Comments on: Charge me (and my batteries) up

Question: Charge me (and my batteries) up

tip120 — Thu, 01 May 2008 13:58:02 -0800

I am charging at least 20, but up to 30 electronic components. Each will draw 300mA at 5-7VDC. What sort of power supply do I need?

I have at least 20 (possibly 30) custom electronic devices that are battery operated. Each object has a 1.1A LiPoly battery and a MAX1555 IC charger. These objects are going to be free roaming during the day, and plugged in at night to charge.

I know the circuit works as expected, and have charged one of them off of usb power (100mA 5VDC).

If all the objects are going to be plugged into one power supply at night, am I correct in thinking that I will need a supply which produces a regulated 9A 5VDC? Where can I go looking for something like this (in Europe preferably). Would an old computer supply work?

By: Krrrlson

Krrrlson — Thu, 01 May 2008 14:13:46 -0800

A standard ATX supply from a computer should be able to provide the current you need on the 5V output.

By: phrontist

phrontist — Thu, 01 May 2008 14:22:48 -0800

The basic idea of adding up currents is sound. So yes, 30 * 300 milliamps = 9 Amps.

It depends on what the supply is rated for, current wise, but a computer supply should work (I think the standard is 30A at 5v, but it should say on the supply itself). If you need to use smaller supplies, you can either put a few devices on each or wire them in parallel. I bookmarked this a while back, and it looks good, but I haven't tried it myself.

By: pombe

pombe — Thu, 01 May 2008 14:28:35 -0800

If you go to one of the big electronics suppliers like Digi-Key, you can find a whole range of power supplies.

By: moonmilk

moonmilk — Thu, 01 May 2008 16:19:50 -0800

I agree with phrontist that the idea is sound, but I'm not confident that you can wire up several smaller supplies in parallel- in my experience, switching power supplies (like those used for computers) shut themselves off if you try to do that. Just get one big supply, or connect a separate batch of devices to each small supply, as phrontist suggested.

Herbach & Rademan (in USA, sorry) is another good source of power supplies (and often much cheaper than the big distributors).

By: flabdablet

flabdablet — Thu, 01 May 2008 20:32:28 -0800

Any old PC power supply would work fine.

9A at 5V is 45W. The cheapest, crappiest second-hand computer power supply you will ever find will be rated at least 200W, of which at least half will be available on the 5V output. If in doubt, consult the output ratings panel on the power supply label, which will tell you exactly what the maximum current rating on the 5V output is. A scummy old 250W supply I just looked at says it's good for 25A at 5V.

All you need to do is convince the power supply that it should be On, which you can do by shorting pins 14 and 15 on the motherboard connector.

Red wires are +5V, black is 0V.

By: theyexpectresults

theyexpectresults — Fri, 02 May 2008 00:12:02 -0800

30A is a lot of current. In the UK at least, your circuit breaker might not even able to handle it. As an example, an electric cooker usually has a 30A fuse. Please be careful; I wouldn't to have 30A going through a single device overnight.

By: flabdablet

flabdablet — Fri, 02 May 2008 02:43:48 -0800

Yes, thirty amps is a lot of current. But thirty amps at five volts is only a hundred and fifty watts. If you're drawing this from a switching power supply that's 80% efficient (and most are better than that) then you're going to be pulling 150W / 80% = 190W from the mains. In the UK, with 240V mains, that means well under one amp of mains current. You could safely run a dozen of these things at full load off a single UK outlet (that's three hundred and sixty amps aggregate, at five volts) and still be using less power than a typical electric kettle.

In any case, tip120 is talking about needing only nine amps at five volts, not thirty amps, which is simply a typical maximum current rating for the five-volt output on a 300W PC power supply. Drawing nine amps from a supply rated at thirty is nice conservative power engineering.

By: flabdablet

flabdablet — Fri, 02 May 2008 04:47:59 -0800

By the way, tip120, the fact that the MAX1555 will run off anything between 3.7 and 7 volts means you don't really need a regulated supply. However, PC power supplies are cheap commodity items. I doubt you'd be able to put together an unregulated supply good for 10A at 5V for less than the cost of a PC supply, especially if you want to come close to matching the performance of the PC supply on efficiency and weight.

By: tip120

tip120 — Fri, 02 May 2008 05:46:05 -0800

Thanks all for your comments. I was much more concerned about using the right math than using the wrong wires. I think a PC power supply will do just fine.

By: tip120

tip120 — Thu, 29 May 2008 01:59:11 -0800

As a follow-up to this old question, I wound up using 6 5v 2.5amp power supplies (cheaper!), each fed 5 objects. Thanks again for all the help.