LED

Question

I have a control panel for a fire suppression system and on the firing circuit output there is an LED with a .68k resistor in parallel for testing. 24V when activated. What specs would the LED be?

shabaz · Answer

Hi Harry, There's not enough information, but I suspect it could be a current loop? If so, that's important information that was omitted. If the current system used an LED and resistor in parallel, then you can buy any generic red LED and it will work. You can search (on say newark.com or farnell.com) for "5mm red LED" and you'll see plenty. Personally I would choose a few, and see which one works best with the resistor you're using. Test it out (i.e. with and without a fault activated). Some LEDs may glow dimly when in the LED off situation, so you can try a different one, to optimise it. Don't stray from generic low-power (20mA or so) Red 5mm LEDs; if you choose a different colour or a more powerful LED then the electrical characteristics are different.

fmilburn · Answer

Typical LEDs turn on at a forward voltage around 2V and have a maximum current that they can tolerate without failing which is specified in the data sheet. Are you sure the resistor is in parallel? In a simple circuit where the current is controlled by a resistor they will be in series. Is it a single panel mounted LED? Please confirm the LED / resistor arrangement. Assuming the issue is the LED no longer lights was there something that occurred that might explain the failure?

EDIT: I have concerns about repairs on a safety system without understanding the circuit or cause of failure.

vapyro · Answer

This is not a repair, I am trying to put together a repair kit for dealers to use. Sorry about poor photo but the LED and resistor can be seen across #3 and #4. 24v is the supply voltage and the voltage between 3 and 4 when activated.

vapyro · Answer

Try this photo. The bias is 2.7V and I tried a blue one with 2.6 bias that failed immediately.

aswinvenu · Answer

Hi Harris, If I understand the question correctly you need to turn on the LED when you supply 24volts right? Please do not connect a resistor in parallel to the LED. Voltage across the LED will be 24 volts. LEDs work in forward bias and when you connect 24 volts parallel diods resistance drops very low. So a huge current passes through the LED and burns it immediately. The high resistor you put in parallel doesn't even matter when you forward bias the LED with 24Volt. To get the results you need to use a resistor in series with the LED. Supply voltage is = 24 Volts LED Forward voltage = 2 volts (take whichever the LED you are planning to use) LED Current = 15mA (again take this number from the datasheet) Now apply KVL and find the Series Resistor value. R = (24 - 2)/(15x10^-3) = 1.1KOhms. Regards, Aswin

vapyro · Answer

I need to explain further. This is a factory set up. I am simply trying to duplicate what they have to use as spare parts. For some reason I can't attach the control spec sheet. The LED and resistor are in the ignition circuit for test purposes and are removed when the extinguishing generators are installed. The system is manually activated to simulate a thermal event and the LED lights up to show that the control is working properly. They are removed when the actual extinguishing generators are installed. Since the circuit is monitored, when testing they also act as a dummy load so no fault is shown on the control. I understand the basics of LEDs and resistors but need to understand what is being used. I contacted the factory but got no response. this system works on 8 to 28V with 2ma monitoring current and up to 1.2a when activated. I can supply further data by email.

dougw · Answer

I think the other comments are correct - the LED needs to be in series with the resistor.

A 6.8K resistor would put about 3 mA through any LED if they were in series across 24V - which will illuminate most LEDs (fairly dimly).

If you want to protect the LED from being inserted backwards, put a regular diode in series with both the LED and the resistor.

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