Building an Isolation Transformer

I finally found an affordable isolation transformer on Ebay. This video shows my project of putting this transformer (which at the time of making this video you can still buy) into an enclosure and wiring it up.
00:35 the transformer
01:35 testing isolation
02:17 schematic
04:47 enclosure
06:42 wiring
07:38 first test
07:57 testing the switch
09:22 testing the fuse
My previous "up to 100V" isolation transformer build is here: • Building an AC power s...
The build video of the VARIAC is here: • Building a VARIAC for ...
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[@wessyrisza9211]

Finally, I build isolation transformer. Thanks

[@laser-on-off]

Great video, thank you. Cheers from France.

[@tonnyodekerken9013]

I would exchange the red and green light. If the secondary side of the transformer is floating you are reasonably save against electrocution. If you however involve the ground onto the secondary side a life threatening situation is created. On the primary side you have the earth leak circuit breaker in your fuse box protecting you from electrocution. On the secondary side however the protection is non existing.

[@TheHWcave]

Its a valid point and I did spent some time on what light to indicate what state. In the end I decided to go with green = earthed. I am trying to standardise on this colour meaning earth even though it means i have to swap out a few green binding posts on my older builds for different colours. But that is totally my slightly obsessive view. Your point is absolutely true and anybody who builds one of these should consider your arguments and decide for themselves.

[@tonnyodekerken9013]

@@TheHWcave Seems we are in opposite corners here. My oscilloscope has a red sticker, meaning watch out connecting since it is earthed. The signal generator however has a green sticker since it is floating. A ground in a circuit can be your friend or your enemy. I guess it’s all relative.

[@SloVu]

bear in mind,transformer vibrates,screws on your amp and volt meter could get loose

[@TheHWcave]

Its a good point and I used spring washers just in case for both. It turns out in this case there is no vibration at all, even at load. The transformer is quite well put together and still relatively small (as transformers go). An advantage of the plastic box is that it would amplify any vibratory noise like the body of a violin.

[@standamartinak3679]

Hello, very nice work! Where do you buy such big electric plastic boxes please?

[@TheHWcave]

This is actually a junction box for electrical installation. Available on eBay

[@standamartinak3679]

@@TheHWcave Thank you!

[@weazeldark3983]

Should wired coils other way around and used the 400v as option

[@TheHWcave]

I considered that but in the end I decided against it. I am using my isolation transformer mostly behind the VARIAC which I already wired to provide almost 270V when turned to "11". If the isolation transformer would step that up even further, the voltages could be seriously dangerous for connected equipment. For repair and test of devices, normal voltage or even lower than normal voltages when dealing with unknown faults are much more useful. And being able to raise the mains voltage to 270V if I have to is more than enough for me. I hardly ever used that so far.

[@zoltankecse991]

Hi! Great video. Please tell me. Can I use the lamp in series after the isolation transformer (230/230) at the same time in the same circuit? Like this: Main power (230) -> isolation transformer -> series lamp -> and socket (outlet) Is this right? ( I want to test audio amplifiers.)

[@TheHWcave]

Yes, a current limiting lamp will work just fine

[@zoltankecse991]

@@TheHWcave Thank you very much.

[@ivanr5315]

Hi there, I have been thinking about this but I do not understand what is the purpose of the floating/grounding switch. The ground reference as shown in the diagram in my view has little relation (aside from capacitive coupling) to the secondary of the transformer. So if the switch was between the secondary ground and earth one could select if the transformer was being operated in pass through (secondary neutral connected to earth) or as an isolation transformer (secondary neutral not connected to earth). What is the purpose for the switch in its current configuration? Thanks for the good videos

[@TheHWcave]

I know what you mean. All I can say is that I looked up isolation transformer schematics and they all do it this way. Earth is not connected to one of the secondary connections (making it sort of neutral) but directly and only to the earth pin of the outlet socket. The switch stops that connection and makes the outlet float free, which is what you need when plugging a scope in. Or (much safer), the thing you want to measure is plugged into the floating outlet and the scope is in a normal wall outlet.

[@ivanr5315]

@@TheHWcaveHi there yes I think I got it. To act as an isolation transformer the secondary must not be connected to a earth reference as we discussed. Connecting the mains earth to the system earth just make sure that the chassis or metallic parts do not float to unknown voltages. This however can be an issue when using an oscilloscope as there could be an electrical path between the oscilloscope GND (referenced to ground) and metallic parts of the DUT. Thanks for coming back on this, it makes sense now

[@alexwade9921]

Hi there. I grabbed myself one of these transformers from eBay. Thanks for the recommendation, it’s a very nice item. I will be using it in a project in permanent series in the same enclosure with a variac, and also at the input a switchable dim bulb limiter. Which way around would you recommend to put the transformer and variac? I’ve seen it done both ways. I was thinking if I use it with the transformer first (1A) and my chosen variac (2A) second, then I could have one amp at 240v going up to 2A capacity at 120v output and below. Some people put the variac at the supply side and the isolation transformer second though, including Mr Carlson, and it appears yourself at the early part of this video (using with your old isolation transformer) What’s the difference?

[@TheHWcave]

Yes, these transformers are a very good deal. Glad you managed to bag one. My VARIAC can do 1000W, the isolation transformer only 250W, which is the only reason for the way I am using both. Having the VARIAC connected to mains allows me to still use the full capacity of the VARIAC when I need more power than 250W but not the isolation. Then again it does not really matter for me since they are two separate units, and I can choose which one plugs into the output of the other... but normally my isolation transformer is plugged into the output of the VARIAC and the VARIAC input directly into mains.

[@BjornV78]

Nice build :-)
Can you also do a voltage measuring between earth and one of the 2 terminals of the outlet socket, while the switch is in floating position?
This is normally way more then 5V of "Ghostvoltage". Grtz

[@TheHWcave]

157V blue terminal to earth and 16V yellow terminal to earth (at 233V input)

[@BjornV78]

@@TheHWcave , you can try with a 1M or higher resistor between the blue terminal and earth, to lower the ghostvoltage. This voltage is very low current (µA range) , but it can kill sensitive components.

[@t1d100]

@@BjornV78 Could you, or Heinz, please tell me more about this issue? Even a video? I DIY'd my iso trans from two identical trans, placed back-to-back. But, I did not consider this issue, due to a lack of knowledge.

[@BjornV78]

@@t1d100 hi, this issue is called ghostvoltage , strayvoltage or phantomvoltage, and is due the capacitive leakage from the primairy side to the core and secundairy side. The amount of leakage is depending on the construction of the transformer, if the transformer has a earthshield between primary and secundairy side, then the leakage is much lower. In most scenario, this leakage is more then 50% of the mains voltage, but has always very low current (in the µA range). A small neon lamp will glow, but a incandesent light bulb doesn't glow. This leakage can be reduced to almost zero if one of the 2 output leads of the secundairy is connected to earth, but a 0 Ohm connection the secundairy side and earth will bypass the all concept of being isolated from earth/ground, so this connection is best to keep as high as possible (1Mohm or more). But the lower this resistance is, the more current can flow through the ground lead of a scope probe if you connect this to the hot of the secundairy side.

[@BjornV78]

@@t1d100 also, if i remember correctly, a back to back setup will also lower this ghostvoltage leakage because this create a extra barrier between mains and output.

[@felixcat4346]

You sound a bit like Dr. Strangelove.

[@TheHWcave]

Not "Merkwürdigliebe" ?? Jawohl ...

[@karlmartell9279]

Dr. Strangelove or: How I Learned to Stop Worrying and love the electric shock, more commonly known simply as Dr. Strangelove.

[@jone1jone145]

How you made the schematic at 2:18 ?

[@TheHWcave]

Libreoffice Impress (same as PowerPoint). I figured it was so simple it was faster than doing it in KiCad