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I have a circuit that is used to rapidly turn an LED on and off. There is a capacitor in the circuit to provide some buffering (C1). What minimum voltage rating is required when selecting C1? I want C1 to be a through hole component with the smallest diameter/height possible.

schematic of led driver circuit

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The capacitor cannot charge higher than the 3.3V supply, so a 4V or 6.3V type would be fine. So, probably you're looking at a 5mm diameter part.

There is nothing limiting the current through the LED other than its own internal resistance and the transistor beta (and a bit of capacitor ESR). I realized that you want a bright flash, but this could be harmful to the LED, the transistor or the capacitor.

Gain of that particular transistor falls off rapidly above 100mA but you're probably looking at a poorly controlled current of somewhere between 100mA and 200mA given the high gain of the BC847. Specs to look at would be:

  • Maximum peak current rating of the LED (should be in the datasheet), and based on a certain length of pulse.

  • Maximum RMS current of the capacitor (calculate from repetition rate, peak current and datasheet ripple current rating). Pick one with a relatively high rating such as 250mA or better for long life.

  • Transistor heating- assume current is limited by transistor and use repetition rate to calculate mW dissipation and thus heating.

If it's a hobby one-off and you don't care about longevity, you can just try it and see how long it lasts, of course. The LED is probably the main issue for low rep rates.

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    \$\begingroup\$ typically you would go with double of the voltage, for safety. so i'd suggest the 6.3V one \$\endgroup\$ – Matze Strawberrymaker Jul 4 '15 at 12:34
  • \$\begingroup\$ @MatzeStrawberrymaker There are few through-hole parts in 4V anyway, and 6.3V are available in 5mm (the smallest common diameter) so no reason not to do that. Also would pick 105°C rating rather than 85°C. \$\endgroup\$ – Spehro Pefhany Jul 4 '15 at 12:42
  • \$\begingroup\$ @MatzeStrawberrymaker thanks for the suggestion, i'd like a part that is < 6.3mm in diameter and < 6mm in height. I haven't found a 6.3V rated part with those limits. Should the 4V part work? \$\endgroup\$ – b20000 Jul 5 '15 at 15:46
  • \$\begingroup\$ @SpehroPefhany Thanks for the answer, I do have a current limiting resistor in the circuit, at least that is what I thought - the 57.6 ohm resistor OR5? \$\endgroup\$ – b20000 Jul 5 '15 at 15:47
  • \$\begingroup\$ @b20000 does it need to be pth? Elsewise you could take an smd capacitor. Here is one example : mouser.de/ProductDetail/Panasonic/EEE-FP0J221AP/… . but you can get capacitors in every diameter and height, you just need to look for them. \$\endgroup\$ – Matze Strawberrymaker Jul 5 '15 at 16:00

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