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A little tester to determine transformer PhaseDots with no scope or signal generator
A little tester to determine transformer PhaseDots with no scope or signal generator
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Old 13th June 2015, 01:06 PM   #21
Mark Johnson is offline Mark Johnson  United States
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A little tester to determine transformer PhaseDots with no scope or signal generator
Quote:
Originally Posted by Ghianni View Post
Just use your digital multimeter at diode function and done. See the attached pic.
Yes I tried the Kingbright LEDs on 3 different multimeters. The meters were cheap ($40), cheaper ($15), and cheapest ($6). Besides giving the expected diode reading on the DMM display, both LEDs lit up on all three meters. The green LED glowed green and the red LED glowed red. Thus the DMM reminds you which end is cathode and which end is anode, and it is an easy way to verify that the water-clear-plastic LED in your hand, glows green. So you should solder it into the green LED position: D4, Windings In Phase.
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Old 13th June 2015, 08:56 PM   #22
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Ghianni View Post
Why you are searching for a tree and lose the forest?

Just use your digital multimeter at diode function and done. See the attached pic.
I'm aware of the fact that most LEDs will light up on a DMM's diode test, but that was not the point. The point was that once the uneven length leads of new LEDs are clipped to the right length, you still have the ability to determine anode and cathode without having to resort to a multimeter.
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Old 14th June 2015, 10:06 AM   #23
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by kasey197 View Post
Hey this is very useful - I have long forgotten custom toroids with multiple secondaries that I can never remember how to wire up for series/parallel as the wires are all the same color! ...............
Use the Mains Bulb Tester you have lying on the shelf.

Use it to check the wiring of any/every mains transformer, especially those that don't have wiring instructions.
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Old 14th June 2015, 10:17 AM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jitter View Post
Well, there is another way if the LED is clear or semi-clear. There's a cup-like structure inside the LED. I guess it's what holds the junction as the light is emitted from it.
With normal LEDs, I have always seen that this structure is connected to the kathode and it clearly takes up more space inside the LED than the anode.

Click the image to open in full size.Click the image to open in full size.
The cup structure/reflector is nearly always the -ve side.
I too gave advice on this in this Forum, until a Member said I was wrong.

I went to my stock of single colour LEDs, and compared short lead, and flat and cup reflector and all except one were consistent. The Bar on the schematic symbol, and the Cup and the Flat and the Short lead are the Cathode side and go to the -ve side.
BTW, you can see the tiny wire from one lead looping into the centre of the reflector. Maybe like the "cat's whisker" of the early diodes/transistors.

BUT !!!!! one diode type had the cup reflector on the anode side.
I don't understand why, or how, this discrepancy can occur.
But the accusation was right. I was wrong. Not all cathodes are indicated by the cup reflector.

This makes it difficult to identify 3mm LEDs (no flat on the side) that have been trimmed. One has to polarity test to check for correct orientation.
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Last edited by AndrewT; 14th June 2015 at 10:21 AM.
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Old 14th June 2015, 01:11 PM   #25
Mark Johnson is offline Mark Johnson  United States
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A little tester to determine transformer PhaseDots with no scope or signal generator
Quote:
Originally Posted by AndrewT View Post
Use the Mains Bulb Tester you have lying on the shelf.
Unfortunately a Mains Bulb Tester won't find every kind of transformer wiring error. I'll let theoretically-inclined readers state a single-sentence rule which describes the kinds of errors it won't find; instead I'll merely exhibit one example (attached).

This is the power supply wiring diagram from the Akitika GT-101 audio power amplifier, when used with 115VAC mains such as in USA. The transformer has two primaries, each 115VAC. It also has two secondaries, each 35VAC. The goal is to deliver 70VAC to the bridge rectifier.

For 115VAC operation, the primaries should be wired in parallel and the secondaries should be wired in series.

However, if you accidentally connect the secondaries incorrectly and test it with your mains bulb tester, you get no indication that anything is wrong. Try it in real life. Or try it in simulation. The mains bulb detector has no idea that the secondary windings are out of phase; it doesn't tell you the dots are wrong.

(if you try it in simulation, remember to install a teeny bit of series resistance in each winding; I used 100 micro-ohms)
Attached Images
File Type: png akitika_gt101.png (27.4 KB, 301 views)
File Type: png dots_error.png (24.5 KB, 309 views)

Last edited by Mark Johnson; 14th June 2015 at 01:17 PM.
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Old 14th June 2015, 01:15 PM   #26
AndrewT is offline AndrewT  Scotland
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One can wire the dual primaries out of phase and the Bulb turns ON.

Or one can wire the secondaries out of phase and shorted and the Bulb turns ON.

Or one can wire both the multiple primaries and the multiple secondaries out of pahse and the Bub turns ON.

All transformer wiring errors are protected from damaging the transformer using a correctly wired Mains Bulb Tester.
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Old 14th June 2015, 01:19 PM   #27
Mark Johnson is offline Mark Johnson  United States
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A little tester to determine transformer PhaseDots with no scope or signal generator
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Originally Posted by AndrewT View Post
Or one can wire the secondaries out of phase and shorted and the Bulb turns ON.
When the secondaries are in series and out of phase, no current flows. Try it.
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Old 14th June 2015, 01:19 PM   #28
AndrewT is offline AndrewT  Scotland
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BTW, your right hand diagram will not damage the transformer.
But once you measure the output voltage you will find near zero volts across the 100r resistor. The resistor will stay cold.

Ah ! one thinks and rewires either one of the primaries or one of the secondaries.
Powering ON this time either brings on the Bulb or gives one the expected 100r voltage. i.e. it gets hot.
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Old 14th June 2015, 01:27 PM   #29
AndrewT is offline AndrewT  Scotland
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It was because of this ambiguity in wiring dual primaries/secondaries that I asked about a "safe" way to check the transformer wiring, since it seemed that guessing and getting it wrong, usually resulted in blowing yet another fuse @ 13p each.

The Forum gave me no useful answers.
It was some time later I discovered that Valve guys used a Mains Bulb Tester to check old equipment before direct on line powering and I realised this one piece of test equipment solved the risk of damage if wiring the mains transformer incorrectly.

The Mains Bulb Tester seems to be fool proof. It prevents catastrophic damage to transformers and PSUs.

It cannot be guaranteed to prevent damage to small components in an incorrectly assembled amplifier/DAC/Pre-amp/Crossover.

But wiring a pair of diodes across the +ve & -ve supplies to Power Ground, at the LV circuit power inputs, does prevent damage due to applying reverse polarity to a small circuit and protects against a mis-wired rectifier that tries to apply the wrong voltages to the +, - & 0 power inputs.
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Last edited by AndrewT; 14th June 2015 at 01:31 PM.
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Old 14th June 2015, 02:15 PM   #30
Mark Johnson is offline Mark Johnson  United States
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A little tester to determine transformer PhaseDots with no scope or signal generator
To disambiguate the phase dots of series windings, you want someone to connect their transformer to a Mains Bulb Tester AND a load resistor AND a voltmeter? The signal generator + oscilloscope method (picture 1 of post#1) is simpler and far safer. So is the PhaseDots tester board.

Personally, I would rather test the transformer and find the correct phase dots beforehand. Then the transformer can be connected to the mains and to the remaining circuitry with confidence that the wiring is correct and no damage will occur, catastrophic or otherwise. I also prefer to test the transformer in a safe, low voltage arrangement that has no live mains wires. Either the signal generator + scope method or the PhaseDots tester + 9V battery method. Why horse around with a live-mains test setup if other options are available and very cheap?

For those who don't own a signal generator and oscilloscope, building a PhaseDots tester on a little piece of VeroBoard (StripBoard in USA) is quick, simple, and inexpensive: 2 CMOS logic ICs, 1 NE555 timer IC, and 2 LED drivers. Parts cost was USD 7.00 per PhaseDots board when I made my buy from MouserUSA back in April. Since there's nothing even remotely exotic on the BOM, you could buy everything from a discount far-East supplier like Tayda or Futurlec, and pay even less.
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