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Author Topic: Comparing PWM LED dimmers  (Read 3785 times)

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Comparing PWM LED dimmers
« on: December 01, 2014, 03:01:15 AM »
Hi all
I am a newbie, this is my first post - hello to everyone.

I am working on some small LED circuits for using in scale models, and found the PWM LED dimmer on this site very useful (http://www.pcbheaven.com/circuitpages/LED_PWM_Dimmer)- so thanks Giorgos for that.

The circuits I have are using a 9v DC battery, and the LEDs are pairs in series with a 100 Ohm resistor in series. I have two PWM dimmer circuits coming off the battery, each with 10 LEDs. Your controller works well.

However, I recently received in the post a PWM LED controller I found on Amazon, very cheap (?2 incl P&P in UK -
http://www.amazon.co.uk/gp/product/B008TNH68E?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00#productDetails ) It was spec-ed at 12v-24v. It seems to work with 9v, but not 4.5v.

It has some extra components - as well as a 555 timer, it has a LM358 op-amp, a D472 FET, and a 78L05 (5v voltage regulator).  I have attached a photo - you can see the circuit and components from it.

According to an Amazon reviewer who seems to have done some technical research on this unit, it is modulated to 1khz, and draws 7.65mA when 'off'.

My question is - what is roughly the difference between your PWM dimmer and this one?
* And is it that the Amazon one has a higher (though un-needed) capacity? (It's rated at 96watts).
* What frequency is yours modulate to?
* How might the two PWMs compare in 'efficiency'?
* The Amazon one says 12v-24v, but yet it works with 9v. Is there anything wrong with using it at 9v - will it breakdown because of this?

I am asking because although I was able to make a PWM controller to your design using stripboard, there are other model-makers in a forum I am part of who also want to use PWM LED dimmers, but are not confident about making a circuit board (even though it's not a difficult one). If there are reasons why Giorgos's design is better or more appropriate, I will encourage them to persevere with making it, if not I guess they may choose to use the Amazon one, because it would be much less work for them.

I hope this an acceptable topic to raise on this forum.

Thanks again for the 555 DWM dimmer design, and all the other projects on the site.

« Last Edit: December 01, 2014, 08:42:30 AM by kam »


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Re: Comparing PWM LED dimmers
« Reply #1 on: December 01, 2014, 09:18:46 AM »
Hello Jim to the forum, I'm Giorgos, with my nickname ;)

So, from what i can tell, the CN circuit uses the 555 to generate the capacitor charge-discharge waveform and the op-amp is used as a comparator to generate the PWM. I guess this is how it works because i do not see pin3 of the 555 being used.

Check out this circuit. I use the same technique to "extract" the nice and smooth charge-discharge waveform out of a capacitor and then use it to fade-in-out LEDs:
Keep this for your reference to know how it works. The difference between the LED breather and the CN dimmer is that there is no resistor (most likely) connected to pin 7 of the 555, so the discharge is done rapidly, effectively generating a sort of curved sawtooth.

Now, the comparator with the potentiometer is definitely used to extract the PWM out of a sawtooth generator. Check out this old circuit of mine. It uses precisely the same technique.
For your reference as well. The 741 (op-amp) at the end of the circuit is used to compare the DC input at the inverting-input (-) with the sawtooth waveform at the non-inverting input (+) and extract the PWM. In your case, the sawtooth comes from the trigger of the 555 (pin 2) and the DC input is directly from the middle of the potentiometer. Read my article on the page i sent you to find out how it works.

So, now that we know how both PWM circuits work, we can compare them. The fact that the CN has the 78L05 is good because it can operate at a wider range, while my circuit is limited to the 555 voltage limitation (that is +15V). The CN has also a reverse voltage protection 9the diode on the right side). Plus, the Cn has a better transistor. Overall, right off the bat the CN is better.
The 555-op-Amp PWM generator is not necessary at all, its waste of money. A simple 555 can do the job just as well. The simple 555 has a small drift in frequency when changing the PWM duty cycle though. But when powering LEDs, this is absolutely unnoticeable. The frequency may change from say 1.5KHz to 1KHz when changing duty cycle, but hey... Can you notice that? The 555-op-Amp circuit on the other hand has fixed and stable frequency, but who cares? If you want perfect PWM, go ahead and make this circuit instead: http://www.pcbheaven.com/circuitpages/Voltage_Controlled_PWM_Generator/
With this triangle wave oscillator: http://pcbheaven.com/circuitpages/Triangle_Wave_Generator/
But... why????

So, if you want to convert my circuit to be better than the CN, you should add:
1. reverse polarity protection by adding a series diode right at the + of the power supply (1N4007 diode corks fine)
2. voltage regulator with the LM7805 to get stable 5V for the circuit operation at higher voltage than the 15V limitation of the 555
3. Different transistor to get as much output power as you want.

Check out this circuit for example:
Its precisely the same, works at higher frequency (to avoid acoustic noise) and has huge output fet, the IRF520 (or IRF540). Its the 2nd circuit that you want to see, titled "What about the 3-wire and the 2-wire fans?". Instead of the T1 2N2222 transistor that i use for the LEDs, replace it with the Q1 IRF520 fet. Remember to change also the pull-up resistor R1 from 4K7 to 1K.

And you're done


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Re: Comparing PWM LED dimmers
« Reply #2 on: December 02, 2014, 12:23:26 PM »
Thanks very much Giorgos,
I really appreciate that explanation and comparison of the two PWM dimmers. Thanks for generously taking time to write, and sharing your knowledge freely.

I think I understand the difference between the two, and how two circuits can do more-or-less the same thing though one has extra ICs. For the use I intend, yours is perfectly fine, those are un-needed extras.

After finishing this, my next project will be to go back to my original intended use of the 555 - making PWM controllers for fans in a silent PC I am building. I can see there's a design for this on pcbheaven, so my next post will be about that.

Take care,