12V from a 9V battery?

[cb]

I'm almost certain I saw a post on this a while back, maybe several years ago. I searched and could not find anything. There are plenty of posts about doubling the 9V, but I need 12V. Suggestions?

[Mark Hammer]

Yes.  You use a charge pump to essentially double the voltage to 17v from the basic 9 (there will be two diodes' worth of voltage to subtract, which is why it's 17 not 18), and you feed THAT to a standard 3-pin 12v regulator.

[cb]

Thanks, Mark. Is the regulator preferable to simply dropping the 17V to ~12V using the old diode trick (would take about 8 diodes I think)? Also, do you know the current consumption (roughly) of a regulator? I'll have to look that up.

[Mark Hammer]

I don't know that it is necessarily preferable, but here's the deal: the diodes will simply subtract X number of volts off the top of whatever you feed it, whereas the regulator will deliver the same 12v until the pumped voltage drops to around 14V.

[earthtonesaudio]

Note that the current coming out of a charge pump is dependent on capacity and oscillator frequency.  High current loads cause the voltage to drop.

[alparent]

Yes.  You use a charge pump to essentially double the voltage to 17v from the basic 9 (there will be two diodes' worth of voltage to subtract, which is why it's 17 not 18), and you feed THAT to a standard 3-pin 12v regulator.

Could I drop the voltage using a zener diode? Would that be better?

If so..............next question.................how would I do that? 

[R.G.]

Upvert and regulate is usually better than upvert and subtract-a-constant, which is what diode droppers and zeners do.

This generalism is based on the fact that upverting with a charge pump adds noise at the switching frequency of the converter and it's harmonics. If these are high enough, it may be OK, and it may also be more or less easy to filter them with R-C networks, but if you start with a regulator that has a lot of noise rejection to start with, it gets easier.

If you simply upvert to 17V, adding, say, 100mV of noise/ripple in the process, a regulator may get rid of 30-40db of that, down to 1-2mV of ripple. A diode or zener that simply wipes off 5V will leave all of the 100mV of noise/ripple there, simply subtracting a constant from the upverted voltage. Usually, less noise is better.

Note to subjectivists: Yes, feedback regulators have their own issues. But for most purposes, regulated voltages with ripple reject and low impedance are preferable.

[alparent]

Now that's a reply!

Thanks for educating me R.G.

[PRR]

Do you really NEED 12V? Or are you just copying from someone who had 12V handy?

How much 12V?

If over 10mA, double to 17V and reg to 12 makes sense. (The regulator steals 5mA in addition to your load. There are thriftier regs but I don't know a part number.)

If under 1mA, resistor+Zener may be just as thifty, even allowing 1mA-2mA for Zener, and perhaps smaller or simpler.

If you need BIG 12V, like 100mA..... first, the average 9V battery will poop-out with a doubler sucking 200mA, and then it is wasteful to reg-away a third of what the reg outputs. If battery cost will matter, you would love a smart up-converter which would transfer just-enough charge to make your 12V without gross waste. However the learning-curve for switchers, even with National's Switcher Workbench, eludes me so bad that I would shop for a different plan (including dog-collar 12V batts, motocycle batts, etc).

[alparent]

Do you really NEED 12V? Or are you just copying from someone who had 12V handy?

Don't know? you tell me!

I was looking at this http://www.diystompboxes.com/smfforum/index.php?topic=70800.0