DC step down units

[hobbit]

Hiya people,

I'm asking these question mostly on behalf of a friend who is living in very rural Wales.

Due to his location and the electricity board wanting too many thousand pound to connect him to the mains he runs his place on a 12v DC system charged by wind and solar.

He has a problem that he has several low drain pieces of equipment, radios, LED lamps etc, but they all take different voltages he runs them off disposable batteries.
He could run an inverter to run a powerpack transformer but it seems daft to the pair of us to convert 12v to AC, step it up to 240v, then back down to the needed DC and rectify, especially as he says an inverter is power hungry, and from my memories that there are losses in every step of the conversion.

I have a little electrical/electronic experience and he has asked me if there is a "box" I can build cheaply for him to give - for instance - a 6v output from his 12v batteries so he can supply these things directly, and for the odd things that won't be convenient or that need to be portable a way to supply a charging voltage to replenish rechargeable cells.

Anybody got any ideas for something simple that I can do, bearing in mind he's going to ask me for different output voltages for each power "box", I'm probably looking to make ones with outputs of 9v, 6v, 4.5v and 3v, and what components will be needed for each voltage.

Any help advice and suggestions welcome,
Thanks in advance, Hobbit.

 

[Reloadron]

Anybody got any ideas for something simple that I can do, bearing in mind he's going to ask me for different output voltages for each power "box", I'm probably looking to make ones with outputs of 9v, 6v, 4.5v and 3v, and what components will be needed for each voltage.

Part of the trick is knowing the actual power requirements, as in the current needed at for example 9v, 6v, 4.5v and 3v. Once that is known then some circuits could be developed to do what is needed. If the current is low as in less than one amp in a few cases simple regulators could be used.

Ron

 

[hobbit]

Thanks for the quick reply, I realise things like this will have to be worked out, but I'm guessing most things will be below the 1 amp continious running (DAB radio, walkie-talkie baby monitor) or could be kept below that for topping things up, like charging up an onboard battery (I'm thinking of his LED "flood-lighting" here, runs on 6v, has daylight sensor and PIR, and uses 2 x 1w Luxeon type LEDs that stay on for ten seconds).
I'll chat more with him about exactly what it it he needs and wants and see if I can work out any higher loads (been reading Mr Al's "Indirect Measurement of the Current Draw of a Device" thread and if it makes sense in the morning I'll use that)., but for the time being and purposes of this enquiry we'll say a load up to 1 amp for each voltage.

Cheers, Hobbit.

 

[dougy83]

While you could make it yourself, there are premade modules that will do the trick. The following adjustable converters (just some examples) provide up to around 2A (3A with heatsink) output - just buy a few, adjust each for each piece of equipment and you're set.
**broken link removed**
**broken link removed**

 

[Diver300]

You can use car chargers for mobile phones. Those are very cheap, and on most of them there are two resistors that set the voltage. Changing one of the resistors will change the output voltage.

 

[hobbit]

Cheers Diver300, just what I like, a plain simple and idiot-proof way to do something. Will pass it on to my mate as simplest solution, followed by the suggestions by Dougie83.

 

[hobbit]

Ok folks, he says many thanks for the info on the step down stuff, but he now has ANOTHER question:
He's got a pair of 18v batteries for a rechargable drill and wants to know the quickest (easiest/cheapest NOT the most rapid) method of charging them from a 24v supply. I'm guessing some kind of load to drop the volts but anyone give me an idea of what sort of value resistance? or would a bulb in line work just as well, if so 12v or 6v? I'm gonna send a link to this site to him and he can ask his own questions!
Ta muchly, Hobbit.

 

[Diver300]

The all-important question is what chemistry is inside the batteries.

If they are NiMh or NiCd, you can charge them at about 0.1 C and a bit of overcharge won't matter too much. So if they are 2 Ah, then aim to charge them at about 0.2 A, taking about 10 hours. A 2.5 W 12 V bulb in series would be about right, and will be slightly better at producing a constant current than a resistor. A 6 V bulb will tend to blow if the batteries are flat. Use two 1.2 W bulbs in parallel if Halfords haven't got any near 2.5W.

This is adequate, but it is not a good way of charging NiMh or NiCd, and it will probably damage them a little bit each time they are fully charged, so you can minimise that by not charging unless they are flat, or by checking the temperature and stopping when warm.

If you want a good charger, there are better, more complicated circuits that cut off when fully charged and detect temperature, the voltage peak when fully charged, or time or any combination of the above. If you have that sort of sophistication, you can charge much faster.

If the battery is Li-ion or Li-po, do not do any of that. Don't even think about it.

 

[hobbit]

Sorry, It's me again, passed on the link but he still comes to me to ask the questions. Keep telling him I'm an electrician and not an electronics bod! I suppose he looks at it as some knowledge is better than none.
He's been using a walkie-talkie type system as a baby monitor for when the kid has fallen asleep in the car, but that has now died and he's asked is there a very simple monitoring system that I can build.
It needs to be a 12v powered intercom basically, wired not wireless, with the car end continually sending as long as it's plugged in. I can "see" it as everything at the car end with the sounding speaker on the end of a long bit of wire. Sounds very Heath Robinson but it'll do the job. He just needs to know when the kid wakes up.
I have a microphone and possibly other parts from a wireless Tomy baby alarm that has stopped transmitting (apparently), I just need a basic circuit so I can build it, and hopefully into a cigarette lighter plug!

And thanks again to Diver300, that was just what was wanted, he says are Nicads and as long as they'll survive about 50 charges he's not excessively bothered about the life of the batteries.

Cheers folks, and one day I may even ask some questions of my own.