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Electronics question

Started by Raduprv 10 replies 1.2k views
Original Post
Raduprv
Raduprv
I noticed that I have too many AC/DC adaptors around here, and not enough plugs. Quite a few of my devices work at 12 volts (router, modem, wireless headphones), so I can feed them from a 12V battery, as I am doing with my sound system. The battery is charged by some solar panels I have in the window (and I just ordered more). But there are some other devices that run at different voltages, such as 4.5, 3, etc. So my question is, how can I reliably and safely drop the voltage to the desired value? Can I just use resistors? I know I can use the Ohm law, but I am a little confused with the amps. I mean, each device can consume a variable number of amps, depending on various factors (such as how much the volume is turned on, how much load it has, etc.) Is there a generic way to, say, get 3 volts from 12 volts, regardless of the amps that device might consume?
fractoid
fractoid
Depends on how much you want to spend.

The most expensive, most versatile, and possibly least efficient approach is to buy one of those car power socket adapter thingies that will boost your 12vdc to 110/240v AC. Then you plug a power board into that and plug all your AC adapters for your various devices into that.

For a bit more tinkering, you can build your own power supplies around voltage regulator ICs. For a low-current application it's probably cheapest to use a linear regulator - these will give you a regulated voltage (5v, 9v, whatever) and burn off the excess power as heat, so for currents above 200-300mA they tend to get hot and waste lots of power. For higher current applications, use a switch-mode regulator such as the LM5008. These are more expensive than linear regulators, but are much more efficient and so can handle much higher currents.
etothex
etothex
You can build your own, but I highly suggest buying premade converters.

If a component calls for 12V, it needs 12V. In fact, what I'd suggest is getting an unregulated 10-14V to 12V converter, because depending on the battery the voltage may fluctuate a bit around. Then, get 12V to 3V or whatever components.

Hardware is cheap, but not that cheap. It's a pain(and a fire hazard) if you smoke your electronics.
Raduprv
Raduprv
Umm, I really do not to use 'hacks' such as using an invwertor, because it's inneficient and it generates unnecesary RF radiation which I can live without. I also don't want to buy convertors and such, and I don't care if 12V devices will be powered by 13-11 volts. They will work ok.
Extrarius
Extrarius
Your best bet would be to buy voltage regulators rated for the voltages you need. The only problem with that is that you generally need to feed them at least 2 more volts than they output, which means for the 12V systems you'd need higher voltage.
It sounds like you don't mind risking your 12V systems though (or maybe they have internal voltage-uppers and regulators), so maybe you'd be fine with just using regulators on the <10V systems.
"Walk not the trodden path, for it has borne it's burden." -John, Flying Monk
Raduprv
Raduprv
My non 12V devices will be non crucial or expensive, such as a radio, a thermometer, etc. I don't really think they need exact voltages, most of the devices have at least a 10% tolerance. If I can get away with some resistors, then I'm going for it.
trzy
trzy
Quote:
Original post by Raduprv
My non 12V devices will be non crucial or expensive, such as a radio, a thermometer, etc. I don't really think they need exact voltages, most of the devices have at least a 10% tolerance. If I can get away with some resistors, then I'm going for it.


If you're thinking of building a voltage divider out of resistors, I wouldn't recommend it. Some devices will require more power than a resistor-based voltage divider can deliver. I know from experience that for CMOS or TTL logic devices, resistor-based voltage dividers don't work.

To power 5V, 3V, etc. devices, you'll either want batteries that can supply the proper voltage levels (they do exist) or use voltage regulators, like someone else suggested.

I believe something like the 7805 (which is rated for up to 1 amp) can take a 12V DC voltage and output 5. The SMSReader uses it.
----Bart
Zahlman
Zahlman
Quote:
Original post by trzy
If you're thinking of building a voltage divider out of resistors, I wouldn't recommend it. Some devices will require more power than a resistor-based voltage divider can deliver. I know from experience that for CMOS or TTL logic devices, resistor-based voltage dividers don't work.


You could always combine it with a current amp... though I suppose that *is* a bit silly :s
lonesock
lonesock
not much...the phone is expecting <= 12 V. There are plenty of single IC Voltage regulators: plug in 12, get 3 out, etc. That, a resistor, and an LED will get you your car cell-phone charger.

As to running all your devices, the safest is to just get a power strip. Anything that plugs into wall A/C should be UL approved. The simple resistor ladder will not work because as soon as you introduce a new load (radio, clock, speakers, whatever) you are effectively changing the resistance, so your math goes out the window. And because they are active devices the load changes (turn off your stereo, blow out your clock kind of thing).

I know that many electronics labs have a wall-A/C to multiple, variable DC out...but I'm betting that is more expensive and bulky than you are looking for.

sorry.
Raduprv
Raduprv
Hmm, ok, I guess I'll go to Radio Shack so they'll rape me a little (as they grossly inflate their prices) and get a voltage regulator...

trzy, I don't want 10 different batteries, I want one battery and multiple devices connected to it.
Extrarius
Extrarius
You could always order them from Mouser or DigiKey. The shipping might make up the difference, but you can order tons of components without increasing the shipping at all(it is generally by weight and this stuff doesn't weight very much at all) so it depends on how many of what you need.
"Walk not the trodden path, for it has borne it's burden." -John, Flying Monk

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