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Author Topic: Dimming 1w leds with transistor and walwart issues  (Read 2312 times)
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Best scenario of using a "bench" computer power supply is that I can power my leds and arduino straight off the 12v rail of the power supply.
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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1. Switch-mode constant current supplies such as the one you linked to are not intended to be interrupted with PWM. It may work, but you may end up putting large current pulses through the LED and shortening its life and/or the life of the constant current supply. Look for a supply with a separate PWM input, such as http://www.led-supplies.com/led-drivers-LED-power-supplies/1-10V-PWM-Dimmable-Constant-Current-LED-Drivers.html or http://www.ebay.co.uk/itm/1W-350mA-Constant-Current-LED-Drive-PWM-Dimmer-DC-DC-Buck-Power-Supply-Board-/140895009844?pt=US_Lighting_Parts_and_Accessories&hash=item20cdff3c34.

2. The reason why your 2N2222 gets hot is that it has a high saturation voltage (up to 1.6V @ 500mA from the datasheet). You need a transistor that has high current gain and low saturation voltage at 350mA. A ZTX851 would be ideal. Even a BC337 would be run cooler than the 2N2222.  You can certainly use a mosfet instead, but you do, look for one with logic level gate drive, not the IRF510 (which is designed for 10V gate drive).
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Thank you very much for the explanation!

So the switch-mode constant current drivers shouldn't be pwm'ed?

So are you saying the ZTX851 would be better to use than a logic level mosfet but still not better than getting a constant current source with a dedicated pwm pin?

Can I build this circuit instead of buying a different pwm constand current driver?
http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step8/a-little-micro-makes-all-the-difference/
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a) Yes.
b) Yes.
c) Yes.

Note: A switch-mode driver will be more efficient in terms of power use than the MOSFET circuit - something to think about if it's running 24/7 (the MOSFET circuit turns the excess volts into heat!)
« Last Edit: December 08, 2012, 07:23:37 pm by fungus » Logged

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a) yes
b) both the ZTX851 and the logic level mosfet are good solutions at this sort of current
c) yes. Use the lower of the two schematics. Calculate how much power the mosfet will dissipate, which is (V_supply - V_leds - 0.65) * I_leds, and provide the mosfet with a sufficient heatsink.
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Alright I got four of the second circuits all setup in eagle.

The mosfet will dissipate (5v - 3.3v - 0.65) * 0.350A = 0.37w
-where did 0.65 come from?  Voltage drop of the Logic Level FET?


According to his formula to find the resistor value for R3 I should be using a 1.5ohm resistor, do these exist?  If I bump it up to 2 ohms my max current goes down to 250mA which is acceptable but still lower than i'd like.

He is using a Fairchild FQP50N06L for the Logic level FET, and a Fairchild 2N5088BU for the NPN transistor with a 100K ohm resistor for R1.  Will all this work as planned?

I appreciate everyones help very much, I am learning quite a bit!
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0.65V is the approximate voltage drop across R3 in that circuit. You may also see it given as 0.6 or 0.7. Its exact value depends on the type of Q1 and on the temperature.

A 2 ohm resistor should give you around 325mA, if Q1 is a typical small signal transistor at room temperature. You can get 1.5, 1.8 and 2.2 ohm resistors easily, and 2 ohm resistors perhaps a little less easily. I think 1.8 ohms will give you close to 350mA.

FQP50N06L is a suitable mosfet to use (as are many others). 2N5088BU is a little obscure and you may wish to use something more readily available, such as BC547, BC337 or 2N3904. Any small signal silicon NPN transistor will do.

PS - if you want fine control of the current, then for R3 use a 2.2 ohm resistor in parallel with a second resistor of about 10 ohms. Adjust this second resistor to get the current you want.
« Last Edit: December 09, 2012, 04:15:18 am by dc42 » Logged

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Quote
So the switch-mode constant current drivers shouldn't be pwm'ed?

The answer is more complicated. Some smps are specifically designed to be pwm'd - they typically come with an on/off pin. Others are not.

Quote
So are you saying the ZTX851 would be better to use than a logic level mosfet but still not better than getting a constant current source with a dedicated pwm pin?

ZTX851 isn't a good part. The drive requirement for 300ma Ic is quite substantial for a mcu, and if you cannot deliver that much current, you see Vce going up and power dissipation up.

I would go with a to220 mosfet, just to be safe.

Quote
Can I build this circuit instead of buying a different pwm constand current driver?

You can, with the understanding that if you are using a mosfet as the regulator, leave it 1v drop at least.
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ZTX851 isn't a good part. The drive requirement for 300ma Ic is quite substantial for a mcu, and if you cannot deliver that much current, you see Vce going up and power dissipation up.

The drive requirement of 15mA base drive current for 300mA Ic is less than half the current capability of an Arduino output pin.
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According to his formula to find the resistor value for R3 I should be using a 1.5ohm resistor, do these exist? 

Yes. Don't forget that the resistor will be a little heater, too. At 350mA it will produce over 1/2 a Watt of heat. I'd get at least a 2W resistor to give it a chance of not burning your fingers.
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Yes. Don't forget that the resistor will be a little heater, too. At 350mA it will produce over 1/2 a Watt of heat. I'd get at least a 2W resistor to give it a chance of not burning your fingers.

Over 1/2 a Watt? 0.35 * 0.35 * 1.5 = 0.18375, so less than 1/4W.
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