Use transformer/variac to power 100V battery charger

Thread Starter

Jahnlee

Joined Jul 2, 2015
64
Hi,
I bought a Panasonic 7.2V battery powered screw driver from Japan. It comes with a 100V input battery charger. My household supply is 230V.
I have a variac that can step down to 100V but when I read the manual, it says do not attempt to use a step down transformer with the battery charger.
Anybody has thoughts why a variac or transformer should not be used?
Is the output of the variac less distorted (sine wave) than a transformer?
Does it make any difference if I plug in the charger first, then turn on the power, then plug in the battery or
plug in the battery into the charger and then into the variac and then turn the power on.
Thanks for your help.
 

dl324

Joined Mar 30, 2015
10,488
Can't see why they'd tell you not to use a step down transformer. You're planning to use the variac to accomplish the same thing.
 

oz93666

Joined Sep 7, 2010
737
The manufacturer may be overcautious ... this from Wikipedia could explain .." An autotransformer (variac) does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output. .."

But anyway using a variac seems rather an inconvenient solution for an item you may use regularly .... I'm pretty sure all you need is a diode on the mains feed ...I could be wrong.
 

ebp

Joined Feb 8, 2018
2,332
A "real" transformer or variac would be OK. If the adapter can tell the difference between the output of a transformer and AC mains at the required voltage, it would be a true technical marvel - which it isn't. I used to design industrial power supplies and regularly used an isolation transformer and/or a variac to adjust input voltage for testing.

I suspect what they are cautioning against are voltage reducers that are not actual transformers - triac-based circuits, simple diodes, etc. - the sort of thing often sold for "travel" use. These could be disastrous, especially a simple diode type. (If the adapter itself uses an iron core transformer and the primary is driven with and waverform that is not well-balanced AC, the core can "saturate" which makes the transformer appear like a low-value resistor - just the resistance of the winding. If there is a local fuse, it may survive. If there isn't it may well burn up before tripping the mains circuit breaker.)
 

Thread Starter

Jahnlee

Joined Jul 2, 2015
64
The manufacturer may be overcautious ... this from Wikipedia could explain .." An autotransformer (variac) does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output. .."

But anyway using a variac seems rather an inconvenient solution for an item you may use regularly .... I'm pretty sure all you need is a diode on the mains feed ...I could be wrong.
Hi, yes inconvenient but I do not know how to use diode on the mains feed to step down the voltage. Anyway, since I already have the variac, I might as well try it.
 

Thread Starter

Jahnlee

Joined Jul 2, 2015
64
A "real" transformer or variac would be OK. If the adapter can tell the difference between the output of a transformer and AC mains at the required voltage, it would be a true technical marvel - which it isn't. I used to design industrial power supplies and regularly used an isolation transformer and/or a variac to adjust input voltage for testing.

I suspect what they are cautioning against are voltage reducers that are not actual transformers - triac-based circuits, simple diodes, etc. - the sort of thing often sold for "travel" use. These could be disastrous, especially a simple diode type. (If the adapter itself uses an iron core transformer and the primary is driven with and waverform that is not well-balanced AC, the core can "saturate" which makes the transformer appear like a low-value resistor - just the resistance of the winding. If there is a local fuse, it may survive. If there isn't it may well burn up before tripping the mains circuit breaker.)
Thanks, this is what I have. I am not sure why there are 2 coils, with a wiper on the smaller coil. There seems to be a small transformer after (?) the variac



 

LesJones

Joined Jan 8, 2017
2,600
Is the thing in the pictures in post #6 what you are calling the "charger" or is it the thing you are calling the "variac" ?
The box seems to contain two transformers. The one you lable "transformer" is s small "E" & "I" construction that probably supplies the electronics on the front panel. The large round item in the top right hand corner of the box is a toroidal transformer. The yellow item with the wiper will be a variac. If this box is what you are calling a "variac" then you will need to post a coppy of the manual for it and if that does not give much information then trace out the schematic of how all the items in the box are connected together.

Les.
 
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Thread Starter

Jahnlee

Joined Jul 2, 2015
64
Is the thing in the pictures in post #6 waht you are calling the "charger" or is it the thing you are calling the "variac" ?
The box seems to contain two transformers. The one you lable "transformer" is s small "E" & "I" construction that probably supplies the electronics on the front panel. The large round item in the top right hand corner of the box is a toroidal transformer. The yellow item with the wiper will be a variac. If this box is what you are calling a "variac" then you will need to post a coppy of the manual for it and if that does not give much information then trace out the schematic of how all the items in the box are connected together.

Les.
Hi Les,
Thanks for your reply. Yes, I called the whole box a variac. The original question though was whether this 'variac' is suitable for use with a 100V battery charger as the manufacturer (Panasonic) stated do not use a transformer to step down the voltage to use with their battery charger. The manufacturer of the variac spec says 0-260V output, max 5A. 1300VA.
 

ebp

Joined Feb 8, 2018
2,332
The box appears to have both a variac and an isolation transformer. There should be no problem using it to power the charger.
 

DickCappels

Joined Aug 21, 2008
6,303
Yours sounds like a good question for the manufacturer. Maybe the best way to ask would be to ask how to recharge with a 240 volt supply, a question they may be prepared to answer.
 

Tonyr1084

Joined Sep 24, 2015
4,369
First, what I'm about to say - - - don't take it as an authoritative answer.

I have a half dozen power supplies that are marked as an acceptable input range of 100 VAC to 240 VAC. When you say your battery charger requires 100 VAC, I am wondering if that's not the whole picture. Speaking of pictures, can you post the picture of your battery charger, the label where it defines the characteristics of the charger? I suspect your charger is compatible with 100 to 240 volts AC. BUT AS I SAID FIRST OFF - DONT' TAKE THIS AS AN AUTHORITATIVE ANSWER! DON'T PLUG IT IN AND SEE WHAT HAPPENS. You could end up hating me if you do.

Post the picture of the battery charger label. And any other parts of the charger you think might be helpful. But I'd like to see the label itself.

Again, don't plug it in and see what happens. Until we KNOW what we're dealing with - let's take it slow and figure this thing out.
 

Thread Starter

Jahnlee

Joined Jul 2, 2015
64
Yours sounds like a good question for the manufacturer. Maybe the best way to ask would be to ask how to recharge with a 240 volt supply, a question they may be prepared to answer.
Hi, thanks for your suggestion. That is a very good way to ask and I will try to find their email and ask them. If I get any response, I will update.
JL
 

Thread Starter

Jahnlee

Joined Jul 2, 2015
64
First, what I'm about to say - - - don't take it as an authoritative answer.

I have a half dozen power supplies that are marked as an acceptable input range of 100 VAC to 240 VAC. When you say your battery charger requires 100 VAC, I am wondering if that's not the whole picture. Speaking of pictures, can you post the picture of your battery charger, the label where it defines the characteristics of the charger? I suspect your charger is compatible with 100 to 240 volts AC. BUT AS I SAID FIRST OFF - DONT' TAKE THIS AS AN AUTHORITATIVE ANSWER! DON'T PLUG IT IN AND SEE WHAT HAPPENS. You could end up hating me if you do.

Post the picture of the battery charger label. And any other parts of the charger you think might be helpful. But I'd like to see the label itself.

Again, don't plug it in and see what happens. Until we KNOW what we're dealing with - let's take it slow and figure this thing out.
Hi, thanks for your reply. Attached are the pictures. I was about to plug it into the variac set at 100V until I saw your message. Anyway, I think I have about 2 weeks of battery life (maybe more) before I need to recharge them.

 

Tonyr1084

Joined Sep 24, 2015
4,369
Thanks for posting the picture. Honestly I wouldn't have believed it if I didn't see it. To be frank, that IS a strange AC input requirement. I've never heard of ANY AC system, American, European or any other system running on 100 VAC. I don't know if China has such a 100 V system. Even still, with the evidence clear as day, I now have to wonder if that's a mis-print. I've dealt with other Chinese suppliers and have found discrepancies in translation to English, and I wonder if this is another case of translation error. But as yet I still CAN'T tell you to plug it into 240 VAC.

Maybe it's time to find a different power supply / charger.

[edit] Do you have a data sheet for this model? I went to the Panasonic website and couldn't find that model number no matter what I entered. I'm now wondering if this is a Chinese knock-off. Honestly, I would have guessed Panasonic to be Japanese, not Chinese. I could be wrong.
 
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Thread Starter

Jahnlee

Joined Jul 2, 2015
64
Thanks for posting the picture. Honestly I wouldn't have believed it if I didn't see it. To be frank, that IS a strange AC input requirement. I've never heard of ANY AC system, American, European or any other system running on 100 VAC. I don't know if China has such a 100 V system. Even still, with the evidence clear as day, I now have to wonder if that's a mis-print. I've dealt with other Chinese suppliers and have found discrepancies in translation to English, and I wonder if this is another case of translation error. But as yet I still CAN'T tell you to plug it into 240 VAC.

Maybe it's time to find a different power supply / charger.

[edit] Do you have a data sheet for this model? I went to the Panasonic website and couldn't find that model number no matter what I entered. I'm now wondering if this is a Chinese knock-off. Honestly, I would have guessed Panasonic to be Japanese, not Chinese. I could be wrong.
The item was shipped from Japan and here's the website
http://www2.panasonic.biz/es/densetsu/powertool/recommend/ez7421/set.html#tab
There's some info in there but it's in Japanese.

This item is meant for the local market in Japan only.

The US version which is a totally different model is available from amazon
https://www.amazon.com/Panasonic-EY7410LA2S-3-6-Volt-Lithium-Ion-Driver/dp/B000RERWXC/ref=sr_1_fkmr1_1?ie=UTF8&qid=1546158230&sr=8-1-fkmr1&keywords=panasonic+screwdriver+ey
 

Tonyr1084

Joined Sep 24, 2015
4,369
As I've recently learned from @bertus, Japan has a 100 VAC system. At this point I would not suggest trying to plug your supply into a 240 line. Not unless you don't mind chancing the possibility of blowing out your supply.

Back to your original proposal: Use of a Variac (variable transformer) - I don't see why it would be a problem. But then again, it's not my money and equipment on the line. At this point I can give you no further assistance. I hope you solve your problem. ME? I'd opt for buying a supply that can work from a 100 to 240 input.

You don't say where you're from. I'm in the US. When I want a 12 volt supply of around 2 to 4 amps I just go to the local cable company and ask them if I can have one of their returns. They don't reuse the old supplies, and I often can get two at a time. Xfinity and DirecTV are a few places I can go to get power supplies for free. If you have such in your area, perhaps you can give that a try. I also find a lot of old printers that have external power supplies. I've found one at 30 VDC, one at 18 VDC and one with two different voltages, I don't recall. I don't know if that will address your needs but it's worth a shot if you can get your hands on something like that. Last year I modified one of those freebee PS's from 12.1 volts to 13.8 volts by varying the resistors on the reference voltage. I use it to keep an old car battery in the garage topped off so that when I'm working on the car I can play an old car radio. Works sweet.

Bottom line, be creative. But also be careful.
 

ebp

Joined Feb 8, 2018
2,332
100 volts has been the standard in Japan for a long time. Like zillions of others sold all over the world, the charger likely uses an iron core transformer and was designed for a specific location. Use your variac box and don't fuss about it. It will work just fine.
 
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