# Mild electronics question

Discussion in 'The Projects Forum' started by CDC, Sep 30, 2010.

1. ### CDC Thread Starter New Member

Sep 30, 2010
3
0
Hi there my name is Michael.

I have a Mains Voltage to 24VDC 2.5Amp transformer. I currently have hooked up to a long skinny LED light~30LED's for the purposes of lighting a fish tank.
Can i dimm LED's like i would a lightbulb? i understand basic electronic principles(ie built a electric motor and transformer, used picaxe and made and programmed a basic chip) but its been a long while since ive done any of it.

The lights too bright and i basicly want to know if i can put a dimmer on it?

2. ### kubeek Expert

Sep 20, 2005
5,378
1,019
Yes you can use a dimmer, using some kind of PWM modulation will get you the best results.

3. ### CDC Thread Starter New Member

Sep 30, 2010
3
0
pulse width modulation talks alot about wave forms, my LED's run of 24V DC. I have no idea how u are going to use pwm to dim my lights, do you?

4. ### SgtWookie Expert

Jul 17, 2007
22,198
1,797
See the attached.

If you're in the States, you can get the components at your local Radio Shack.

The LM317 is a voltage regulator. R1 and R2 set the output of the regulator to ~12v.

The LM555 is a timer; the way I've wired it up allows a reasonably constant frequency output where the duty cycle can be varied from perhaps 5% to around 98% duty cycle. With VR1=10k, the frequency is roughly 1kHz. You can use a 20k or 50k pot instead; the output frequency will be lower (~220Hz for 50k), but you will have a slightly wider PWM range.

LED1 and RLED represent your string(s) of LEDs.

I do hope you have used a resistor in series with your LEDs to limit their maximum current.

Calculating the maximum number of LEDs that you can wire in series:
LED_count = INT(Vsupply / Vf_LED)
where:
Vsupply = 24v - (24v/10) = 24v-2.4v = 21.6v
Vf_LED = The typical Vf of the LED at a specified current.
INT = The rounded-down integer value of the result of the equation within the parenthesis.

Calculating RLED:

R >= (Vsupply - (Vf_LED * LED_count)) / DesiredCurrent
where:
Vsupply = 24v - (24v/10) = 24v-2.4v = 21.6v
Vf_LED = the typical Vf specification at the current rating of the LED.
LED_count = the number of LEDs that you have in series.

If R is a negative number, you must reduce the number of LEDs that you have wired in series.

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5. ### CDC Thread Starter New Member

Sep 30, 2010
3
0
thank both of you for your interest in helping me.

sgt.wookie i cant make sence of your equations although i think i under stand the idea behind them. 1 thing i didnt mention is that thei got the LED light with its power supply ex shop front display.

maybe i should scrap the idea of using and electronic solution to my problem and just get some frosted glass or plastic to case the light it. A diffuser or something

6. ### SgtWookie Expert

Jul 17, 2007
22,198
1,797
Oh, ok - it's a pre-made string. Don't worry about my calculations then. Just use a 50k pot instead of a 10k pot.