Looking for opinions on LED dimmer circuit

Joel Moore · 2005-08-27T04:04:51+00:00

Let me start by giving credit where due. I found this attractive LED dimmer design on the following site:I'm not at all familiar with MOSFETs (and to be honest my general electronics skills are very limited...which is why I'm here) so I was a little uncertain about how to proceed in adapting that design for 24VDC.So I did some reading on MOSFETs and after getting even more confused about them I simply changed the layout of the LED array to work with 24V (6 LEDs and a 120 ohm current-limiting resistor in series) and plugged it in. No problem despite the fact that the MOSFET gate is now seeing nearly 24V rather than 12V. Then I connected 11 more of these LED strings in parallel and ran that. Still no problems.Here's where I'm a little uncertain. According to the article one IRF510 MOSFET should be able to handle up to around 16 LEDs before you have to get something more powerful. But in my case I have 72 LEDs running through a single IRF510. The chip doesn't even feel warm. Should I not be surprised that I can support so many more LEDs simply by moving to a 24V design or is the author of that article either mistaken or being overly cautious about his MOSFET?Is there anyplace I can read a simple description of how to design with MOSFETs? The back of the MOSFET package has a long list of parameters, none of which mean anything to me. What's important to consider when working with these things?And can anyone see a possible flaw in this design? It almost seems too simple to me. I don't want to implement this if it's likely to fail a few months down the road. Commercial LED dimmers can cost hundreds of dollars yet here we have a circuit for less than $10 (including a 555 timer which I'm not using in my case) which seems pretty effective to me. So what's wrong with it?Here's an ASCII version of my circuit:24VDC ----------- | | | | 10K LED ARRAY | | | | | Drain | | -- Gate...

Aug 27, 2005 24 Replies

Rate this thread:

Fred Bloggs 20 years ago

Well- just in time to not use it. You really don't have enough headroom to string 6x LEDs and a resistor in series with 24V. The HLMP datasheet shows the Vf as lying anywhere within 3.6V to 4V. If you go with 5x LEDs in series then you can get much better regulation and certainty of 20mA through the LEDs. A circuit that you can modulate with either your PLC or the 555 would look like shown below- just strap 555 pin 4 RST low when you use the PLC input- labeled EXT OC IN. Radio Shack sells a 20 pack of 2N4401 for $2.: View in a fixed-width font such as Courier.

. . 24VDC------------------+-----+------+------------------+ . | | | | . | | - - . | | v v . | | - - . | | | | . | | - - . | | v v . all transistors | | - - . | | | | . 2N4401 | [1.5K] - - . | | v v . | | - o o o - . [10K] | | | . | | - - . | | v v . | | - - . | | | | . | c - - . | |/ v v . EXT OC >-----------+-----+---| - - . IN | | |\\ | | . | c e c c . | \\| | |/ |/ . | |---+----+------- o o o --| . c /| |\\ |\\ . |/ e e e . 555 OUT>--[10K]--| | | | . |\\ | | | . e [1.5K] [150] [150] . | | | | . | | | | . GND ---------------+-----+------------+------------------+ .

Vote

Joel Moore 20 years ago

Fred Bloggs wrote in news: snipped-for-privacy@nospam.com:

Nice. I'll give that a try as I experiment some more. We still have around 200 PCBs here from the previous 12V design so it was really easy to modify them for 24V with a couple of cuts and jumps. Changing to groups of 5 LEDs will require a new board design (which will happen anyway for other reasons) but it sounds like it may be the right way to go.

And I like your idea of a circuit that can use either the 555 or the PLC output. Even when using the PLC we'll need a board for making the connections (LED boards are separate from that) so it would make sense to include the 555 controller right on there.

Thanks again.

Vote

Joel Moore 20 years ago

Fred Bloggs wrote in news: snipped-for-privacy@nospam.com:

I'm finally getting a chance to try this. Hopefully your newsreader software will bring this old thread to your attention because I have a couple of questions:

1) What is the purpose of the 100uF cap between Vcc and GND?

2) I want to understand the design around the zener diode. Obviously the goal is to supply the 555 circuit with 12V. How do you calculate the biasing resistor value? I'll probably be powering 2 or 3 timers in this circuit so I imagine the current draw will be 2 to 3 times higher.

And what about that .47uF cap in parallel with the zener? What is that doing?

Thanks for any advice,

Joel Moore

Vote

Richard Crowley 20 years ago

Both capacitors are there for "noise reduction" (electrical noise on the power supply rails). Always good design practice to eliminate any odd sources of circuit malfunction. Kinda like your grandmother reminding you to put on both your jackets and mittens before going out in the snow.

I have found this web page:

formatting link

to be helpful for calculating the values of the dropping resistor in a zener shunt circuit like this.

Vote

Joel Moore 20 years ago

"Richard Crowley" wrote in news:dfpoo4$8f2$ snipped-for-privacy@news01.intel.com:

Ok, that's what I kind of figured. I guess the values then are fairly independent of the rest of the circuit (i.e. frequency of PWM, etc.)?

Bookmarked. Thanks.

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required