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SCR latching circuits and LEDs Answered

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LEDs and I just can get along...In another project I had a solenoid blowing out LEDs. Now this: I have a number of switches hooked up so they trigger an SCR latching circuit to turn on (and keep on) some LEDs. The SCRs are wired in parallel to each other and have a 1K resistor on the gate. Power is applied to the whole circuit via a timer relay (not hooked to the SCRs). The power is a 12v, 200mA wall wart. And the SCRs I'm using are NTE 5465. The LEDs last a while but burn out or go to a weird blinking state (when the switch is depressed) after about 2-3 days. In my previous project I needed a diode to help with the solenoid voltage spike. But I don't think that it is the case for this project since the SCRs aren't inductive. Do LEDs connected to the switch part of a relay need a diode too? The LED package I'm using is purchased from Happ Controls (see hideous grainy picture). It's rated for 12V but they can't give me a mA rating on them. But they say that no inline resistor is required. Any thoughts? I think this is another case of my inexperience causing me to overlook the obvious.

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AndreasC2 (author)2015-03-20

If i understand this question .....the user he/she wondering why leds are burnt with

12v supply.Ok all you need is a 2,2k resistor in series with led.... its simple .....PERIOD.

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gmoon (author)2007-09-04

Unregulated wallwart? They don't always produce the advertised voltage...Check with a VOM under load. LEDs are very susceptible to over voltage. Even 12.2 volts could kill them fairly quickly (unless they have built in regulators, or are rated for automotive use, 14V+). I would add the spike diode regardless, anytime you're connecting semiconductors to a relay. Why SCRs and not just a cheap transistor? Most of the SCRs I'm familiar with are rated at 400V or more. A transistor (std NPN or a FET) will do the job, and only cost 10 cents.

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robotninjasquid (author)gmoon2007-09-05

Yes, the power supply is unregulated. Or at least it doesn’t say that it is. It reads 15 to 16 vDC when under load. I used SCRs because I needed them to “latch” and stay on until the relay reset. I didn’t think transistors did that (but I could be horribly wrong).

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user

When I suggested you check the voltage you said it was "OK on the voltage". It is only of academic interest now but under what exact load conditions were you getting the 15 to 16 Volts?

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When it read 15-16V I had up to three to four lights on. In other news: I have a regulated power supply on it and the LED that burns constantly (on the On switch) is holding. We'll see how it goes. Thanks again to all for your help.

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Did you see my post lower down? Your experience with LEDs is growing. Frying components helps you understand what you can't do;-) I've certainly fried my fair share in the past! Best of luck, Pat. Pending

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gmoon (author)robotninjasquid2007-09-05

RE: the SCRs--OK, that makes sense. And you did say that in the OP, sorry...I just didn't read... 15-16V, even under load. Regardless of voltage vs. current, that's pretty far beyond the LED's voltage rating. Yes, I would try a regulated supply. Or build my own regulator.

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Nothing wrong in using SCR's in that way.

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Patrick Pending (author)2007-09-05

You said the relay is not hooked to the SCR's do you mean that this is a separate circuit with its own independent power supply? This looks like it could be an LED cluster array i.e. several LEDs wired in series with or without a built in resistor. LED clusters (internally wired in series) are usually very forgiving (as the array Vfmax is a sum of all the individual Vfmax's). Can you see individual LEDs within the package or is it a single LED? Even assuming that they are standard type LED's you could expect the forward current to be about 20mA. If all 16 LEDs are lit then your power source would need to supply 320mA! . The voltage supplied by your 200mA supply is likely to be varying greatly between no load and full load. It could be that when a single LED is lit then the voltage across it is greater than Vfmax for this device. Try measuring the: no load voltage, one led lit, and full load (all LEDs lit). I suspect the problem is the power supply and it certainly looks like you need a bigger one. You should either get one with good voltage regulation or alternatively add a voltage regulator circuit. Cheers, Pat. Pending

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Correct, the relay has its own power supply. Here is a picture of the guts of the LED cluster. There is a blue resistor looking thing in there but it has too many bands: the colors are brown, green, black, blue, brown. There should never be more than 8 LEDs on at once. I am under the impression that amperage killed LEDs but it sounds like it is voltage. Or is it both? So the consensus is get a better power supply or a voltage regulator to keep it at 12VDC. I’ll give that a whirl. Is it a good idea to only use regulated wall warts? Seems like they give everyone a lot of trouble. Thanks everyone.

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How many LEDs were in the cluster?
Are you sure you got the colours right on the resistor? - here's a 4, 5, and 6 band resistor calculator

It is generally the heat that fries the LED's which is a function of the power. Power = Volts X Amps. As the voltage across a forward biased diode changes very little between normal operation and totally fried it is often best to consider the LED primarily in terms of current.
If you want to learn a bit more about LEDs than here is an excellent Wiki - there's no need to read every word (skip the boring and overly technical bits). Unfortunately the Wiki doesn't include a graph of current against voltage which I believe is the key to understanding how an LED operates. If I find a decent graph I will post it.

Cheers,

Pat. Pending

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Patrick Pending (author)2007-09-04

No you shouldn't need a diode across the switched contacts of the relay. Do you have a test meter that you could check the voltage across the LED. A rough drawing of the circuit might shed some light on your problem. Pat. Pending

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Ok on the voltage. I also made a circuit diagram. I only show 5 SCR/LED units but in reality there are 16. Brian

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