Suppose I have a transformer with an output of 13.8v (sort of thing that powers a CB) and I want to take it down to 12v and be capable of a load up to 5A, what's the best way of doing this?
First though is a few diodes in series to create a volt drop, and other is a potential divider. I would like to use a voltage reg, but I think 13.8 is below the minimum voltage.
Graham
Practical but why do you need exactly *12* volts ?
Non-starter for any current.
See low dropout regulators.
Graham
a low drop-out regulator could probably manage that...
but before that, how close do you need to get to 12V?
Bye. Jasen
-- Transformer???
Typical 'ordinary person speak' for wall wart.
Graham
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The idea of the circuit is to replace a dozen warts with one high current PSU and free up a lot of wall sockets. In additon, I can easily turn on or off whatever widget without plugging/unplugging stuff as nearly all warts don't have switches on them. I can also add a degree of current protection which would be better than there is at the moment. I have a load of other stuff I want to use, but don't because I have more warts than I really want to have. Nearly everything is 12v, hence the need to tweak the output down a touch. I doesn't need to be super-accurate, but reasonably close. I did toy with the idea of using my PC PSU to power stuff, but I don't want to load it unneccesaily.
Graham
If by "transformer" you mean a regulated DC supply of 13.8 volts and at capable of least 5 amps, the best way is to modify the supply. The easiest way is to put a couple of diodes rated at say 8 amps in series with the output. That will give you roughly 12 volts, and the voltage will vary depending on the load. Given your application, that variation is not a problem.
Ed
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That's what I thought. I just wondered if there is a better or right way of going about it.
Graham
-- Buy a regulated 12V supply capable of handling the total load.
It seems to me that a small (1/2 to 1 amp?) wall wart offers better overcurrent protection than a large five amp one would do. Unless you are planning on using a number of regulators, each dedicated to just one load? That's an excellent way to taylor the voltage and current capabilities to each one.
There are a wide range of low-dropout regulators on the market that will do this job nicely. The LM2941 is one that I use for this kind of application. See the data sheet here:
But this is just one of hundreds of low-dropout regulators you can choose from. It's worth your time to consider the entire range and choose carefully.
It's unlikely those wall warts are regulated. You should be putting "12v" in quotes. It's a nominal rating -- the typical cheap wall wart puts out voltage all over the map, depending on how much you load it. If you look at the print of almost any cheap wall wart, the voltage is specified at a certain current. Open circuit they will produce a much higher voltage, loaded heavily the voltage will drop precipitiously. That extra 1.8 volts you're worrying about is much less than the voltage variations the wall warts will exhibit.
In message , Grey writes
If it's exactly 'the sort of thing that powers a CB' then it's highly likely to already be regulated, you may be able to adjust it down to 12v or fairly close. There might be a pot inside if you're lucky, if not you could draw out the schematic and either work it out for yourself or ask someone here to help, it's probably only a component or two to be changed.
The diodes are do-able, forget the divider, the output voltage will vary according to the current drawn. You could use a low dropout regulator but why re-invent the wheel if the supply already has a regulator?
-- Clint Sharp
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