trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Is there any way I can check that there really is a voltage between source and drain, in circuit, with power on?

You did measurement with multimeter before:
Source to Drain

Voltage = 335 V DC.
Voltage = 0 V AC.

Are you expect some other reading? Is mosfet tested good?

I have purchased a Vellerman HSP5 handheld oscilloscope, and it has a bandwidth of 1MHz and max voltage of ~ 600V, so that I don't have problems with grounding my CRT analogue oscilloscope.

Yeah, this will be gamechanger...

It is nice have a different equipment, but oscilloscopes are usualy used to see how stuff works, and not how to not work in lack of power.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I mean voltage going through the transistor source to gate, not parallel with it (possibly only going through the multimeter).

[CapLeaker]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Sounds like the mosfet doesn’t wanna switch. You got power full bore after the bridge rectifier into the main filter cap. So you can have few problems. R1 open (or going higher in resistance than it should), the cap failed, diode shorted, or U1 is buggerd up. Make sure you get proper Vcc to pin 5. Look at the spec pdf what it needs. And measure DC voltage between pin 5 and main filter capacitor ground for proper voltage. If you deterrent that the voltage is too low at the chip, replace the resistor, check that diode. Still doesn’t work, repale u

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

OK. We are heading towards trying to verify if U1 is broken. By measuring VDD to ground of U1. I don't want to desolder U1 to test resistance to avoid damage.

I have performed some more tests.

Theory

1. If Q1 (mosfet) is not working, it won't switch on the source drain voltage and no current will flow to the primary coil. If it is working it will switch on current to the primary coil.

2. If I bridge Q1 source to drain with a wire, Q1 will be permanently switched on source to drain.

3. If I apply 3 V battery power to Q1 gate to drain it will switch on source to drain and conduct current source to drain.

4. With a current limiter bulb connected in series with this adaptor, the bulb will light when current is drawn, and the bulb will not light when current is not drawn.

5. Previously U1 gate to Q1 gate voltage measures -0.1 V to 0.9 V changing rapidly. No voltage measured at the primary coil.


I bridged source drain of Q1 and measured voltage at primary coil


Result

Primary coil voltage
1.1 V DC.
0.6 V AC.
The current limiter light bulb goes on.


I removed the bridge on Q1 source to drain and put 3 V battery power to Q1 gate to drain (to switch on Q1 mosfet)

Result

Primary coil voltage
1.1 V DC.
0.6 V AC.

The current limiter bulb lights up.


I removed the 3 V battery power to Q1 gate to drain

Result

Primary coil voltage
0 V DC.
0 V AC.
current limiter bulb goes out.


Summary

With or without Q1 source to drain bridged, applying 3 V battery power gate to drain switches on Q1 and causes the current limiter light bulb to light up and the primary coil to measure a voltage. Removing the 3 V battery from Q1 gate to drain causes the light bulb to go out and the primary coil measures 0 voltage.


Conclusion

It seems that it is possible (but not certain) that the pulsing voltage from U1 gate to Q1 gate measured previously ~ 0 to 0.9 V DC, is not enough to switch on Q1 or there is a problem with the feedback or current sensing so that U1 won't switch on Q1. But then why any pulse?


Discussion

Do you think these tests show that the Q1 mosfet works?
Can we discount Q1 as the cause of no voltage at the output?
Even though there is voltage to the primary coil, it is very low, and I wonder if everything else (especially U1) were working properly and a changing voltage being produced, would that be enough to produce 6 V DC at the output. Or maybe the voltage is low because there is no load on the output.

Could U1 being broken account for the low voltage AND Q1 not switching on and then off?

Any advice is appreciated.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

just measure ohms U1 VDD to ground .

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

U1 VDD to U1 ground resistance measures 61.9 ohms.

Incidentally U1 VDD to the near side of D4 measures 62.5 ohms and continuity.

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

2. If I bridge Q1 source to drain with a wire, Q1 will be permanently switched on source to drain.
...
I bridged source drain of Q1 and measured voltage at primary coil

You are pushing your luck, even with light bulb current limiter (60w of power!) you can damaging various component on short circuit pathway! You are efficiently connected +335v with ground throu some non relevant resistance...

4. With a current limiter bulb connected in series with this adaptor, the bulb will light when current is drawn, and the bulb will not light when current is not drawn.

The bulb will light when excess of current is drawn, so it prevent some nasty things can happen if you accidently touch wrong place. Light bulb is not miracle ultimate protector for everything bridgeing on your mind, you can damage all undersized components along it.

But now, after testing it seems the mosfet is ok...
Mosfet in this configuration is not intended to be permanently switched on, he must pulsing very fast.


Look this video, its explain how your device work, it is almost identical, but mosfet is in same package with ic...

https://www.youtube.com/watch?v=lM_JZ6_YELY
This SMPS Power Supply Has No OptoIsolator or TL431. How Does It Work? Switch Mode Repair

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Thank you.

You are correct it was a little bit of a risk.
I did check min and max input voltage of Q1 and it's 2 V to 4 V, so 3 V should be ok.
Also, I checked that, when Q1 is powered up, source to drain is supposed to be almost no resistance, so bridging is similar to no resistance.
I checked the junctions of Q1 after and they are fine with no short circuits.

I used wires, croc clips, wore an insulated jacket, and heavy duty rubber gloves, and wasn't anywhere near the circuit when powered up.

If there were anything else wrong with the circuit, and I powered it up, then as you say, smaller components might have been fried. I am happy that I have managed to get some power to the primary coil, and have proven the open circuit to that coil is the mosfet, and the cause of that may be U1 not providing enough voltage at the gate.
C3 is new and so are all of the other capacitors, since I replaced all of the electrolytics. I have not found a single diode shorted or conducting reverse biased, or oc sc resistors, so U1 is looking like the most likely broken part.
I have not checked the sensing circuit or the feedback. I don't know much about PWM but I am guessing a broken sensing circuit or feedback circuit might cause U1 to not provide enough voltage at Q1 gate. That seems like the only other possible cause and I am not betting on that much since all those components are smd capacitors and resistors, and a SMD rectifier bridge (in one package). I have checked all of the resistors and no oc or sc. I doubt the smd cap (grey one) is sc oc but could bridge that too.

So, really, the SMD bridge rectifier (RS1M) is the only other realistic possible cause. Correction it's a rectifier.


I would like to know how to deduce if U1 were broken had I not thought of this test though. Pete's conclusions did not come with much explanation.

RS1M has a cathode mark so should test like any other diode.


Testing D8 (RS1M)

Testing red probe to cathode = 0.697 V, black to cathode 0.064 V but there are other paths to the circuit.
both sides of this diode measure continuity to wires 1,2,3, and 5 of U1 and continuity from each side of the diode, but there are other paths.
I think this test shows that RS1M (D8) works fine.

That leaves U1 as the broken component, I think.

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

yes U1 isnt looking good

[petehall347]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

check out this voltage divider circuit .you will soon see why the voltage is low at the ic . input 2.4 meg . and 62 ohms . and 335 volts
https://ohmslawcalculator.com/voltag...der-calculator

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Result Power On Reading U1 wire 1 to wire 5

Voltage ~1.29 V DC, (data reads it should be 9.5 V to 15V)
Voltage = 0 V AC.

(with and without bridging R2)

Test 2 wire 1 to wire 2. That is U1 ground to U1 gate.

Voltage = 0.4V DC to -0.1V DC, (changing quickly from one value to another).
AC = -0.4V (ripple?)

...

Testing D8 (RS1M)

Testing red probe to cathode = 0.697 V, black to cathode 0.064 V but there are other paths to the circuit.
both sides of this diode measure continuity to wires 1,2,3, and 5 of U1 and continuity from each side of the diode, but there are other paths.
I think this test shows that RS1M (D8) works fine.

So you are bringing conclusion via elimination proces... I think.
At least you can check and confirm where is this component with 62 ohm resistance. When I see the pulsating output, I suspect for external short and IC is fighting against them... but may be intenal also. 62R is unusual value for short circuit.

Be sure to unplug from mains voltage, and put 12Vdc directly on IC** and you may notice a hotspot, around 2.4W disipation of heat on a problematic component. You can spot it via finger tip or with spilled alcohol who evaporated quickly on that problematic component.

**it will be idealy that you can control and monitor passing current, you can use 12vdc from any wall adapter rated over 300mA, and monitor current with multimeter.

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

check out this voltage divider circuit .you will soon see why the voltage is low at the ic . input 2.4 meg . and 62 ohms . and 335 volts
https://ohmslawcalculator.com/voltag...der-calculator

Thank you. So, the then R1 and U1 wires 1 and 5 are acting as a voltage divider. R1 is 2.4 MR and U1 is 62 R so it takes less voltage. But it should take more if it were working I think.
Thanks for the link.

I'm glad that I found which componet was broken. The purpose of this was to learn about basic electronics fault finding, and I think I learned some theory and practical.

[Tarot Superstars]

Re: Learning to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

So you are bringing conclusion via elimination proces... I think.
At least you can check and confirm where is this component with 62 ohm resistance. When I see the pulsating output, I suspect for external short and IC is fighting against them... but may be intenal also. 62R is unusual value for short circuit.

Be sure to unplug from mains voltage, and put 12Vdc directly on IC** and you may notice a hotspot, around 2.4W disipation of heat on a problematic component. You can spot it via finger tip or with spilled alcohol who evaporated quickly on that problematic component.

**it will be idealy that you can control and monitor passing current, you can use 12vdc from any wall adapter rated over 300mA, and monitor current with multimeter.

Thank you.

The advice on using a 12V power supply is very helpful since that would make it safer. I have a laser pointer thermometer and protective goggles, and the thermometer that measures up to around 400 C, and can measure the temp on U1 when I get it wired up.

I have a universal 12 V supply but I am afraid if I connect this to U1 it will break. Are you sure I can connect this adaptor to U1 VDD and ground and not break it? I might prefer 1 or 2 last mains powered measurements rather than risk breaking my power supply.

I would like to get a temperature reading just to confirm the short in U1. Now I know how to switch on Q1 I think U1 overheats and will show.

About other components. I have tested all the other components so it must be U1 internal. I am so much convinced U1 is broken I am no longer trying to find another cause. All the diodes and resistors that feed U1 are working so it has to be U1, since Q1 was proven to work with the proper gate voltage.

I might look at reading a current too. What sort of amps should I be looking for?

I am thinking of changing the title of this thread to "Learning to fix..." since I don't think I want to spend the money to replace U1. Anyone know how to change the title?

Edit: I got the handheld oscilloscope so I will probably take a frequency measurement on the gate.

[harp]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

I have a universal 12 V supply but I am afraid if I connect this to U1 it will break. Are you sure I can connect this adaptor to U1 VDD and ground and not break it? I might prefer 1 or 2 last mains powered measurements rather than risk breaking my power supply.

From theoreticaly calculations I am prety sure, but in practice things may go wrong. Say, with good electrical component, are you sure that do not fail right now? You may guess and hope to they not fail, but he fail at one moment. With broken component who has 62 ohms (some inside carbonised burns), it is unpredictable if he go next second on 500kiloohm, or on 0.01ohm.
Your adapter also may be good designed (with protection) or crapy, with proper or "only working yet" components...
I suggest if it is only adapter you have, and it is so valuable, that you dont using it on irelevant purpose like this.

I using small adapter from some router, 12v 1.2A with chinese module who limit current and can lowering voltage. Nothing fancy. If it stop working for any reason I do not drop any tears...

This tips for connecting a limited current to circuit with low resistance is common method to finding short component. The set voltage must not exceed maximum ratings of any component connected on that rail, lower voltage can be obtained if is left enough power to disipate heat.

I might look at reading a current too. What sort of amps should I be looking for?

So, it is about 60R, and with applaying 12v into it, it will drow about 200mA, but if it unexpectedly shorts it can pull out maximum current of your supply. Because, it is always good idea to have some current limiter circuit while testing, and a very bad idea is to use car battery directly, hopeing to circuit stay consuming only 2.4W...

[Tarot Superstars]

Re: trying to fix a 6V 0.5A T1 (Transformer) AD050120T2 power supply

Hi.

I intended to do some temperature and frequency measurements.
The circuit had some more damage to traces and I attempted to repair them.
However, when i powered on the circuit for the second set of measurements, there was a short, open circuit, or other issue.
R0, R8, and Q1 were damaged. R0 is open circuit, R8 is open circuit, and Q1 is short circuit.

On the first set of measurements, there was no frequency measured (possibly because of an open circuit trace).
On the second there was the issue I mentioned, so there were no readings for temperature or anything else.

So, that's the end of this project. I was sure U1 was the issue after F1 and a bulged capacitor.
Now there are some more broken components.
I have learned a lot about fault finding and that was the purpose of this project.

Thanks for advice.