If have a circuit that needs 5V, and I have a battery that generates 9V, I can wire a couple of resisters into a voltage divider, and have a 5V supply. But the big "but" is that you're throwing away nearly half the energy in the battery, dissipated as heat in the resisters.
Do voltage regulators do the same, internally? Do they limit the current supplied externally by throwing away the excess? Or are they more friendly to battery life?
Second - batteries come with two ratings - Volts and Amp-Hours. (Or milliamp-hours in the usual sizes.)
Strictly by dimensional analysis, the energy stored in the battery is the product of the two - Watts = Volts * Amps, so Volts * Amp-Hours = Watt-Hours.
Suppose you have a 12V battery with a 60 mAH rating. If you plug it into a DC circuit with 600 Ohms of resistance, you'll get a 20 mA draw. And you can expect about 3 hours of battery life.
But suppose I have a circuit that has wants 3V and a 20mA draw, and I use a voltage regulator to drop the voltage. It's still a 20mA draw, so should I expect 3 hours of battery life? Or could I expect 12?
You have some good advice here. When you search for switching regulators, specifically look for DC-DC converters. A quick searrch on google should take you to a number of companies that make devices that will accomplish your goal for you. Based on your input, output, and current requirements, I would be surprised if you can't find a device in the < $10 (US) range.
That's a class of devices I'd never heard of before, and they provide some interesting options.
Consider the MAX619CPA - puts out a regulated 4.8-5.2V, from a 2.0-3.6V input. Supplies 20mA, with an input of 2.0V, or 50mA with an input
And with a shutdown pin.
Quite a neat little device - lets you run TTL logic off of a pair of primary cells, instead of those clunking 9-Volts. The next time I try to build something in a palm-sized box, I'll very much keep them in mind.
There's still 20 mA coming out of the 12V battery, so, yes, you get
3 hours battery life - 3/4 of the energy is thrown away as heat.
That's why they invented the "switching regulator" - it turns the supply on and off rapidly, storing the energy between pulses to feed the load smoothly - that _does_ increase the efficiency, but you still won't get 100%, but 80% to 95% are not uncommon.
Voltage regulators waste energy just like a resistor
AmpHrs is based on current coming from the battery. If 20mA comes out of the battery it doesn't matter how much of reaches the load.
The only exception is if the regulator is dc to dc convertor - a small switching power supply - This has the possibility of providing a lower voltage with greater efficiency however the gain may be negligable at low current levels.
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You can, if the current through the two resistors is very much more than the current needed by what is attached to the divider point. Otherwise, not so good. Another often not so good way would be if your 5V device current requirements are constant and then you can just use a single dropping resistor.
You'd be throwing away a lot more than nearly half, using a divider. Let's say your 5V device needs 10mA at 5V and that this 10mA is _very_ stable and doesn't fluctuate. If you simply use a dropping resistor, you could get away with "nearly half" by using a (9v-5v)/10mA or 400 ohm resistor. But with a divider, and you might use any quiescent current through it you want and calculate from there, you might pick a divider current of 20mA, knowing that half of it will go through your device. Somthing like this:
In this case, the power is 9V*20mA or 180mW. Your device needs
5V*10mA or 50mW. As you can see, 130mW is wasted. Way more than half. It's only in the limit-case where R2 is infinity, that less than 1/2 of the power is wasted.
They effectively have a dynamic R1 that changes its value according to the time-dependent device current demands. Roughly speaking, R2 is nearly infinite in linear regulator cases (almost.) So they do waste power and they would, in the case you cite, waste almost half.
Not the excess current, as their is no excess current. But they throw away the unneeded voltage. And to do that, they have to dissipate power.
Nope. Not unless you look at switchers, which can put some of the energy into magnetic or electric fields for repeated short times instead of tossing it away as heat.
Energy is the watt-hours thing. Yes.
If that's close to the draw they used in coming up with the 60mAh rating. Batteries do not have the same Ah-rating at all current draws you might propose and manufacturers pick the better figure (the peak or near-peak) of the curve over current loading. So if your draw is much different, expect that rating to be optimistic.
With a linear regulator, yes. With a switcher, you'd probably get better life than with the linear regulator. As always, mileage varies.
Batteries don't provide precise voltages. And while electric motors are fairly tolerant of imprecise voltages, digital circuitry isn't.
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