12V 10A CCTV camera system power supply pulsating

[stj]

Re: 12V 10A CCTV camera system power supply pulsating

does the input to the diode show AC voltage?

[R_J]

Re: 12V 10A CCTV camera system power supply pulsating

The feedback pin #2 should also be higher. Without the optocoupler this open circuit voltage should be about 6v due to an internal pull up resistor, so something is causing it to drop and the only component on that line is the optocoupler output.

[CapLeaker]

Re: 12V 10A CCTV camera system power supply pulsating

No, it won’t. You answered your own question. There is nothing on the feedback optocoupler. I think the PWM tries to start but it shuts down for some reason and goes into under voltage lockout and starts pulsating on that starter cap. You may see a quick blip on voltage on the output while you plug in the PSU, but that’s all you going to see.
If you have a CC CV bench power supply, back feed some voltage into this PSU secondary, with the mains unplugged.

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Will check everything mentionned and report back. Thank you gentlemen!

Capleaker: I do have a bench power supply, not sure where I would inject voltage however and how much 12V ? Presume I should be injecting it at the secondary output terminals, but what should I be looking for, amp draw?

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

does the input to the diode show AC voltage?

It actually does, pulsating from .6V to 2.3V AC ??

[R_J]

Re: 12V 10A CCTV camera system power supply pulsating

No, it won’t. You answered your own question. There is nothing on the feedback optocoupler. I think the PWM tries to start but it shuts down for some reason and goes into under voltage lockout and starts pulsating on that starter cap. You may see a quick blip on voltage on the output while you plug in the PSU, but that’s all you going to see.
If you have a CC CV bench power supply, back feed some voltage into this PSU secondary, with the mains unplugged.

Then how does the optocoupler control the ic? by magic?
Page 5, Vfb_open open loop voltage 6V.

OB2268/9 self adjusts the switching mode
according to the loading condition. At from no load
to light/medium load condition, the FB input drops
below burst mode threshold level (1.8V). Device
enters Burst Mode control.

[CapLeaker]

Re: 12V 10A CCTV camera system power supply pulsating

Then how does the optocoupler control the ic? by magic?
Page 5, Vfb_open open loop voltage 6V.

Sorry RJ, I should have quoted post #20. Wasn’t meant for you.
Your post is correct.

[R_J]

Re: 12V 10A CCTV camera system power supply pulsating

Not a problem, There is an equivalent ic, SG5841/J that shows the internal workings a bit better,

From Post #19

When you say C4 gets its initial charge via R10, you mean via pin 7 of the chip, because I don't see how C4 could get charged via R10 ?

If you look at the sg5841 internal (page3) you will see how pin 7 gets its startup voltage from pin3

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

So any ideas on what other testing I should do? Capleaker mentionned something about injecting voltage into the secondary, not sure what to check and at what levels of voltage should I inject?

[CapLeaker]

Re: 12V 10A CCTV camera system power supply pulsating

Just put rated secondary voltage about an amp and back feed it, see if it draws any current. Hopefully not. Then you can concentrate better on the other things. Like the primary, the feedback and the transformer. A missing voltage reference also could make your day sour fast same as a shorted snubber cap.

If I remember someone said something about a load on the output, but that won’t do anything since the darn thing won’t even start. I’d put my DMM into manual range and watch the secondary output while plugging the PSU in. See if you get a little blip in voltage or not.

By any chance did you verify all the resistors in the primary having rated values? I see some Chinese resistors in there and I am always sceptical on these, as they like to change values going higher in resistance. Even seen resistors change value until a PWM drops out. But they were measuring ok while the PSU was off.
Maybe measure the voltage on all pins of the PWM referencing to the primary bulk capacitors negative.

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Just put rated secondary voltage about an amp and back feed it, see if it draws any current. Hopefully not. Then you can concentrate better on the other things. Like the primary, the feedback and the transformer. A missing voltage reference also could make your day sour fast same as a shorted snubber cap.

If I remember someone said something about a load on the output, but that won't do anything since the darn thing won't even start. I'd put my DMM into manual range and watch the secondary output while plugging the PSU in. See if you get a little blip in voltage or not.

By any chance did you verify all the resistors in the primary having rated values? I see some Chinese resistors in there and I am always sceptical on these, as they like to change values going higher in resistance. Even seen resistors change value until a PWM drops out. But they were measuring ok while the PSU was off.
Maybe measure the voltage on all pins of the PWM referencing to the primary bulk capacitors negative.

Ok, so I put voltage and fed it through secondary at 1 amp, but no current draw, so I guess that's a good sign.

When I connect the psu to the main, I get a pulsating 4V - 6V DC on the output so it is giving some output but not constant - don't see it when I put a load on the output like a 12V lamp.

So I started to concentrate on the resistors, and especially R10. Very very weird, but maybe it's normal R10 is a 1.5Mohms resistor. When I measure it with probes on each side, I get a reading of 1.239Mohms. When I turn my probes around, believe it or not, I get no reading (see photo of my multimeter).?? What is going on?? Thank you gentlemen for all your help, learning a lot from this little experience!

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Ok, so I removed R10 and it measures 1.5Mohm both sides. On the board, if I put the black probe going to pin 3 from that resistor side and the red probe on the other side of where the resistor connects I get 435Kohm reading. When I switch the probes around, I get 10Mohms reading.

Not sure what that means.

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

By any chance did you verify all the resistors in the primary having rated values? I see some Chinese resistors in there and I am always sceptical on these, as they like to change values going higher in resistance. Even seen resistors change value until a PWM drops out. But they were measuring ok while the PSU was off.
Maybe measure the voltage on all pins of the PWM referencing to the primary bulk capacitors negative.

Checked all resistors of the primary section and they all correspond to their marking.

Voltages on the PWM relative to the negative of the bulk capacitors:

Pin 1: GND
Pin 2: .1V to .3V oscillating
Pin 3: 16V to 19V oscillating
Pin 4: 0V to 1.3V jumping around sometimes to 3.2V
Pin 5: same as pin 4
Pin 6: 0V, sometimes shows .1V or .2V then 0V
Pin 7: Oscillating between 11V and 15V
Pin 8: .001V

[lotas]

Re: 12V 10A CCTV camera system power supply pulsating

Try replacing TL431 with another one, check the trimming resistor.

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Try replacing TL431 with another one, check the trimming resistor.

Replaced TL431, no change. Checked the trimming resistor and I doesn't appear to be working, stuck on 13.32Kohms no matter which side I turn it...could that be the culprit?

[R_J]

Re: 12V 10A CCTV camera system power supply pulsating

Looks like it should be a 1K (102) so it might be the problem

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Should I try and jumper it with a 1k resistor?

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Ok, so jumpered a 1K resistor on the trimming resistor and still pulses, but with a higher output voltage i.e. was before hovering between 4-7V, is now pulsating between 8-10V. Wow, this one is a hard one!

[rddube]

Re: 12V 10A CCTV camera system power supply pulsating

Tried it with a 12V load, but no output. When I turn off the mains, the led still pulses for about 2-3 seconds after power off.

[R_J]

Re: 12V 10A CCTV camera system power supply pulsating

Ok, so jumpered a 1K resistor on the trimming resistor and still pulses, but with a higher output voltage i.e. was before hovering between 4-7V, is now pulsating between 8-10V. Wow, this one is a hard one!

This is a potentiometer, it will vary the resistance between 0Ω and 1,000Ω, If the pot is set to the middle, that would be aprox. 500Ω so try a 470Ω resistor

If the pot is open, the TL431 reference ic, would not have its voltage divider circuit complete, R19 is supplied by the output dc voltage, R18 is in series with the pot completing the circuit to ground, with no path to ground the reference would see the full dc voltage.

Something to remember when checking resistance in circuit, if there is any other voltage, even a volt or two in that circuit it will change the resistance reading on the meter, so if the output cap has any charge it could effect the reading across the pot. Same goes for checking the 1.5meg in the primary, if the main cap had a few volts of charge the reading would change because one end of the resistor is connected to the cap.