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Author Topic: DC dimmer circuit  (Read 2921 times)
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In theory, if I did pull the gate of a p-channel mosfet to ground, would that not turn it on rather than off?

If the gate of the P-channel mosfet is grounded while the source it at +12v, then that would turn the mosfet on. However, a pulldown resistor on the gate will not drive it to ground in that circuit, because the output resistance of the TC4429 is much less than 10K.
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Why did you choose a target time of 50ns? We're not talking about switching at several MHz here, just a few tens of kHz.
There is no qualified reason to design for less than optimal performance other than cost/size (power, drive circuit) and so any PWM design irrespective of base frequency should aim at minimizing switch time. The practicality of this is typically limited downwards to somewhere in the 20ns-60ns range for any given MOSFET. I suggested 50ns as a target in a previous post and 80ns is what is used for characterization in the datasheet.

Other than that it seems as if the dimmer is shaping up nicely.
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Well I FINALLY had a chance to get this circuit together. smiley-eek

Life is what happens to you when you are trying to work on electronics projects... smiley

The circuit is now set up as in the attached diagram. However something is not quite right. For testing, I am running the circuit on 11.9 volts. When D10 is high the output from the TC4429 reads 11.65 volts on my multimeter and 11.9 when D10 is low. Obviously, that is not enough to turn the IRF5210 on.

May I once again appeal to the experts for some advice?

Thank you for your patience.


* DC-dimmer-circuit-3.jpg (44 KB, 874x700 - viewed 22 times.)
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You have drawn the mosfet the wrong way round, however if you have wired it up according to the S G D labels in your schematic then you have wired it correctly.

You haven't shown the pin numbers you used for the connections to the TC4429. Check these connections, in particular make sure you have connected both ground pins and both Vdd pins (the datasheet says this is required for proper operation of the device).
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Hello dc42!

Well, I have quintuple checked the connections multiple times. (My brain has involuntarily been going over every little detail for hours)

I wired the IR5210 according to the datasheet where pins 1, 2, and 3, are G, D, and S, respectively. (See attached)

WRT the TC4429, I have all the pins connected except for 3 which is supposed to be left unconnected. The chip is mounted upside down on my circuit (again see attached), so the pin numbering is clockwise from the top-right.

Even now my gut feeling is that something is "misconnected," but I can't see anything wrong.


* Screen Shot 2012-07-17 at 09.17.14.png (83.89 KB, 715x472 - viewed 14 times.)

* Screen Shot 2012-07-17 at 09.19.15.png (390.35 KB, 552x532 - viewed 13 times.)

* Screen Shot 2012-07-17 at 09.16.10.png (36.85 KB, 347x189 - viewed 11 times.)
« Last Edit: July 17, 2012, 11:54:06 am by Pokey » Logged

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The resistor shown as 10K in the schematic is actually 1K in the photo, but that doesn't matter. The wiring to the chip in the photo looks OK as far as it goes, assuming you have the chip the right way round (as it is upside down, I can't see which end has the pin 1 marking). Can you post a photo of the whole setup, so I can see how the chip is connected to the mosfet, Arduino and power supply?
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Thanks for your time dc42.

Here is the whole circuit. The violet wire from the top is 12v and the red wire (top-right) is ground, both from the power supply. There are several other things going on too, but i hope you can make out the connections for the mosfet.

I should note that the middle pin (pin 2, D) on the IRF5210 is bent forward a little, it is not attached to the resistor.


* full-circuit.jpg (368 KB, 1200x796 - viewed 17 times.)
« Last Edit: July 17, 2012, 04:21:25 pm by Pokey » Logged

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I can't see anything obviously wrong with the wiring in your photo. There is the possibility that the mosfet gate oxide may have been damaged by static charge. Try disconnecting one end of the 47 ohm resistor and measuring the output from the TC4429 again. Also check that the ground pins of the TC4429 really are at 0v all the time, in case you have a bad joint.
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Try disconnecting one end of the 47 ohm resistor and measuring the output from the TC4429 again

Okay, I tried that and I get the same result 11.99v when input is high and 11.65v when input is low.

Quote
Also check that the ground pins of the TC4429 really are at 0v all the time, in case you have a bad joint.

It reads a perfect 0v.

Do you think the TC4429 is bad? Luckily I ordered two; I will try replacing it.

TNX
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Hooray! It works!

Replacing the TC2249 solved the problem.

Thanks, dc42 for helping me track this down.

(I'll add another one to your tab. smiley)
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Well, the new TC2249 is now exhibiting the same problem. The only thing I did was to set it to output a 32 μs pulse width. I was getting strange readings so I set it back to a simple digitalWrite HIGH.

Now I get a similar 11.7v on the TC2249 output instead of the expected 0v. I was careful to ground myself when working on it. Is it possible that 30kHz is burning something out?

Is there a more robust way to drive a mosfet?

TNX
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I would expect the TC2249 to be quite robust. The only ways I can think of causing it to burn out and exhibit the symptom you report are to short its output to the +12v supply, or to drive continuously at a high frequency with a substantial capacitive load.

What value resistor do you have between the TC2249 output and the mosfet gate?
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What value resistor do you have between the TC2249 output and the mosfet gate?

47 ohms.
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What value resistor do you have between the TC2249 output and the mosfet gate?

47 ohms.

That should be sufficient to protect the TC2249 from just about any load. I guess the other thing that could destroy the TC2249 is a large negative-going transient on the +12v supply. Such a transient could possibly occur when the LED switches on or off. To avoid such transients, add a decoupling capacitor between the -ve side of the LED and the source of the mosfet.
« Last Edit: July 20, 2012, 04:27:10 pm by dc42 » Logged

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How about using a motor driver such as an SN754410?

Or a TI UC2714?

My opinion of the TC2249 is at an ultimate low. I am not sure I want to order more...although its failure is probably a result of my inexperience.

Thank you for your help, dc42.
« Last Edit: July 20, 2012, 06:40:23 pm by Pokey » Logged

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