Re: Looking for opinions on LED dimmer circuit

On Fri, 26 Aug 2005 23:04:51 +0000, Joel Moore wrote:

> Let me start by giving credit where due. I found this attractive LED
> dimmer design on the following site:
>
> http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10
>
> 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 IRF510
> | |
> | Source
> | |
> 470K |
> | |
> | |
> -----------
> |
> Sinking
> PWM
> Output
> |
> GND---------
>
> Thanks for any input on this.
>
> Joel Moore

Joel,

One very important rule with MOSFET's such as the IRF510 which you appear
to be breaking is that the Voltage from gate to source must not exceed
some critical value. This is usually 20 Volts. If you are really applying
almost 24 Volts to the gate when the source is grounded, then you are
playing with fire. The data*** or package will call this Vgs max or
something similar. You can fix this by tweaking resistor values so that
the gate voltage is lower. For example, change both resistors to 10 k,
then the maximum gate voltage will be around 12 V which should be fine for
you. How fast is your PWM going, by the way? If it is really fast, then
you might use smaller resistors than 10 k, but if it is slow, you could
use larger resistors.

But more importantly, I don't think your circuit makes any sense. The
MOSFET is nearly useless, if you really built the circuit as drawn above,
because when the PWM output is low, the MOSFET will be on, but the PWM
will be sinking all the current that flows through the MOSFET, and when
the PWM output is high, there will be no potential from the drain to the
source, and hence no current flowing. So you could just as easily remove
the MOSFET from your circuit above, and replace it with a short circuit
and there would be no change in behavior. The whole point of the MOSFET is
to handle large currents, or possibly to act as a variable resistor
capable of handling some power dissipation. In a PWM circuit, I would
expect the former.

This leads me to believe that you didn't really wire it that way. You
probably have the PWM output going to the gate of the MOSFET, and the
source of the MOSFET is directly grounded?

The LED array should also contain some kind of current limiting resistor
setup, as mentioned in the article. It might be helpful if you drew out
the details of the LED array, since they matter.

Also, is your PWM circuit getting hot at all? If you DID really wire the
circuit as shown, I imagine there is a lot of current going through it,
and I would expect it to get warm.

Anyway, if you use the MOSFET as a switch, you can put a lot of current
through it (two amps maybe for the IRF510? I don't know the exact number
off the top of my head, but it would be called Id max). As long as you
are not exceeding Id max, then you can keep connecting LED's until the
FET feels warm. It may be possible in some situations to exceed Id max
without the MOSFET getting warm, so you want to do some calculations to
verify that you are not doing that. (To do this, we would need to know
details of the LED array box.)

--Mac

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