics is
included in the
subject line. Or, you can contact me via the Feedback Form in the
FAQs.
 
A

alphamnemonic

Jan 1, 1970
0
[email protected] wrote in [email protected]:









That is my question. Why do you have output A and output B (pin 11 and 14)
grounded?

Also, pin 13 is VC in for the transistor collectors. Why do you have this
connected as an output?- Hide quoted text -

- Show quoted text -

Yes the sawtooth is on pin 13. Pin 13 is being used as an output and
pins 11 and 14 are grounded because the chip is being used in single-
ended supply mode (see http://www.datasheetcatalog.org/datasheet/motorola/SG3525AN.pdf
p.6 for an example of this configuration). I can see why you find that
strange--I did too!
 
A

alphamnemonic

Jan 1, 1970
0
I'd avoid the funky setup you have. In my opinion I'd connect pin 13 and 15
together and use either output A or B.

I'm not sure what Q4 is doing, I've never seen it used this way, but you
must know more than me since you designed this elaborte circuit.

Keep us posted at what you do to resolve the problem.- Hide quoted text -

- Show quoted text -

It's not my design. I am trying to repair a commercial unit. I will
keep you posted.
 
It's not my design. I am trying to repair a commercial unit. I will
keep you posted.

First, I've never scoped pin 13 so don't know for sure what should be
there.

But, my guess is that one of the 3 transistors is not behaving
properly.and you're
seeing one of the caps charging.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header
above is
ignored unless my full name AND either lasers or electronics is
included in the
subject line. Or, you can contact me via the Feedback Form in the
FAQs.
 
A

alphamnemonic

Jan 1, 1970
0
I have been looking at the circuit and the datasheet.

I'm willing to bet the schematic is drawn wrong. It doesn't make any
sense to have pin 13 driving the base of a transistor.- Hide quoted text -

- Show quoted text -

The schematic is drawn correctly. This mode of operation is described
on p.6 of the datasheet (link is given in my post from July 3). I
removed the board and re-soldered R1 and R2, and the supply is now
working properly again. A rather boring fix, but if it works long term
I won't complain... If anyone else is having trouble with this supply,
I can send you waveforms captured with FlukeView from the working
unit. Thanks for the suggestions everyone.
 
A

alphamnemonic

Jan 1, 1970
0
I'd like to see them. Do you have them in a format you can easily email?-Hide quoted text -

- Show quoted text -

Yes: .rtf
 
A

alphamnemonic

Jan 1, 1970
0
I phrased that incorrectly.

I meant can you post them on here for downloading or on a site?

Thanks- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

I don't have a website, but I will post them here if you tell me how.
 
A

alphamnemonic

Jan 1, 1970
0
   This isn't a binary newsgroup, and you can't post pictures through
Google Groups.

   Sign up for a free photo hosting service and post the link to the
pictures.

--http://improve-usenet.org/index.html

If you have broadband, your ISP may have a NNTP news server included in
your account:http://www.usenettools.net/ISP.htm

Sporadic E is the Earth's aluminum foil beanie for the 'global warming'
sheep.

The pics are up at http://sites.google.com/site/scientificelectronicsrepair/
 
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