Slimy38
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| posted on 8/7/17 at 10:07 AM |
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LED current limiting resistor calculation
I have four LED's that are 1.35V and 100mA. They need to be powered off USB, so 5 volts. Normally I've used a current limiting resistor to
protect things, but I can also see that 4 x 1.35 is more than the supply voltage.
So first thing, is a current limiting resistor required if I run them in series? If it's safer to have a resistor, what combination of series
and parallel would be best, and how do I calculate the value of the resistor? I have a decent range of resistors, but they're all 1/4 or 1/8
watt so I'll need to keep the overall power down. It'll also save my USB port too!
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ReMan
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| posted on 8/7/17 at 11:08 AM |
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try this
http://led.linear1.org/led.wiz
www.plusnine.co.uk
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AntonUK
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| posted on 8/7/17 at 01:57 PM |
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quote:
Originally posted by ReMan
try this
http://led.linear1.org/led.wiz
after sitting here for 5 mins with a calculator, i should of clicked your link first. nice find thanks
Build Photos Here
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coyoteboy
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| posted on 8/7/17 at 09:59 PM |
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Worth noting that the more you can put in series the better as the total power wasted in the resistor will be lower, however production variation can
limit this (when you get to 5+ is series you can see serious variation across them). In this case I'd be going 2x2
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ReMan
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| posted on 8/7/17 at 10:52 PM |
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Agreed
In fact, depending on these leds, you may get away with 4 in series and no resistor
www.plusnine.co.uk
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Slimy38
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| posted on 8/7/17 at 11:00 PM |
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quote:
Originally posted by ReMan
Agreed
In fact, depending on these leds, you may get away with 4 in series and no resistor
That's actually what I've done, seems ok at the moment...
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coyoteboy
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| posted on 9/7/17 at 12:27 PM |
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Just be wary that with no limiting resistor, if one led goes into thermal runaway you will pop the supply.
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gremlin1234
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| posted on 9/7/17 at 12:47 PM |
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quote:
Originally posted by coyoteboy
Just be wary that with no limiting resistor, if one led goes into thermal runaway you will pop the supply.
could put a 250ma fuse in series
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MikeRJ
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| posted on 9/7/17 at 05:06 PM |
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quote:
Originally posted by ReMan
Agreed
In fact, depending on these leds, you may get away with 4 in series and no resistor
Very bad idea unless running from a supply that has a suitable current limit. The forward voltage of an LED reduces with increasing temperature, and
with no current limiting you very quickly reach a thermal runaway state.
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coyoteboy
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| posted on 9/7/17 at 09:41 PM |
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quote:
Originally posted by gremlin1234
quote:
Originally posted by coyoteboy
Just be wary that with no limiting resistor, if one led goes into thermal runaway you will pop the supply.
could put a 250ma fuse in series
Fuse is too slow to protect most electronics. You should consider fuses just wire protection.
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