Appropriate transistor for LED controller - Page 1
Forum section: electronics / beginners

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 03, 2023, 05:14:06 pm
content: Hello everyone,With my limited knowledge of electronics, I have created a LED intensity controller that uses a 555 timer in astable mode to generate a PWM signal. This signal is then passed to a 2N2222 transistor which supplies power to the LED array.The LED array draws 1.1 A when connected directly to 5V.My design works, but the problem is that the transistor gets extremely hot. This is not surprising, because the voltage drop between the emitter and collector, as determined with the oscilloscope, is about 1.1V. This means that the power loss is about 1W. I did not expect such a large voltage drop when reading the 2N2222 manual.Could you please suggest another common NPN transistor that has a lower voltage drop and heat dissipation?Best regardsMarko

author: Mazo
author_rank: Regular Contributor
timestamp: April 03, 2023, 05:32:05 pm
content: Use a transistor rated for atleast 2A of collector current,so it has high enough Beta at 1A and low enough Vce(sat)@1A.Transistors don't like being pushed to their boundaries and punish you for that(you are asking way too much of your poor 2n2222)Jellybean part you can throw at the circuit is TIP41C(aim for atleast 50mA of base current).Your 555 can't source that much from the Q output from memory,so you need a darlington,which will raise the Vce(sat) back to the levels you are seeing now,so it's a no go.Either reconfigure the switch as a Sziklai pair,use a mosfet or throw a non-jellybean part for the transistor(FZT690B).

author: Benta
author_rank: Super Contributor
timestamp: April 03, 2023, 06:29:50 pm
content: Begs the question: why do you think it's appropriate to draw >1 A through an 800 mAsmall-signaltransistor?

author: Zero999
author_rank: Super Contributor
timestamp: April 03, 2023, 07:35:14 pm
content: The base drive is inadequate, hence the voltage drop.As mentioned above, the 2N2222 is only rated to 800mA. Use the TIP121 which is a Darlington and has a higher current rating.This circuit will cause the 555 to get hot. There's nothing to limit the current through the discharge pin, when the variable resistor is turned all the way up.

author: mariush
author_rank: Super Contributor
timestamp: April 03, 2023, 08:05:09 pm
content: Lots of npn transistors you could use, but you need something higher current and higher betaSome examplesDSS4240T-7 40v 2A with hFe 300 :https://www.digikey.com/en/products/detail/diodes-incorporated/DSS4240T-7/2044509FMMT619 50v 2A hFe 100 :https://www.digikey.com/en/products/detail/comchip-technology/FMMT619-G/6138902But wouldn't it be easier to just get a LED driver IC?For example PAM2808 (allows dimming by doing PWM on the enable pin, 200 Hz recommended frequency) :https://www.digikey.com/en/products/detail/diodes-incorporated/PAM2808BLBR/4033259or AL1782 if you don't mind a maximum current of 750mA per channel (has 2 channels) and pwm support (recommended 4kHz or higher) :https://www.digikey.com/en/products/detail/diodes-incorporated/AL1782T16E-13/9658808

author: Eraldo
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:07:06 pm
content: Better use a mosfetElse use a power transistor with another smaller transistor in darlington configuration.But if possible just use a mosfet. It's entire purpose of its existence is switching fast with little heat dissipation. In your configuration you need an N channel mosfet with a gate voltage threshold lower than the output voltage of your 555timer

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:52:00 pm
content: I have P-channel mosfets IRF4905 and IRLZ44N, N-channel mosfet NDP6020P and Dalington TIP120 at hand. Which one would you recommend in this situation and which resistor would be appropriate at the gate/base? If I understand correctly, using P-channel mosfets would simply reverse the direction of the increase with the potentiometer.

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 03, 2023, 08:59:16 pm
content: Quote from: Zero999 on April 03, 2023, 07:35:14 pmIs it possible to adapt the circuit to avoid that? Or it is not critical?

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2023, 09:26:04 pm
content: Quote from: Pygmalion on April 03, 2023, 08:52:00 pmOut of those you listed I would expect the IRLZ44N to offer the best performance by a significant margin. The TIP120 is a Darlington transistor and should also work well, but it will have a higher forward drop so you will need to attach it to a heatsink.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2023, 09:26:56 pm
content: Quote from: Pygmalion on April 03, 2023, 08:59:16 pmAdd a fixed resistor in series with the top end of the pot in order to set the minimum resistance.

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 03, 2023, 09:33:40 pm
content: Quote from: james_s on April 03, 2023, 09:26:56 pmThis is an obvious solution, but it will prevent maximum brightness of the LEDs.

author: Zero999
author_rank: Super Contributor
timestamp: April 03, 2023, 09:47:18 pm
content: Quote from: Pygmalion on April 03, 2023, 09:33:40 pmThen reduce the series resistor value to compensate. If you use a low value, just enough to limit the current, then the maximum duty cycle will be over 99%, which will be indistinguishable from 100%. I would increase RV1 to 100k, reduce C1 to 1nF and add a 1k resistor in series with the discharge pin.

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2023, 11:16:08 pm
content: If you want it to go all the way up to 100% duty cycle then the 555 is not the best part for the job. It would be very easy to do this with something like the attiny13 microcontroller.

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 04, 2023, 05:19:56 pm
content: I have just replaced the 2N2222 with IRLZ44N. There is no more heating and even the TLC555 chip is cool.I also checked the voltage on the IRLZ44N with the oscilloscope - even when the potentiometer is at 100%, there are small spikes in the signal. Either that or the fact that the TLC555 is a MOSFET-based timer ensures that the timer chip does not heat up.

author: barshatriplee
author_rank: Regular Contributor
timestamp: April 04, 2023, 05:35:04 pm
content: I think TIP120 will be good. It has a collector current of 5A. You can use a MOSFET like the IRF540N instead.

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 05:38:55 pm
content: What voltage are you powering the 555 with? 2.5V is really low to drive a mosfet with. Ideally you want close to 10V.

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 04, 2023, 05:49:27 pm
content: Quote from: james_s on April 04, 2023, 05:38:55 pm5V, just as shown on schematics. Fortunately, IRLZ44N opens at 1.5V, I regularly use it with my Raspberry Pi.

author: Eraldo
author_rank: Regular Contributor
timestamp: April 04, 2023, 09:17:01 pm
content: Quote from: Pygmalion on April 04, 2023, 05:19:56 pmI'd say it's propably because of the tolerances of the components. For example the potentiometer not having perfect 0/100 ratio even when fully turned or other stuff.If you're not satisfied with it try changing the potentiometer (or maybe try a capacitor in the timer output or even the mosfet output. but make sure it doesn't disturb the circuit too much. Just enough to remove the spikes. Maybe play a bit with itbto find the ideal value)

author: james_s
author_rank: Super Contributor
timestamp: April 04, 2023, 09:59:40 pm
content: Well if it works it works. There are a lot of things you can get away with for a one-off hobby circuit that would be very poor practice when designing a commercial product. When I was a kid I built a power supply that had 3 7812's just wired in parallel to get 3 amps. That's not supposed to work very well but it did and I used it like that for about 20 years.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2023, 10:16:49 pm
content: Quote from: Pygmalion on April 04, 2023, 05:49:27 pmThe scale on your oscillograms is 500mV/division, so the gate drive is 2.5V, assuming its correct.Quote from: Pygmalion on April 04, 2023, 05:49:27 pmIt depends on the drain current. The IRLZ44N will pass an amp or so, with a gate drive of a couple of volts, but not much more, without a large voltage drop. At least 5V of gate drive is required to get the full drain current, with minimal losses.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2023, 10:59:32 pm
content: "The scale on your oscillograms is 500mV/division, so the gate drive is 2.5V, assuming its correct."As no one knows what the trace shows, a guess could be the drain voltage.

author: Pygmalion
author_rank: Regular Contributor
timestamp: April 05, 2023, 12:47:50 pm
content: Quote from: Zero999 on April 04, 2023, 10:16:49 pmQuote from: Zero999 on April 04, 2023, 10:16:49 pmThe oscilloscope screenshots are drain voltage. Multiplied with maximum current it gives heat dissipation of MOSFET, which is for open state about 0.2V x 1A = 0.2W.All in all the voltage drop on MOSFET of 0.2V is pretty small for these currents and I think it is satisfactory.