120v ac to 18v 2-6A DC circuit no stepdown Transformer

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120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby FourthDr » Sun May 13, 2012 12:18 pm

Hi:

I need to power a project I'm working from the 120v AC main lines. Ideally it would output 18v DC at 2A up to 6A. It needs to be some kind of step-down switching circuit that does not use a large step-down transformer or large heat sinks. I've looks around and have not really seen anything that would work. I've been told to: buy a laptop power block...LOL. But that won't fit into my project which is tight and irregularly shaped. Any ideas or suggested circuits?
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby Bertho » Sun May 13, 2012 1:09 pm

You'd want a transformer in there somewhere (as part of the switch PSU) to ensure isolation, but that is standard in most line-powered step-down PSUs. Then, 6A@18V is about 110W output. With 85% efficiency (and that is high for a 6.667:1 ratio), that means about 130W in. So, you need to dissipate 20W and that means a lot of heat.

So you want a lot of power minimal size; can be done, but that is not going to be cheap. Do you have to feed it from the AC line directly? What is the sizes you are talking about? Can you dissipate 20W?

If you want to design it yourself, then your biggest headache will be to create a proper switch-transformer and getting rid of the heat. All other stuff should be (relatively) easy.

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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby tayken » Mon May 14, 2012 1:49 am

This might be helpful for you Transformerless Power Supplies: Resistive and Capacitive, an app note from Microchip. I found this when I was checking out another app note: Low-Cost Electric Range Control Using a Triac.

As always: AC is dangerous sh*t and stay away if you don't know what you are doing!!!
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby arakis » Mon May 14, 2012 2:20 am

you could build a simple buck dc/dc converter, but like it was mentioned there is no way any 110W PS design is not gonna need large heat sinks...
best regards FIlip.
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby FourthDr » Mon May 14, 2012 9:20 am

Maybe I should explain my application? I basically want to take a cordless power tool battery pack, add a PS inside the empty pack and power the tool directly from 120v AC. 6A might be a bit much for something that is being powered by sub c's @18v dc. So I could probably get away with 2Amps output @18v.
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby JanW » Mon May 14, 2012 11:28 am

I'm guessing even 6 Amps is a bit on the low side for powering cordless tools. The stall current of a cordless drill is much higher (~ 20 Amps).

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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby markus_b » Mon May 14, 2012 12:19 pm

You need something like this: 12-V-15A-180W-Switch-Power-Supply on Ebay. Smaller ones (less Amps) have not enough power.

Just look at the Watt rating, you want well above 100W for the tool to be useful.
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby Adam » Mon May 14, 2012 10:04 pm

A couple of things come to mind for this idea:

  • You will need a high stall current - 6A may not be enough. If your stall current capability is not high enough then your power tool will stall easily.
  • Your power supply will need to provide safety isolation from mains voltage, as a battery powered tool will not be designed for Class II safety (double insulation)

I don't recommend trying to design your own switch-mode supply unless you know what you are doing with high-voltage electronics and transformer isolation systems. You should probably source one which is standards compliant so that you can be sure of the safety isolation.

A power supply that is optimized for your mains voltage (110 or 230) will generally be a little smaller than a wide-range input one for the same power output.
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Re: 120v ac to 18v 2-6A DC circuit no stepdown Transformer

Postby Sleepwalker3 » Tue May 15, 2012 6:34 am

It might be worth taking a look at this article which comes from the popular Australian electronics magazine "Silicon Chip", which deals with exactly what you are talking about it. The also did articles some years back about rebuilding the battery packs and made a special charger that was far better than most cheap power tool chargers and they explained why.

http://www.siliconchip.com.au/cms/A_112247/article.html

Unfortunately, if you aren't a subscriber then you have to pay to see the rest on the net - unless you happen to have the print copy of course ;)

Certainly keep away from any 'non-isolated' solutions like the cap method from Microchip, Adam was right, your tool just won't be designed with suitable insulation ratings and it will be a deathtrap - not that it would be easy to get that type of circuit to pump out the required currents anyway. On the subject of safety, even a normal 'isolated' SMPS might not be ideal. Most have their 0V line floating with respect to Earth, but have leakage currents and this can end up giving you a minor 'tingle' and sometimes a bit more when you contact it, some people may have experienced this when they touch the metal case of a non-earthed Set top box, DVD player, or similar (actually Silicon Chip mag did an article on fixing that problem some years back also). It's also best to ensure the SMPS case is Earthed, as they usually have small value suppression caps for EMC compliance running from the Active and Neutral leads down to Earth (the case) and so the case will have a fair potential unless properly Earthed. If using an SMPS, it would be best to get one that is rated for medical use, as this will have better isolation and less leakage and aren't that hard to come by.
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