# LED to drop voltage?

I'm working on a project that runs on 4.5 to 3.3 volts from 3 AA batteries. I just realized that one component can only handle Vcc = 3.6 - 1.9 volts. I really, really, don't want to use a power regulator.

Can I stick an LED and resistor in series of the Vcc to the component and use the forward voltage as the Vcc.

Say for example:

LED can handle 20mA. Component draws 12mA. With a 10K resistor at 4.5v, the LED will draw .45mA and the forward voltage will be 2.4v

4.5v--- LED---10k---Component Vcc

Will this work?

Huh... Or will a voltage divider be a better way?

Depends what this "component" is.

If you need to drop a volt or two, a silicon diode will drop the voltage by around 0.6V. If you need to drop more, you can connect diodes in series.

Without knowing what the component is, can't be sure it doesn't need a stiffer supply, perhaps with decoupling?

Sorry I forgot to call it out. The component is an nRF24L01+ radio.

The voltage divider will not work because of the load... or so I read.

I'm thinking that a zener diode may do the trick.

Why not just put a 3.3V regulator in? Footprint will end up being the same as all the other components you're considering adding.

The proper solution is probably an LDO (low drop-out voltage) regulator.

But, you deserve credit for a creative idea...

IF the device really draws 12mA from the power supply it might work... Double check that. I popped-up the data sheet, and it says it transmits & receives 12mA. If it draws too much current, you could blow the LED, and if the LED blows it could short-out and put the full-supply voltage across your nRF24L01. (In reality, the nRF24L01 probably won't get fried with a couple extra volts.)

But, you don't need the resistor. When you put things in series, the voltage gets divided depending on the resistance. (The "resistance" of the LED depends on the voltage and is not constant). And, the current is the same in all of the series devices. With the nRF24L01 "limiting" current to 12mA, the LED will also have 12mA flowing through it, and will glow less-brightly than at 20mA.

I'm thinking that a zener diode may do the trick.

I believe the lowest voltage zener you cna find is 4.7V. As John suggested, regular diodes in series will drop ~0.6V each, so 3 diodes should do it.

CrossRoads: Why not just put a 3.3V regulator in? Footprint will end up being the same as all the other components you're considering adding.

I'm trying to keep a very low power draw through the entire circuit. Most of the regulators that I found drew at least 5mA by themselves. As it stands now, I should be close to .7mA for the sleep/stand-by current on my project. Adding 5mA to that flushes my low power plan down...

DVDdoug: I believe the lowest voltage zener you cna find is 4.7V. As John suggested, regular diodes in series will drop ~0.6V each, so 3 diodes should do it.

I just need to chop 1.0 volt, so really 2 will do but if I'm reading the data sheet right... (maybe not!) the NTE134a will work. It has a 1.2 forward voltage drop and can pass 200mA.

http://www.nteinc.com/specs/5000to5099/pdf/nte5061a.pdf

Many modern LDOs only draw a few uA quiescent current. Have a look at the MCP1700, 1.6uA.

The 1v2 forward drop on that Zener is at 200mA, there's no graphs to show what it is at lower current but I would expect a lot less. Any form of unregulated drop will vary if the current varies, does the module always draw the same current?

Rob

Graynomad: Many modern LDOs only draw a few uA quiescent current. Have a look at the MCP1700, 1.6uA.

nope... while sleeping it goes from 13mA down to uA.

Graynomad: Any form of unregulated drop will vary if the current varies, does the module always draw the same current?

Rob

Ooooooooooo... now we're talking! Too bad I didn't find this when I ordered last week!

while sleeping it goes from 13mA down to uA.

I can't see that in my data sheet, all the graphs talk in uA. Also it's a 3-pin reg, there is no sleep/shutdown pin.

Can you point me to the data?

Rob

Sorry for the confusion. I was talking about the radio (load) not being constant. The voltage regulator is perfect! Just ordered 10!

Good luck.

Rob

Assuming these are LED’s by the title of the thread, why not just wire them all up in parallel so that your voltage doesn’t drop if you were to wire it up in series. That way you can put a resistor on the positive terminal and then all the leds will wire up to that. The resistor will have to be calculated so just go here and scroll down to calculate it http://ledz.com/?p=zz.led.resistor.calculator

That way the voltage won’t drop although the current will. So the batteries won’t last as long although you won’t need as many of them at once to power the whole thing.

I believe the lowest voltage zener you cna find is 4.7V

Not true, I have a load of 3V3 ones, you can get lower if you want.
If you use a zenner or LED, then there is a lot of noise generated so be sure to use a good capacitor across your load.

Yup, lower is available. Digikey shows 1.8V parts: http://search.digikey.com/us/en/products/MMSZ4678T1G/MMSZ4678T1GOSDKR-ND/1140204