Whats wrong with my Transformer? - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 04:01:00 am
content: I have an In-Lite transformer (220VAC-12VDC) that only puts out 4.4V without load. When i add wery small load, the voltage drops. An 3V LED diode will almost not light up at all.When i meassure the resistance of the black and white wires on the primary side i ger 42ohm. Is that normal?I get 12.9VAC to the PCB, but only 4.4VDC out. I measure 12.9VAC all the way to the green varistor (look at image under). After varistor i do not get anything.Some ideas whats wrong here?

author: vu2nan
author_rank: Contributor
timestamp: September 29, 2021, 04:44:43 am
content: Across which points are you measuring the DC voltage?

author: james_s
author_rank: Super Contributor
timestamp: September 29, 2021, 04:47:28 am
content: Is that varistor in series with the output? If so that may have just failed.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 04:47:39 am
content: between black and white wires on the third picture to the right. That is the wires that goes to the DC jack output that should give 12VDC.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 04:49:03 am
content: Quote from: james_s on September 29, 2021, 04:47:28 amYes, it is in series.But why do i get 4.4VDC out if the varistor is failed?

author: james_s
author_rank: Super Contributor
timestamp: September 29, 2021, 04:55:33 am
content: It sounds like it has gone way up in resistance, so it's forming a voltage divider with the input impedance of your multimeter.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 04:57:28 am
content: Quote from: james_s on September 29, 2021, 04:55:33 amOkay, can you please explain what you mean in the simpliest way?

author: james_s
author_rank: Super Contributor
timestamp: September 29, 2021, 05:01:29 am
content: https://en.wikipedia.org/wiki/Ohm's_law

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 05:03:22 am
content: Quote from: james_s on September 29, 2021, 05:01:29 amI mean why has it gone up in resistance?

author: vu2nan
author_rank: Contributor
timestamp: September 29, 2021, 05:07:32 am
content: The DC voltage is to be measured across the electrolytic capacitor.The varistor, intended for over-voltage protection, would be across a power source and not in series with it.

author: james_s
author_rank: Super Contributor
timestamp: September 29, 2021, 05:09:09 am
content: I assumed he meant it was a PTC or polyfuse for over-current protection, which would be in series with the output. Drawing a schematic of the circuit might help.

author: bdunham7
author_rank: Super Contributor
timestamp: September 29, 2021, 05:25:04 am
content: That is a 27 volt MOV, which seems a bit unusual. OP--please try to trace out the circuit or at least post good photos of the entire top and bottom of the circuit board. And when you say you are measuring voltages, please tell us where both leads of the multimeter are. Saying you have voltage 'to' the MOV but not 'after' actually doesn't mean anything.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 05:26:23 am
content: Thanks. i will try to do that later today

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 29, 2021, 06:45:53 am
content: Quote from: ElectricPower on September 29, 2021, 04:01:00 amthat 'varistor' is a MOV connected in parralel with your AC voltage to protect from spikes bigger than 17Vac, in normal condition he is 'open' from the multimeter point of vue.it's clear you have the correct AC input in that voltage stabiliser, not correct voltage output.what's fried? maybe some rectifier diode, maybe the ic regulator, maybe something else, without some details we cannot guess what.trace that AC to DC rectify first by measuring DC voltage on first electrolytic cap, then move forward if the DC is at least 13Vtracing the schematic is simple for your pcbbtw, after what i see on that lab table(the test gear), i'm surprised you ask such simple questions

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 29, 2021, 07:00:53 am
content: As i said, i will post some better pictures later to day

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 12:23:00 am
content: Here is ome better pictures of front side and back side of the board.The black cable that goes in to the board top left in the first picture (top right on the backside picture) and should be 12VDC output, but meassures 4.4VDC.I messure 13.4VAC between secondary input from transformer and the left leg of the green varistor on the first image.I have meassured all the four rectifier diodes and shows 0.54V in diode mode on the multimeter.I meassures 15.4VDC when i probe each leg of the green capacitor to the left under the four rectifier diodes.When i turn the unit on and set the wheel to ON posistion i can hear the relay clicks, so i think that is working...

author: timenutgoblin
author_rank: Regular Contributor
timestamp: September 30, 2021, 02:06:30 am
content: Try reflowing the solder joints of Q1. Also, what is SR+ and SR- ? Is that a sensor for ambient light? Maybe you need to test this garden light power supply in a dark room to test it properly.I can see a blue wire OUT1 connecting to the relay. Where does the other blue wire go to? Have you tested the screw block terminal?That SMART device might be a PLA (I think).

author: james_s
author_rank: Super Contributor
timestamp: September 30, 2021, 02:43:25 am
content: I would check the voltage across the relay contacts with the relay active and a load on the output. It's possible the relay contacts have failed. I'm not positive but it looks like that green capacitor is actually a snubber across the relay so if you're seeing the same voltage (relative to what?) on both pins that suggests the relay is ok.Something I find useful for this sort of thing is print out the picture of the back of the PCB and use colored pens to trace the voltage from the input working toward the output. If the relay clicks that tells you that the circuitry driving it is at least doing something so your problem is somewhere between the input, through the relay to the output. I expect the rectifier is only used for powering the controller, if this is a low voltage lighting transformer the output will typically be straight AC from the transformer.

author: radiolistener
author_rank: Super Contributor
timestamp: September 30, 2021, 03:16:54 am
content: are you sure that your circuit is working ok and don't consume too much current?Try to disconnect your circuit and attach some powerful resisistor instead to simulate the required load. Does voltage still drops?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 03:36:47 am
content: Quote from: james_s on September 30, 2021, 02:43:25 amThat is just spot on!Just forget the rubish i posted over. I have missunderstood the transformer unit completely, and in a very stupid way. I said i meassured 4.4VDC at the output, but that is not an output, but an sensor input.I can measure 12.8VAC out of the transformer and if i change mode to "detection" and add an movement detector to the box, i can hear the relay clicks when it is movement and voltage to the output turns on/off. When i set it to "ON" i can meassure 12.8VAC all the time.But when i try to light up an 12V 0.95W LED, the led lights up, but the voltage drops to 7-8VAC. This is my friends transformer and he said that the spot lights in his garage does not light up at all. I think it's because he have 3x 2W spot lights that will create an big voltage drop. The max limit for the unit is 56W.So the question (i think) is why the voltage drops with just a tiny load. Transformer issue?

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 04:01:55 am
content: Quote from: perieanuo on September 29, 2021, 06:45:53 amAnd I'm surprised by other peoples stupis statements.Do you mean that a lot of equipment automatically makes you an engineer?I bought the equipment because I want something to grow with and provide opportunities to learn things faster.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 30, 2021, 04:40:08 am
content: Connect a moderate load (perhaps a car stoplight bulb) and see what gets hot.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 04:41:52 am
content: Quote from: BrokenYugo on September 30, 2021, 04:40:08 amMaybe an very stupid question, but can i attatch an car bulb to an AC output?

author: bdunham7
author_rank: Super Contributor
timestamp: September 30, 2021, 04:50:56 am
content: Quote from: ElectricPower on September 30, 2021, 04:41:52 amAs long as it is an incandescent (with a filament, not LED or something else) bulb, you should be fine.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 04:53:51 am
content: Quote from: bdunham7 on September 30, 2021, 04:50:56 amOk, thanksMaybe try with an ordinary 12V halogen car bulb too? The unit has an max output power 56W and a bulb like that is 55W.

author: bdunham7
author_rank: Super Contributor
timestamp: September 30, 2021, 04:55:22 am
content: Quote from: ElectricPower on September 30, 2021, 04:53:51 amThat's perfect if you have an appropriate holder for it. Those get really hot and can burn you or break violently.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 04:56:40 am
content: Quote from: bdunham7 on September 30, 2021, 04:55:22 amThanks. I will try that later today

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 30, 2021, 05:01:02 am
content: Yeah, don't operate a halogen headlamp bulb out of it's housing. They get hot, they can explode, and being quartz they put out some UV light.

author: james_s
author_rank: Super Contributor
timestamp: September 30, 2021, 05:46:27 am
content: Quote from: ElectricPower on September 30, 2021, 03:36:47 amIf the voltage drops under load that means you have resistance somewhere, it could be a faulty relay or something else. If it's dropping to only 7V with a moderate load that suggests quite a lot of power is being dissipated somewhere so something should be getting hot.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: September 30, 2021, 01:49:37 pm
content: Quote from: james_s on September 30, 2021, 05:46:27 amGood point. I will try some load and look for the heat

author: james_s
author_rank: Super Contributor
timestamp: September 30, 2021, 08:17:41 pm
content: Quote from: BrokenYugo on September 30, 2021, 05:01:02 amI do it all the time, never had an issue with it. I just use clip leads on an old H4 bulb that still has a working high beam filament. If I'm going to operate it for an extended period of time I dangle it into a glass jar.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 01, 2021, 05:10:58 am
content: I found out that it was a bad relay that caused the failure.I changed it and now it works again

author: james_s
author_rank: Super Contributor
timestamp: October 01, 2021, 05:42:51 am
content: Glad you got it working, thanks for posting the update to let us know. I figured the relay was a likely suspect, there just isn't much else in the circuit path.

author: perieanuo
author_rank: Frequent Contributor
timestamp: October 01, 2021, 08:00:47 am
content: Quote from: ElectricPower on September 30, 2021, 04:01:55 amexcuse me sir, it's your constitutional right to burn your scope and be impolite. people like you doesn't deserve to be helped, you made bad measurements, incomplete description, we just tried to adjust this, we didn't mention the word stupid for describing you, maybe we should.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: October 01, 2021, 04:05:42 pm
content: Quote from: perieanuo on October 01, 2021, 08:00:47 amI just answered. You are suprised that i cant understand because i have a lots of equipments? That is for me a stupid statement. I also answered why i have the equipment, so you now next time i post stupid questions.