Help in identifying an optocoupler

[imp]

Hi,

I´m currently repairing a monitor for a colleague and what I thought was an easy job became a bit more difficult ... She bescribed the "usual symptoms" - the screen goes blank a few seconds, then restarts, flickers in back light on/off, well you know what I mean

It´s a Viewsonic VP191s monitor from 2003 I think.

After replacing all the capacitors (none bulging or leaking) I thought I was done, but no ... The startup problem is still there! After some research I discovered that the power supply wasn´t delivering the 12V required but if I waited for like 30 seconds it did. All the capacitors on the power supply (except the SMD) are replaced with Rubycon ZLJ, one Suncon WX and one Rubycon KXW (primary cap). The capacitor for the PWM is probably a SMD since the whole chip is and there´s no obvious "47 uF 50V" on the board.

To make a long story short:

I believe the problem is the optocoupler for the feedback circuit on the power supply and would like to at least try to replace that. The problem is that I can find ANY usable information what to replace with ...... Sorry for not having any pictures (the unit´s at work) but nevertheless I can read out it´s a L-4050 optocoupler.

The power supply is manufactured by Tiger Power and the model no. is TOF-5001 Rev 0E, part no. 44.61103.002

Perhaps there´s some generic part that will do? It´s a 4-pin DIP circuit.

Thanks!

Edit: mistype

[Th3_uN1Qu3]

Re: Help in identifying an optocoupler

Optos used in power supplies are pretty standard parts, an EL817 should do. You can find those basically anywhere.

[imp]

Re: Help in identifying an optocoupler

Thanks for the tip! Trial and error is allowed in this case, but it would be fun to get it running again

I must say that you are one of the most respected persons for me on this forum, I read a lot and I ALWAYS learn something from your posts! It´s incredible to see how helpful and service minded some people are. Keep it up!

[PlainBill]

Re: Help in identifying an optocoupler

Before replacing the optocoupler there are a few things that would be interesting to check.

1. Does the 12V supply ramp up slowly or abruptly transition from some low value to 12 volts?

2. What is the voltage across the input to the optocoupler as this happens?

3. What is the voltage on the REF input of the TL431?

PlainBill

[imp]

Re: Help in identifying an optocoupler

Plainbill,

the output is zero to begin with and momentarily rises to 12V after 10-30 seconds. There´s also no problem to put some load on the device, I tried 3A with success. Heating or cooling the pcb and the circuits make no difference, neither knocking on it with a screwdriver (no loose solder point).

I´ll measure the input to the optocoupler when I´m back at work on Tuesday.

Where can I find the TL431? Is this the PWM-chip perhaps?

[PlainBill]

Re: Help in identifying an optocoupler

Plainbill,

the output is zero to begin with and momentarily rises to 12V after 10-30 seconds. There´s also no problem to put some load on the device, I tried 3A with success. Heating or cooling the pcb and the circuits make no difference, neither knocking on it with a screwdriver (no loose solder point).

I´ll measure the input to the optocoupler when I´m back at work on Tuesday.

Where can I find the TL431? Is this the PWM-chip perhaps?

The TL431 is an "Adjustable Programmable Shunt Regulator". It is commonly used as part of the voltage regulator circuitry. If you look at the board, it is a three lead device close to the optocoupler.

While it's got a long name, and is fairly complex, it's operation is quite simple. It does not conduct current until the voltage at the reference input exceeds 1.25 volts. At that point it conducts much like a zener diode.

Referencing the attached drawing, the unlabeled resistor at the top is an optocoupler led and resistor. When current flows through the LED, the output of the optocoupler conducts, reducing the output of the SMPS controller.

PlainBill

[imp]

Re: Help in identifying an optocoupler

PlainBill,

I think I know which one you mean. I haven´t measured anything yet though, but this time I can supply some photos.

Is the TL431 situated on any of the attached pictures? There´s a 3 pin chip on both sides near the optocoupler, the hole mounted one is difficult to spot/identify. There´s also a TL431 near the output.

Perhaps you can do some marking in any of the photos? I´ve also attached a photo of the PWM-chip.


//Mikael

[PlainBill]

Re: Help in identifying an optocoupler

PlainBill,

I think I know which one you mean. I haven´t measured anything yet though, but this time I can supply some photos.

Is the TL431 situated on any of the attached pictures? There´s a 3 pin chip on both sides near the optocoupler, the hole mounted one is difficult to spot/identify. There´s also a TL431 near the output.

Perhaps you can do some marking in any of the photos? I´ve also attached a photo of the PWM-chip.


//Mikael

It's located on the top side of the board shown in the lower right corner of the third picture. Note that there is a circuit running from one side of the opto-isolator, through R28, and to one pin of the TL431 and C10. R05, R26, and R27 form the voltage divider for the reference input.

PlainBill

[imp]

Re: Help in identifying an optocoupler

PlainBill,

sorry for being so slow in replying ... I actually replaced the TL431 near the output (since I was ordering other stuff from my distributor), but no difference in operation, the PSU is still slow-started. Now I´m more into that a tiny capacitor (0,5 nF) which I believe is the startup-cap for the PWM-circuit, may be the problem. However, all the datasheet for the PWM-circuit that I´ve found has a different pin configuration than the circuit on my PCB, so I decided not to care about this anymore .... I simply replaced the internal PSU with a external 12VDC 5A one, works perfectly! I was lucky that the monitor only needs 12V from the PSU, otherwise it would have been more difficult.

Thanks for your help and time anyway!