Joined: Fri Aug 30, 2002 1:09 am
Posts: 8542
Location: Southern California
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Quote: Garth, what options exist if you want greater current drive on the +/- 12V rails? Would a series-pass bipolar transistor be sufficient to provide additional drive, and if so, how much can be expected? Without delving into it to see if I'm forgetting something, I think all the switched-capacitor voltage converters have four switching FETs; so a single transistor won't be enough. These are just a really easy solution for when you need a negative voltage or a higher voltage at not a lot of current or tight regulation. You can parallel a couple of these though, if you need just a little more current than you can get with a single one. It's still small, cheap, and easy.
I have in front of me a page from EDN magazine's "Design Ideas" section from 14 years ago for using a 7406 and a 20µH inductor, a diode, and a few common resistors and capacitors, all amounting to less than a dollar, to derive +12V from +5V, which I've gotten the parts out for to make a tiny module and try it. I'll try to remember to post here about it when I'm done. [Edit: It works, but the regulation is terrible.]
From there the next step would be switching regulators. For hobbyists, I would recommend ready-made ones like the PowerTrends integrated switching regulator modules. TI now owns PowerTrends. Actually there are lots of companies making this kind of thing, often with three connections: power in, ground, and power out. If they have more pins, there may be redundancy, and/or a pin to set the output voltage with resustors, and/or a shutdown pin, etc.. I can look through a fat file and list them here if you really need it.
There are loads (at least hundreds, maybe thousands) of switching power supply controller ICs on the market, many from Maxim, Linear Technology, and of course other companies; but I would not be very enthusiastic to recommend them for hobbyists. The reason is that the extremely high di/dt's and other things that go with the turf make PC board layout very critical. If you don't do it right (you won't on your first try!), you'll get a supply that's noisy, inefficient, and possibly unstable. They cannot be breadboared and expected to work well. I just did an SMT board layout for our company using one of these that only comes in a tiny package with leads on .020" centers. It worked very well, but it definitely would not have if I had tried it when I was still new to switching supplies. If you want to try one, get something with a lowish switching frequency. For commercial products, high switching frequencies are usually preferable, because they can get by with smaller inductors and capacitors; but they're not friendly to hobbyists. One I've designed into several products that's available in an 8-pin DIP is the MAX732 with 12V output. Input range is approximately 4.5V-9.5V. If you decide to try it, contact me and I'll tell you the tricks to making it behave well.
For another part of the same product, I designed a switching regulator for charging a lead-acid battery using a TIP125 transistor, an LM339 quad comparator, an inductor, and a bunch of cheap discretes; but the different set of requirements allowed using cheaper parts and save several dollars per unit.
Quote: I'm just asking because, I'd think that a 100mA drive current would be better, as it'd allow for daisy-chaining multiple 12V-dependent peripherals.
I feel like I'm kind of beating around the bush; but there are quite a few ways to skin a cat, and here the best way might depend on what power source you have to start with and other factors. If you have something particular in mind, run it by me and I'll see if I can do you any good.
Quote: There are 1U ATX power supplies with 0.8A of power max for the -12V rail.
That's only for plugging into the wall though, right? There would need to be some other provision if you need to be able to operate portably from a battery. Your application may not need it, but I like to try to leave possibilities open in order to avoid painting oneself into a corner._________________ http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?
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