LED

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

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.

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.

Also, those photos were really unusable. Sharp, in-focus, showing color bands is important.

Any mobile phone can do that, as long as there is decent lighting. Appreciate it may be hard to do that when working in areas with limited accessibility.

If that resistor is really in parallel to present a load, the LED may have a built-in resistor. This would be the case if the voltage stays at 24V even with the LED in place. Such LEDs may be substituted by any LED with a resistor in series.

What should the voltage be across the LED when it is active?

If that resistor is really in parallel to present a load, the LED may have a built-in resistor. This would be the case if the voltage stays at 24V even with the LED in place. Such LEDs may be substituted by any LED with a resistor in series.

What should the voltage be across the LED when it is active?