SMPS no voltage past opto-isolators - Page 1
Forum section: electronics / repair

author: fullbridge
author_rank: Newbie
timestamp: November 26, 2021, 08:09:03 pm
content: I have a number of SMPS boards that should give me 12V and 24V on the output side but only give 12 and 1.2. I can't figure out what in hell is wrong with them. I measured all semiconductor components, capacitors, resistors and they are OK. I tried replacing the circled ones and it didn't make any difference.You can see 3 distinct sections, only 1 of them is working.The high voltage capacitors are OK.The fuses are not blown going into all of the 3 sections.The diodes are OK.I measured the electrolytic capacitors with an ESR meter, they are fine.The switching transistors are good.The resistors are good.I measured all of the small SMD components and they are OK as well.The opto-coupler on the working side has 1.2 volts on the input.The opto-couplers on the other 2 broken sides have 0.5 volts or something like that - the voltage is too low (at least from what my multimeter can tell).I tried replacing the small 8-pin chips with new ones but that didn't solve the problem.I tried taking an opto-coupler from another board but it didn't change the voltage reading on the input side either.Something appears to be broken between the switching transistors and the opto-coupler but I can't tell what. It isn't the small chip. All the SMD resistors read good.

author: fullbridge
author_rank: Newbie
timestamp: November 26, 2021, 08:17:47 pm
content: This is how one of this SMPS boards looks like from above

author: fullbridge
author_rank: Newbie
timestamp: November 26, 2021, 09:35:12 pm
content: There is no voltage on the output side of the transformers which leads me to believe the problem is with the oscilator/driver part of the circuit. There's no voltage on the opto-isolator to regulate the duty cycle because there's nothing going out in the first place. But is it not going out because there's no feedback?

author: timenutgoblin
author_rank: Regular Contributor
timestamp: November 27, 2021, 02:42:35 am
content: Quote from: fullbridge on November 26, 2021, 08:09:03 pmI don't think there is anything wrong with your SMPS. At the bottom-centre of the top-side of the PCB (second photo), there is a connector labelled 'CONTROL'. I think the connector requires feedback to enable the normal operation of the SMPS. 4 of the pins go to optocouplers (output side, i.e transistor/switch) and 3 of the pins go to zener diode circuits.Can you test the zener diodes? Which pin is anode and cathode? It looks like the top pin is the cathode.I think the reason that the +12V rail is working is because it doesn't require a trigger or switch to enable it.You could try using a pull-up (or pull-down) resistor (10k ohms) between the 3 connector pins and GND (or +12V). The resistor is for current limiting purposes.I noticed that some of the fuses are missing or are they not required?Without a full schematic it's very difficult to know what to do.

author: magic
author_rank: Super Contributor
timestamp: November 27, 2021, 06:54:12 am
content: Quote from: fullbridge on November 26, 2021, 09:35:12 pmThat's what it appears to be. The optocouplers are only turned on to reduce primary PWM duty cycle when the output reaches regulation and they should be off below regulation.What's the primary controller IC? You say you replaced one already so you should know.You say there is 1.2V on the secondary. Is it steady? Are there any peaks above 1.2V (needs a scope)? What if you load the secondary, will it still be there?If you have a scope, post the waveform on the secondary winding (before any diodes).(And don't connect it anywhere on the primary, as usual).

author: fullbridge
author_rank: Newbie
timestamp: November 27, 2021, 11:33:09 am
content: If I short the opto-couplers on the faulty side I get voltage on the output that fades. If I short the middle 'vtech+' opto-coupler output then the LED at the front lights up and I get 6volts on the output, then when I let go the voltage slowly fades.

author: fullbridge
author_rank: Newbie
timestamp: November 27, 2021, 11:53:36 am
content: Quote from: timenutgoblin on November 27, 2021, 02:42:35 amI think you're right, I'll test it with the 'control' cable plugged in.I tested all the diodes twice with an ohm meter.I put back the fuses after taking the photo, they are required for each of the 3 distinct strips to operate.The top IC is 3845B. The bottom IC, past the opto-coupler is 431AC.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: November 27, 2021, 12:42:54 pm
content: Quote from: fullbridge on November 27, 2021, 11:53:36 amThanks for reading the part numbers, they're difficult to read from the photo.The zener diodes that I was referring to are the sot-23 package devices that are located on the output of the SMPS on the back of the PCB. There are several of them.The +12V rail has a zener diode on the right hand edge of the output pad designated as Z101 connecting to resistor R119 and then connecting to light emitting diode (LED) D114 on the top side of the PCB. Some of the zener diodes are responsible for driving the status LEDs on the top-side of the PCB. There are other zener diodes which may serve a different purpose triggering/enabling switch circuits.