could I use this transformer backwards, to drop 120 to 100 vac?

[mudfud]

since I have a couple machines from Japan with 100vac to 24 vac transformers that will be too high with 120 input, could I use this transformer backward to get an appropriate voltage?

https://www.surpluscenter.com/item.a...-1169&catname=

[daverob]

Technically, yes you could use this transformer in reverse to drop approx 15v from your line voltage, but there are a couple of things you need to be aware of.

Firstly, it's an 'auto-transformer'. This means that it only has one single winding, and there is no isolation between it's input and ouput. The voltage you see coming out of this transformer will still be 'live' and should be treated with the same respect as you would with normal line voltage.

Secondly, I'm not sure about regulations on your side of the pond, but over here this couldn't be used in reverse without modification to the arrangement of plugs and sockets. The reason being, if you simply connect up to this transformer in reverse, you will have live voltages present on the exposed prongs of the plugs if they were ever to become disconnected. Even if your regulations don't specifically prohibit this, I'd think that it should be common sense that it's not a good idea.

The input connector (which will be your output connector, if the transformer is used in reverse) appears easy to deal with, you simply cut off the plug and wire it to a socket. But it looks like the transformers output connector (ie your input connector) may be integrated into the transformer itself, and more difficult to change.

If you can't change this socket, then I think that according to the regulations we have over here, the proper solution would be to put the entire assembly into an earthed metal box. I would say that mechanically fixing the plug into the socket (ie not simply glueing it in) should also be acceptable. The regulations would also require the affixing of the appropriate warning labels for both of these solutions.

[mudfud]

well. since I have 2 Japanese transformers I don't want to waste, I'll get one and see how it works.

my electronics guru also suggested adding some windings to the toroid or adding an inductor in series. experiments that I will do with his supervision next day at the shop

[BigBearSteve]

Try one of these

100 WATT STEP UP / STEP DOWN 100 VOLTS - 120 VOLTS JAPAN TRANSFORMER

[mudfud]

here's a picture of the transformer. i ordered 2 to play with, and who knows, maybe I'll have a saggy line voltage some day?

[daverob]

In the past I've used regular step down transformers to adjust the line voltage for various reasons. A couple of examples are...

On my Sega OutRun machine I needed to replace the monitor, the original Japanese monitor ran off a 100v output on the isolation transformer, my replacement Hantarex monitor needed 120v, so I used a small 120v to 24v transformer I had in my junk parts box and connected it to the 100v supply in the game to give the extra 20v needed to make my monitor work. It's been like this for about 12 years now and still works fine.

Also my test rig for the dongles has a similar set up, so I can check that the dongles work fine and all the internal voltages are correct when supplied with the regular 24Vac and also with 21.6Vac/26.4Vac (+/- 10% of the nominal voltage) to make sure that they will work correctly even if the supply voltage is high/low.

This rig uses two 240v to 24v transformers wired up with a three position switch. The first transformer converts the 240v line voltage to 24v, the second is wired to the output of this transformer to convert the 24v into a 2.4Vac output which can be added or subtracted from the output of the first transformer depending on the position of the switch.

[mudfud]

Thx for the detailed info daverob. I love to study your posts.

BBS. It figures that someone boxed a transformer for people with japan electronics that moved here.

I ordered the surplus ones and will do some more experiments. I even have a real lab now to play in

Also, good point about lack of isolation. With the autotransformer is it just one winding and you connect at 2 different points?

p.s. I have a hunch that much of my wiring is not UL listed

[daverob]

With the autotransformer is it just one winding and you connect at 2 different points?

Correct!

Word of warning about the transformer BigBearSteve linked to. This one is only rated at 100W, so could be a bit on the small side for the modern power hungry pachinko machines produced in the last few years. I'd recommend looking for one rated at 200W if you're trying to run a post 2005 pachinko from it.

100W power handling is only equivalent to a 4A transformer at 24v, and we know that some modern machines can cook a 4A transformer quite nicely, and will struggle with anything rated less than a 6A.

The boost transformer mudfud linked to is rated at 750VA (1VA can be considered to be roughly equivalent to 1W in these circumstances) so one of these should be good for 3-4 pachinko machines.

[mudfud]

thanks daverob!

good point about the wattage being way too low on that converter. mine is big enough for several way better to over-engineer, in my opinion.

[mudfud]

auto-transformer rewired, testing still pending.

I bought 2 of the transformers, and they are sturdy and should be great for dropping the voltage from 120 to 100 VAC.

was going to do some complicated reverse the plug, add a female, then realized I could open her up and swap the wires. the pix show this done.

thing is, I was surprised to have continuity across all the pairs of wires... then I realized that daverob pointed out this is NOT an isolated transformer, so I should have continuity across any pair... right?

letting the liquid electrical tape dry before I try it

will post later if I blow a breaker

[mudfud]

OK. no fires and no breakers popped.

output is 107 volts, output from Japanese transformer is then 27 volts.

is that low enough for the moderns now?

[daverob]

output is 107 volts, output from Japanese transformer is then 27 volts.

is that low enough for the moderns now?

On paper it's a little bit high as the tolerance is usually +/-10% so the upper limit should be 26.4v, but you'll probably find that it drops a volt or so under load when the pachinko is connected up.

I'd give it a try for a quick test to check voltages under load. It's very unlikely to damage anything short term, and long term it'll only be stuff running off the unregulated voltage lines in the pachinko that will be affected, ie motors might run a little faster/warmer, and filament bulbs will be a little brighter and consequently have a slightly shorter lifespan.

thing is, I was surprised to have continuity across all the pairs of wires... then I realized that daverob pointed out this is NOT an isolated transformer, so I should have continuity across any pair... right?

That's right, the transformer is effectively a single continuous winding that is just tapped at the places needed for the change in voltage, so you should see continuity across all input and output connections.