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240V AC to 120V AC stepdown converter

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borisk

New Member
Hi there

I'm looking for a schematics to build 240V AC to 120V AC step-down converter. I need to power up 350 Watts battery charger designed to run of the US 120V AC mains from 240V. I don't want to use a huge transformer and I'm not sure if a simple triac light dimmer type circuit would do the job.

Any suggestions?

Thanks
 

Diver300

Well-Known Member
Most Helpful Member
The battery charger is basically a transformer, so either buy a new one or a suitable transformer. It is unlikely that you could use a light dimmer type circuit, because the peak voltage would not be reduced. That would be fine for a heater or lamp, where the only important thing is average or RMS voltage, but not for a transformer.

You can use an autotransformer which will be a little smaller and cheaper. HAMMOND|170C|Auto Transformer | Farnell United Kingdom

Are you sure that the battery charger won't work on 240 V? A lot if switch mode supplies (like laptop power supplies) are rated 110 - 240 V
 

borisk

New Member
I was hoping to get some schematics... or some ideas. There are compact units available from various online outlets that provide required functionality (for around $50 or so). From the look of them it's unlikely that they are based on an transformer/auto transformer. I am an electronic engineer just haven't worked in this area for quite some time so i was looking for a hobby project to build one. If you came across any schematics please let me know
Thank you
 

Mr RB

Well-Known Member
If the battery charger has a standard (big heavy) transformer then a typical fix would just be to buy a new 240v transformer and replace the internal 120v one.
 

borisk

New Member
The charger (Makita DC18RA) is based on SMPS technology. No transformer inside. It is a hi-tech device with CPU controlling charge process. It has battery cooling fan and other bells and whistles. However it is stated on a box that it only accept 110-120v input and I don't have the schematics for it so I have no idea how to adjust the input voltage. More over I know that the same charger does exist in 240v modification and I would imagine that it is very small change. But as I said without factory diagrams I would not be able to modify it.
 

Diver300

Well-Known Member
Most Helpful Member
That modification looks OK. The charger will probably still work on 120V afterwards.

The 2SK3683 is rated at 500V so it will probably be OK on 240V AC.

I'm sorry this forum wasn't a great deal of help to you. What I don't understand is why Makita make separate versions for 120 and 230 V. All the laptops power supplies I have seen are rated for both.
 

MrAl

Well-Known Member
Most Helpful Member
Hi there

I'm looking for a schematics to build 240V AC to 120V AC step-down converter. I need to power up 350 Watts battery charger designed to run of the US 120V AC mains from 240V. I don't want to use a huge transformer and I'm not sure if a simple triac light dimmer type circuit would do the job.

Any suggestions?

Thanks


Yes, buy a new charger :)

I dont usually take that above advice either ha ha but many people would
want to do that because it is the simplest, easiest, and most of all
the safest.
I do see that the charger looks like a good one, so you may try to sell
the old one.

I also see that the conversion in the link shows simply changing the
MOSFET and the cap with a cap of higher voltage rating, and that might work,
but another thing to think about is
the ripple current in the filter capacitor. If it is too high for the cap
rating the cap will get hot and burn up. It may survive just the same,
but it would be a good idea to keep an eye on the temperature of
that component just to make sure. What i would do is stick a thermometer
on there for at least the first 12 hours run time and watch it to see
if it goes up high. 70 degrees C might not be too much but i wouldnt like
that either, but many caps are rated for 105 deg C so maybe it will be
ok if the charger isnt used all day and all night for many days.

For higher voltage like that using the SAME coil there is also the chance
that the coil will saturate. That will screw things up badly, but again
it may get by. A good idea would be to measure the current with a
scope to check for inductor saturation.

In any case, i wish you the very best of luck with it.


ADDED LATER:
I just noticed that they post that that mod is not recommended by Makita.
I also noticed, more importantly, that the charger can be used with Li-ion cells.
You should know that Li-ion cells have issues of their own and any problem with the
charger (such as higher output ripple voltage due to a higher input voltage)
can cause the cells to burn up or explode violently. I would think twice about
doing this, or at least check over the design more thoroughly. If you have a
schematic or can make one by tracing out the circuit board we can take a look
and help to ensure nothing blows up in the end.
 
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