Simple blinking light triggered by a 555 and powered by a transistor. - Page 1
Forum section: electronics / projects

author: PhilippeASP
author_rank: Contributor
timestamp: July 28, 2017, 04:45:50 pm
content: In an earlierthreadI mentioned I was trying to remember how to use a transistor, and this is what I was planning for it.In the test circuit I'm using a voltage divider and feeding 6v from my main power supply. But I've also tested the circuit using a LabJack controller to feed 5v into the 555. The test lights are a series of little LEDs powered by 12v, but eventually I'll want to power much larger lights (hence the use of big beefy transistors). I ended up buying some IRLB8721PbF (https://www.infineon.com/dgdl/irlb8721pbf.pdf?fileId=5546d462533600a40153566056732591) and RFD14N05L (https://www.fairchildsemi.com/datasheets/RF/RFD14N05LSM.pdf) transistors. Both claim high power capability, and can be driven by ~5v at the gate. I know they're probably overkill, but I wanted powerful ones in case I want to use them for a more powerful project one day.I must be doing something wrong though, the 555 is outputing a clear square wave that goes from 0 to 4.5v, but it's not activating the transistor properly. Using either of the transistors, I end up seeing 3.4 to 3.8 volts at the source pin. Either using the voltage divider or the controller, it's just not working. I tested the lights using an ampmeter and the 12v source directly to see how much resistance I should put on them. 1.5 Kohms had them use 3 mA, which seemed decently bright. Using a single LED, giving it more current than that caused it to burn. So I don't think that the LEDs would cause any problems with the transistor, but that's the only thing that I can think of. Am I missing something obvious?

author: Howardlong
author_rank: Super Contributor
timestamp: July 28, 2017, 04:49:55 pm
content: Move the resistor & LEDs to between the 12v & drain, not the source & GND.

author: nugglix
author_rank: Regular Contributor
timestamp: July 28, 2017, 05:11:04 pm
content: Quote from: Howardlong on July 28, 2017, 04:49:55 pmThis!Every LED has a so called forward voltage (see datasheet).In your case the voltages lift the source of the MOSFET up from ground.Your 4.5V signal on the gate needs to overcome this voltage *plus* the turn-onvoltage of your MOSFET (see datasheet).Example:LED forward voltage 0.7V per LED4 * 0.7 -> 2.8V4.5 V signal - 2.8V -> 1.7V (not enough to turn on FET)PLUS!If the FET starts to conduct a current would flow through the resistor,generating a voltage drop which would lift the source of the FET even further,thereby reducing the turn-on voltage even more.Better luck next time!

author: PhilippeASP
author_rank: Contributor
timestamp: July 28, 2017, 06:11:03 pm
content: How peculiar.Thanks for the assistance, it did work. But only when I put the LEDs connected to the Drain, and the resistors on the Source side. Still not sure why, I mean the tension will get stepped up and down around he load components regardless of what side of the transistor they're on. But I'm glad it works.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2017, 07:35:37 pm
content: What sort of MOSFET is it? N or P channel?The previous comments about connecting the LEDs between the drain and +V assume that the MOSFET is N-channel, as that's the most common type of MOSFET. However, if you have a P-channel MOSFET, the source needs to go to +V and the LEDs between the drain and 0V.

author: DaJMasta
author_rank: Super Contributor
timestamp: July 28, 2017, 08:34:49 pm
content: What kind of LEDs are you using? Different colors use different kinds of junctions and have different voltage drops - white LEDs can have more than 3V drop each, so I assume they're these, but still, it could be limiting how bright they get.Also worth mentioning that any old transistor will probably be fine for most LEDs - the power rating for a transistor is how much can be dissipated by the transistor itself, so in the case of a mosfet, it would be the Rds on times the current - and when running a series string of LEDs, the current through one LED is the same as the current anywhere in that part of the circuit.So if you had a regular 5mm LED, you'd probably top out at 20-30mA, which means even with an Rds on of a few ohms, which is high for most mostfets, you would still be well within the power rating for a TO-92 or similar low power package. Now if those LEDs are something like 3-5W high power LEDs, then you'd need a decent sized transistor to drive them, but if you're using a mosfet with a low Rds on, then it probably still doesn't need a beefy package. With your current 12V supply and 1.5k resistor, you've only got 8mA if the LEDs were just shorted wire, meaning that infineon mosfet when driven at 4.5V on the gate (and the on resistance will be lower with a higher drive voltage) and passing 8mA will be dissipating 0.1 milliwatts - and given a 62 degree per watt junction to ambient thermal resistance, its temperature would only rise by 0.0065 degrees C. You've got a mosfet that is WAY overkill for the application.

author: DrGonzoDK
author_rank: Regular Contributor
timestamp: July 29, 2017, 09:51:34 am
content: What's the model number of the MOSFET? Is it a BS1XX or something entirely different?Also - to be helpful, I quickly mocked up a few simulations of source-side and drain-side switching of LED's on NMOSFETs and PMOSFETs on Falstad's circuitjs -http://tinyurl.com/ydgyahx6The specific values of voltages, resistances etc aren't authoritative, just a quick bodge to get a result that was illustrative. I hope these layouts can help - as well as help you identify if you've got an NMOS or PMOS transistor.Also, are the LED's on when the 555 output voltage is high or low? That may help, too...

author: PhilippeASP
author_rank: Contributor
timestamp: August 02, 2017, 04:01:06 pm
content: @Hero999The mosfet is indeed N-channel.@DajMastaGreen LEDs, but those were just to test things out. My real light is a 25v light to warn people a machine is operating (hence the 555's blink, to catch people's attention). Thankfully the 25v (if it does use LEDs) has internal resistors, so I just need to worry about passing 25v through it. No worry about adding resistors myself.@DrGonzoDKI ended up going with the fairchild one I mentioned in my first post: RFD14N05L (https://www.fairchildsemi.com/datasheets/RF/RFD14N05LSM.pdf). I had one mysteriously fail on me, but the second one is doing fine. The light blinks like a champ. Thanks again for all the help people offered in the thread.