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# how can you do this:

Discussion in 'General Electronics Chat' started by dana, Jun 3, 2008.

1. ### dana Thread Starter New Member

Jun 3, 2008
3
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I need help! I have a project that uses a small generator roughly 12v 2 amps. What kind of circuit do I use to power a bank of LED lights that use 25milli-amps? Knowing that LEDs are sensitive to the amount of amps how do I protect them and also, not put a lot of draw on the system? Is there a way to have an electrical circuit that generates the above current and voltage and light up the LEDs without harming them and without drawing a lot of energy using resisters? I looked on line and came across something called a current limiting resister. But I am not sure this is exactly what I am looking for.
Thanks
Dana

2. ### redacejr Active Member

Apr 22, 2008
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get a sensitive multimeter and a rheostat. set meter to millamperes. hook probes to power supply and rheostat in series and vary the resistance to obtain 25mA. turn meter to resistance and check resistance across rheostat and exchange the latter with a resisor with resistance greater by 50 ohms.... now you are set with no extra calculations...... i always do it like this and saves time

( something useful would be to attach a zener diode in series [10v-15v if supply needed is circa12v] with the leds to protect from current peaks, or a 2.1A fuse)

hope i helped

3. ### Wendy Moderator

Mar 24, 2008
21,838
3,047
Need some input as to their configuration. If the said LEDs are in series then you can reduce the resistor power dramatically, but you can't really get rid of the resistors (although you can replace them with current regulators).

Another approach might be a super simple switching power supply, which converts voltages extremely efficiently, to drop the 12V to a voltage a lot closer to the LED's forward voltage. Again, this reduces the power the resistors will disappate to a much lower value. Something as simple as a 555 oscillator with a variable duty cycle would get you there.

The less voltage the resistors drop the closer to your goal you will be. I've never built the above circuit, but the concept should work. Just stay aware of the 555 200ma current capability, although it will source a lot more after the conversion. If the bank of LEDs is using 600 ma say, the 555 and generator may be sourcing 150 ma (remember there is a conversion)

I've thought about trying to design a switching current regulator, but never have. Never seen one either, but I'm pretty sure it is doable. It would be extremely wasteful for one to three LEDs though.

Last edited: Jun 3, 2008
4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Along with the LED's current rating, you need to specify the typical Vf (forward voltage) - otherwise, it's difficult to impossible to specify an optimal configuration.

You also need to specify the range of voltage that the generator might put out. If it varies considerably, that will negatively impact the optimal configuration.

Jun 3, 2008
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6. ### dana Thread Starter New Member

Jun 3, 2008
3
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Just want to start off by thanking everyone. I am sorry I havent explained my situation; I have very little electrical experience. This may help. I am working on a patent project that generates energy from ocean wave swells. This is a miniature prototype that uses small dc pitman motors (used as generators). Depending how fast they are spun regulates how much voltage and current they produce. This device is so small that there isnt enough energy produced to run a plc to control it, so I have designed mechanical system to regulate the speed to produce constant dc energy. I want to be able to add additional loads on the generator to keep it in time with the rest of the machine. I am trying to use leds to give a visual representation of the loads.

Am I looking at the resisters correctly in this method: that if you have 10 volts dc and want to reduce it down to roughly 3volts for a LED, you lose a lot of energy in the resister, right?

Where would I find a current regulator? How do you measure the efficiency of this? Is this something radio shack has?

Thanks
Dana

7. ### Wendy Moderator

Mar 24, 2008
21,838
3,047
So far you are dead on with the resistor and loss. A current regulator isn't much better, it is nothing but a variable resistor.

A switching power supply will convert the voltage, as opposed to dropping the excess voltage through a resistor. The above scheme I drew is not a regulator, it doesn't give a fixed voltage out but is a ratio of the voltage in. A true switching regulator would do what you want, as it will adjust the output to match to a specific voltage that doesn't vary. The difference between a switching regulator (or convertor) vs. a linear regulator or current regulator is efficiency, a switcher is a LOT more efficient.

If there was such an animal as a switching current regulator that is what you need. The link shown in BkkChris's post is pretty close.

Are you looking for something like a bargraph display perhaps? Visualize the output you want, then let us know.

With the design I drew above, very low ohmage resistors can be used, say 10 ohms. They would drop around .25 volts, which would be a low level of loss compared to a regular resistor scheme, while doing the job. They would absorb about 6mw each, with the LEDs using around 62mw (assuming a Vf of 2.5 volts). You need to pay close attention to the LEDs and thier specs, in this application you can't mix and match different types. The other link is even more efficient I suspect.

Calculating efficiency revolves around wattage used. In the above example I calculated the LED used around 62mw, with a 10V power supply this translates into 6.2 ma, even though the LED is getting it's 25 ma.

What browser are you using to make your posts? They are pretty hard to read.

Last edited: Jun 4, 2008
8. ### BkkChris New Member

Jun 3, 2008
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Ah, given your additional info you may want to consider not trying to vary the current in the LED based on generator output but rather as a visual indicator perhaps varying a pulse frequency to the LED. This would allow supplying the generator output directly to the LED but as generator output increases the higher flash rate would make the LED appear brighter. Off hand without more details I'm not sure how you would want to go about this but it may be as simple as charging a capacitor from the generator and then letting it discharge through the LED. LED displays often use such pulsing in multiplexed displays like the old fashioned calculators and the duty cycle of the pulses controls the apparent brightness. ie. more on than off = more bright. Again the details of such a setup depend on details of your generator and probably some experimenting as well. Some power would be used to run an oscillator outputing to the LED (but that could be much less than what the LED would use) so most of your power out would go to the LED. By pulsing it you can put more curent through it and with choice of pulse width and frequency decide how to limit that power. Just an idea that maybe someone else help could extend.

9. ### Norfindel AAC Fanatic!

Mar 6, 2008
325
25
Why not use a voltmeter to check the output voltage?

Anyways, seems like your best bet is a switching power supply that can be powered from the generator, and output a constant voltage as efficiently as possible.