I've been reading about resistors vs. voltage regulators, but can't settle on which would be better in my case. I have a power supply for a mic preamp that has 15V and 48V. I want to add an LED to the power switch. Would it be best/most efficient to use an appropriate 1/2 Watt resistor, or a voltage regulator? I have both so I'm not worried about cost, but which is the "right" way to do something simple like this? The regulator I have on hand is the LM317T. I also have a 7805CV, and I think any of the 3 options will work fine with the 15V, but if someone could briefly explain which works best here or point me to some literature on the topic, I would appreciate it!
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\$\begingroup\$ And this is why I'm having trouble...the first 2 answers are opposing...and if voltage regulators are the way to go, is there a better one to use than what I listed? I don't mind ebay-ing a bunch of new ones if something works more efficiently. \$\endgroup\$– BrianCommented Nov 13, 2015 at 0:09
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1\$\begingroup\$ The part that confuses people is that your title is "15V to 3V". If you were trying to create 3V from 15V, then you would use a regulator. Since you are trying to light a single LED from 15V, then the resistor is a better choice. Even if you used a regulator, you would still need a resistor too. \$\endgroup\$– David DrysdaleCommented Nov 13, 2015 at 0:29
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\$\begingroup\$ Here's the power supply to help clear things up: imgur.com/an3C88Q. The switch you see the back of it I will change to a double so it can also turn an led on when I switch the power on. I'll come off of that +15V line into my resistor/VR \$\endgroup\$– BrianCommented Nov 13, 2015 at 0:34
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\$\begingroup\$ Bear in mind you may only need 2 to 5 mA rather than the full 20mA for a useful "power on" indicator. A regulator would be overkill. \$\endgroup\$– user16324Commented Nov 13, 2015 at 0:34
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\$\begingroup\$ The LM317 does have some snags . It is featured in the www .badbeetles.com website . \$\endgroup\$– AutisticCommented Nov 13, 2015 at 8:54
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4 Answers
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I want to add an LED to the power switch.
Any linear regulator or any resistor will "lose" the same amount of power when dropping a higher voltage to a lower voltage. If the input voltage is 15V and the output voltage is 2V then the power dissipated as heat is 13V x current. If the current is 20mA (standard red LED) then the power given off as heat is 260 mW. If you are happy to dissipate that power to activate a LED then a resistor is by far the superior choice because it current limits the LED naturally.
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1\$\begingroup\$ Heck, size the resistor for 10mA at the led Vf, bright enough for a power indicator, and use a 1/4th W resistor. \$\endgroup\$– PasserbyCommented Nov 13, 2015 at 2:35
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It depends.
What are the characteristics of the 15V supply? is it a nice stable 15V or does it vary? If the supply voltage is unstable is corresponding instability in the brightness of the LED a good thing (because it indicates a problem to the operator)? a bad thing (because it's a distraction to the operator)? or something you don't care about?
The resistor solution is simpler. The linear regulator solution isolates the LED from variations in the supply voltage which may or may not be a good thing.
The linear regulator solution and the resistor solution are both very inefficient (approximately equally so). The efficient solution would be a switched mode LED driver but that comes at a significant cost in terms of extra complexity and is probablly not worth it for a low current indicator LED.
answered Nov 13, 2015 at 0:15
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\$\begingroup\$ Here is the power supply: imgur.com/an3C88Q. There's not much to it. \$\endgroup\$– BrianCommented Nov 13, 2015 at 0:43
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If you are just driving an LED, you want to control the current, as the LED will set the voltage, so a resistor is most suitable. For almost anything else, where you want to set a fixed voltage, then a voltage regulator is required.
answered Nov 13, 2015 at 0:07
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Resistors Vs Regulators ...They waste the same amount of power as does an analog constant current source .The expected efficiency is 3/15 or 20% of the options so far.In fact this will make your LED less efficient than the incandescant lamp of last century.If efficiency doesnt bother you then stick to the resistor .If you think a SMPS is an overkill you are probably right.Well you have volts to burn in your circuit so place the LED in series with something that doesnt mind losing 3V and already has a sensible LED current already flowing .Double check that there wont be big current surges and now you have actually powered the LED for free .I have used the input of a lightly loaded 7805 so the idle current + the load current runs the LED .Remember that the human eye responds in a logarithmic way to brightness just like the ear registers sound .This means that the current doesnt have to be precise.
