EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Elf Six on January 30, 2014, 01:16:59 am
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Hi guys
This is a sort of general question. If a voltage is to be lowered from, say,
40V or 50V to 12V or 20V for a short duration, would a voltage regulator
or a resistor(cement high-W) be best? What are the pros and cons, and
do either induce any time of delay?
I assume that the 78xx will be plenty for lamps etc, but can they also drive
a strong 20V coil? Aren't they about 1 amp?
Any insight is welcome
Thanks
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The voltage regulator has a limited current capacity, and as most globes are lower resistance when cold the surge could be more than it could handle without a npn or similar to dump the wattage and handle the current (look at a lm317, they have example circuits of this)
the resistor is fine if your not concerned with it being be all and end all accurate, just be aware those cement resistors may be rated at a few hundred degrees for full load, and i have seen them desolder themself from the wires feeding in,
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The problem with a resistor is that it obeys Ohm's law. In other words, the amount of voltage drop that it produces is proportional to the current. That might work if you have a steady input voltage and a steady current, but if not, your output voltage is likely to vary widely.
A voltage regulator uses active elements and feedback loops to produce a relatively constant output voltage, regardless of variations in current, and regardless of variations in supply voltage, as long as you stay within the limits of the data sheet. A linear regulator sort of behaves like an automatic adjustable resistor, where the automatic machine watches the output voltage and quickly adjusts the resistance to provide the specified output voltage.
Yeah, the 78xx is good for about an amp, when properly heatsinked. You can use a pass transistor (or several) to provide additional current capacity.
Going from 50V or so to 20 V or so means dropping around 30V. At an amp, that's going to be around 30W. That's a lot of power to get rid of. You're getting into territory where a switching regulator is probably a better fit, though it has its own set of issues to deal with.