EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: questOn on January 15, 2013, 02:55:16 am
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I was tearing apart device with 3,6,9Vdc outputs all powered from a sealed 12V battery..I thought Id find a couple linear regulators( or something more complicated) but to my surprise it was just a string of 12 diodes.. tapped every 4 diodes ( guessing forward bias is about 0.7~0.75V ) so each tap gives a three volt drop..clever but Is it? How does this compare to having, say a couple of buck switch modes each configured to give a separate output? I would guess that the voltage out would be more constant with a smps (as well as the load current) as the input voltage varies (battery drains).
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It's not very clever because the output voltage will not only vary a bit and power is wasted in the diodes, depending on the current (how much the thing you connect to it pulls).
The diodes will get hot and their voltage drop will change with the heat increase (usually) and you're just wasting power in the diodes when you have a battery with limited power available.
Linear regulators pretty much would work the same, but they're provide much smoother output, but they'll still waste the volts difference as heat. No efficiency increase (if you output 9v out of 12v, your efficiency is basically 75% while for 3v it would be just 25%), just more stable output voltage.
Buck regulators (switching controllers) would be better, and with a smart selection you could get as much as 85-90% efficiency, but they require more components such as an inductor, a schottky diode, 2-3 capacitors and resistors.
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Usually they do that for PRICE ! 12 diodes cost about $1.00 (in small bulk), ANYTHING clever is a few $s, plus you now need a PCB.
Many items can handle a small Voltage variation, so in those cases there's NO need for anything fancy. And by fancy, I mean an
LM317, 6 resistors and 2 caps + PCB.