I'm using 12v to 3.3V transformers, plus a small resister, to supply 2 volts to 100 LEDs. So about 3 amps.
Goal is to protect them should the transformer blow. What do I add before the LEDs to notice a voltage spike (> 3 volts) and shut down so they don't get fried?
Everything that I read about fuses limits AMPERAGE, but not VOLTAGE.
LED power is not really dependent on the applied voltage. Have a look at a data sheet and check the tolerances. That's why it is not a good idea to control the LED voltage.
Why don't you connect 5 LEDs in series and a resistor for direct 12V supply?
Here is the thing about your post.
1.transformers operate on AC voltage
2. What are you suspecting a transformer will do when it "blows"? I don't know that term relating to transformers.
3. What do you consider a voltage spike to be and how long will it last?
4. Understand that detecting a voltage spike and doing something about it means it is already too late to protect a subsequent device from the spike.
5. Fuses are in devices to protect the insulation on the wiring from burning. They cannot protect solid state devices. Yes, fuses are current limiting devices.
paul
Thanks much for the quick reply. I'm admittedly an electronics novice.
Using Buck Converter 12V to 3.3V Step-down Voltage Regulator Transformer 3A/10W Power Supply Module, attached to 12V power supply.
LEDS are standard 20ma, 2-2.2V. Did them in parallel simply because I thought that the right way to do it.
Don't understand 'LED power not dependent on voltage'. When I use a potentiometer instead of a resister, see high voltage (e.g 3 V) makes them brighter and low makes dimmer.
Very high (e.g. 5V) burns them out. That's what I'm afraid a failing Buck Converter will do, pass through up to 12 volts. That's what I want to avoid, and what I mean by a voltage spike.
Haven't had it happen yet, so maybe not possible. Any maybe not possible to detect quick enough to protect 100 LEDs.
Since that's what you want, it's a straightforward solution. 100 leds at 20mA is 2A, so a 3A quick blow/fast fuse would be suitable.
That's the wrong way. If you don't understand why then at least believe it.
It's not optimal given 12V in, but who's to say what's "wrong". It works.
My reasoning was based on the 5 meter LED ribbons you can buy, where all 300 LEDS are in parallel. Should have researched further, I guess.
Thanks FASHKO ... very helpful!
Thanks, Koraks. I thought that might be it, but was confused about limiting amps instead of volts.
Those are a compromise that allows the buyer to cut the strips to any desired length without having to feed a different voltage to them. It's an inefficient solution overall.
Well anyone with an ounce of knowledge about electronics would say that. Simply LEDs in parallel do not share current evenly.
Yes you can buy them wired like this, but they will only work a short time and the people selling them have a vested interest in selling you them again. You probably use them for a month in December and then nothing else for a year.
OK lets get this clear you probably haven't got a transformer.
This is a transformer
If you want a component that behaves like a fuse for voltage then you need a Varistor.
I believe you are referring here to WS2812 "NeoPixel" strips. Each LED in these is actually a set of three LEDs with a complete controller IC included to provide a constant-current drive to each LED (which is the correct way to drive them) as well as PWM brightness control.
Clearly in no way related to your project.
If you have a 12 V supply to start with, then clearly you need to wire the LEDs in series groups of four or five, each group having a current limiting resistor.
As stated before there are better ways to power 100 LEDs from 12 V. But I would not rebuild it if I had it already assembled. The current imbalance is too overweighted. Even if a few LEDs burn out who cares? (Of course this depends on application.)
If you decide to keep the project as is just use a fuse. LEDs are damaged by overcurrent but they should survive some short peak. Hopefully longer than the fuse.
Another possible problems:
3 A for 100 LEDs is 30 mA per LED. Too much for the "common" 5 mm signal diodes.
The current limiting "small resistor" dissipates 1.3 V * 3 A ~ 4 W! It needs to be a (physically) huge resistor. Resistance should be 1.3/3 ~ 400 mOhm. Is this what you have?
To address your initial question, the answer is a 'crowbar' with a fuse. (power supply, then fuse, then crowbar, then protected circuits) The crowbar senses the over voltage , causes a short in microseconds, then blows the fuse. The simplest crowbar is a Zenner diode that is 10% above your nominal operating voltage directly across the power feed. Another is an SCR/ Zenner diode/ resistor circuit. The necessary parts and wiring are easily found on the Internet.
Thanks to all. Dang ... wish I'd asked this question years ago. Going to rebuild it with LEDs in series ... the right way.
BTW ... "it" is this, a 5 foot dragon head stage prop (soon to have a neck added)
That's really cool 
thanks, koraks. I use Arduino to control eye, eyelid and lip movement. Sit behind and make it dance to the music (which helps to drown out annoying servo whine).
Lots of fun building and rebuilding it over the past 10 years. Big hit at Halloween!
I imagine so, sounds like fun on both ends of it!
