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This isn't my strong point by far so please bear with me!

I have some 5mm blue flashing LEDs 4.5v - 6v operating voltage and my power source is a 12v 1A mains adapter (for CCTV camera) over CAT5e. I want to install an LED to an external junction box used for CCTV camera connections for added visual security. I've got 400 Ohm resistance (resistors) and the LED flashes perfectly for several minutes then stops flashing and goes static. Why does this happen and can you suggest a possible fix please?

FYI - I'm using same LEDs with a 6v AA battery pack and 100 ohm resistor and this setup works perfectly.

Any help and advice gratefully received.

Many thanks.

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    \$\begingroup\$ Please edit your question (using the edit link below it) to add in a link to the datasheet for the "flashing LEDs". There isn't a standard for these so we need the datasheet. \$\endgroup\$
    – Transistor
    Mar 12, 2019 at 18:41
  • \$\begingroup\$ Unfortunately a datasheet wasn't provided by the seller, although I've requested one with no joy. 4.5v-6v and I've assumed 20mA. Sorry. \$\endgroup\$
    – JJBW
    Mar 12, 2019 at 19:04
  • \$\begingroup\$ @JJBW, have you tried a higher resistance instead of the 400 ohm? \$\endgroup\$
    – user199402
    Mar 12, 2019 at 19:10
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    \$\begingroup\$ @JJ: I was afraid that would be the case. We advise, "No datasheet? No sale!" \$\endgroup\$
    – Transistor
    Mar 12, 2019 at 19:20
  • \$\begingroup\$ While by no means definitive, one thing you can sometimes do is to try to find data sheets for parts of similar function the idea being that whatever you bought surplus might be similar in its intended usage \$\endgroup\$
    – Chris Stratton
    Mar 12, 2019 at 20:01

1 Answer 1

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If it works well with a 6 volt battery, but not 12 V power supply, then the voltage is too high. Putting a resistor in series drops some voltage, but it depends on the current draw of the LED. Since the LED blinks, then the current likely varies; when it's low (i.e. in the off part of the cycle), most of the voltage is across the LED and not the resistor. To fix that issue, you nee a voltage divider, as shown below.

Voltage divider from https://www.allaboutcircuits.com/tools/voltage-divider-calculator/

Without knowing the current draw of the LED, a guess would be to add another 400 or 470 ohm resistor in parallel with the LED, keeping the 400 Ω resistor in series, as well. The voltage across the LED will still swing, but not above it's rated voltage. Theoretically, a electrolytic capacitor of ~100 µfd could go across the LED to smooth the voltage, but that will probably not bee neded.

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  • \$\begingroup\$ Alternatively, you could use a linear voltage regulator, which has a lower output impedance and less wasted power when the LED is off. \$\endgroup\$
    – Hearth
    Mar 12, 2019 at 20:22
  • \$\begingroup\$ @Hearth, 800 Ω across 12 V = 0.18 W. This is an insignificant drain for a wall wart or the environment, especially compared to the continuously-on CCTV cameras. It hardly warrants the extra cost of an IC. \$\endgroup\$
    – DrMoishe Pippik
    Mar 12, 2019 at 22:41
  • \$\begingroup\$ My main point was the lower output impedance, anyway, essentially eliminating that voltage swing you mention. And how much extra cost is a single LM1117 or 7805, anyway? 40 cents or so? \$\endgroup\$
    – Hearth
    Mar 12, 2019 at 22:52
  • \$\begingroup\$ @DrMoishePippik - I've put a 470 ohm resistor across the LED and another 470 ohm resistor in series and now the voltage on the LED fluctuates between 3.3v and 5.99v when flashing. The LED flashes continually now and the brightness is perfect! I've purchased a new multimeter and put it across the LED when powered down and it's 4.4v. Many thanks for all your help, it's very much appreciated. \$\endgroup\$
    – JJBW
    Mar 13, 2019 at 15:13
  • \$\begingroup\$ @JJBW, make sure to accept this answer as the solution if it solved your problem. \$\endgroup\$
    – user199402
    Mar 13, 2019 at 20:32

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