Electronics question - transistors

Electronics
#1

Hi all,

I’ve made an Alexa controlled LED string light controller based on an ESP8266 and some transistors and it mostly works!

based it on an online tutorial somewhere that I can’t find anymore so needing some input from the electronics gurus.

basically I have a 24V source into the board; goes through a pair of capacitors and a 5v regulator to run the ESP8266.

The 24V also runs the LED strings through 4 transistors (RFP30N06LE). One each for Red, Green, Blue, and power (for dimming). The RGB ones work great, and the power one works for about 3 days, before it fails closed (bright / power passing through to full brightness).

What am I missing that’s causing the power transistor to burn out?

Here’s a pic of the board (sorry I don’t have a current one). The pinouts from the ESP run to the left leg of the transistors. The center pins of the transistors run to the LED strip. the right pins run to Ground for the RGB pins and 24V for the power.

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#2

Since the power transistor is handling 3x the others, my guess is that you are asking it to handle too much current. How hot does it get when everything is on?

#3

How many amps does the LED string draw?

I noticed that you do not have a heat sink on it; does that transistor get hot when it is running?

#4

Another possibility is load dump inductive spikes. Do you have a cap on the input?

#5

Thanks all for the replies, sorry I haven’t responded back. On the road for a few more days before I can check on all that.

It’s a 6ft string of LED’s spaced out about 1.5" per dot; will measure load next time I’m home.

I didn’t notice it getting hot and specifically touched the backs of each of the transistors while it was on to see if they were getting hot… but that was probably only after running for about 5 minutes - not after letting the lights run for an hour or so that they were on once I took it from the bench to the headboard where the string is.

There are 2 caps, a 10uf and 1uf on the board.

#6

15 seconds would be enough to tell without heatsinks.

If they aren’t getting hot, it’s likely a switching transient. We’ll need to peek at it with an oscilloscope.

Yeah, but it’s how they are used in the circuit that counts. Proximity doesn’t…

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#7

Your power MOSFET is killing your circuit. I’m amazed it even works that long. You have a body diode always connecting your VCC to D3 and when D3 is low creates a short circuit. Seems as VCC is higher than the ESP32 can handle?

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#8

I would add to that what are you current limiting resistor for your led? at 5v I usually to draw 5mA. So i use a 1k resistor in series with the single led and a smaller transistor. Ever consider the app evercircuit hobby simulation? I believe it lets you save to html or the original website.

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#9

I suspect the problem here is pretty basic. Your not driving the gate with enough voltage. Specs state gate threshold is 2v. Note that RDSon is spec’d with gate at 5 volts. Your ESP8266 is a 3.3v device. IO pin spec says 0.8(Vsply) min out. You’re barely turning the pwr fet on. You have 5 volts available. Insert a non inverting level shifter between the ESP8266 IO pin and the gate input. And it wouldn’t hurt to put all the pwr fets on a heat sink. Just make sure you insulate the tabs.

Details buried in the spec sheets matter. Ignore at your own risk. Learning what to look for is a process. Don’t ask me how I know this…

#10

I missed that the drawing under the photo was a connection diagram. Ivanko and Art are both correct.

The power control transistor has no effect, because of the way it is connected. The body diaod will always conduct. You need a P-channel FET in that position.

Also note that the body diode, while rated at 30A peak, is tested at 2% duty cycle, or ~600mA average. I suspect what is happening is the body diode is being just overdissipated when the FET is “off” and shorting.

Art is strictly correct - you are not guaranteeing enough drive voltage to saturate the FETs in the on state. You also are not guaranteeing they turn off.

You indicate none of the parts are getting warm to the touch, so FET saturation is probably not a critical issue, but one that should be addresssed.

#11

Wow, ok y’all rock!

Been working and haven’t circled around to this, but should have some time in the next few weeks to play with it.

That said, I was mostly copying what I found online in one of those instructional things (which I actually can’t find anymore). Would y’all mind dumbing this down for me a bit and help me get this thing built out? Would be willing to bring a 6-pack of your choice for some of your time to walk me through it (like I’m 5).

I have some small heat sinks that I’m ready to throw onto the transistors, but it sounds like I don’t even have the right components to start with.

Eventually, I’d love to actually sketch this out and have a PCB made for it as I’d like to make several of these around the house.

-Ian

#12

I should also say that when you use a PFET to what you label “VCC” which is 24VDC, you will, as Art explains need the FET gate pulled up to that 24V to turn it off, so you will need an N channel FET and a couple of resistors between it and the EXP8266 to make it work.

#13

https://calendar.dallasmakerspace.org/events/view/21261

#14

Got a model / part number for the LED strip?

May have missed something here - but…
The schematic and the description given don’t match. Schematic has gate and drain pins swapped.