A Fuse Q, Morr

[POPEYE]

Hi Maurice, I've aimed this question mainly at you as its to do with led's but if
anyone else can help please feel free, As i said it's to do with the led's im putting in the instruments, I would like to protect them with a fuse on the supply, I know these things run on miliamps and i would not be able to get a
standard blade fuse low enough so could you tell me what size (rating) fuse
i should use and what type if poss,
I've got a lot of glass fuses of different ratings some of em small quick-blow
and some fuse carriers from maplins, Idealy i'd like to use a panel mounted
type fuse holder (like the ones you get in battery chargers etc) There easy
to change and could be fitted on the dash, The two i've got take a large
glass fuse though, Thanks for any help or thoughts, Ian.

[jaky cakes]

I have only ever fitted 1/4 w resistors to the positive leg (1 K ohm for red and 470 ohm for green amber yellow) to limit the current they will draw.
As your average 12v LED will draw some where in the region of 20-25mA - you would need to install and run between 40 and 50 LED's to draw 1amp current!

[POPEYE]

I have only ever fitted 1/4 w resistors to the positive leg (1 K ohm for red and 470 ohm for green amber yellow) to limit the current they will draw.
As your average 12v LED will draw some where in the region of 20-25mA - you would need to install and run between 40 and 50 LED's to draw 1amp current!

Cheers jaky, Well all told i guess im up to around 120 leds in this set up so
a fuse of say 2amp would be ok? The lowest blade fuse from VWP is 3amp,
Ive used 470ohm resistors (supplied with leds) on all colours, These are through 7 switches for different colours or any combination or all on (doubt
i'll do that much though) but for safety's sake i'd like to fuse for that
possibility, Ian

[Morr]

Cheers jaky, Well all told i guess im up to around 120 leds in this set up so
a fuse of say 2amp would be ok? The lowest blade fuse from VWP is 3amp,
Ive used 470ohm resistors (supplied with leds) on all colours, These are through 7 switches for different colours or any combination or all on (doubt
i'll do that much though) but for safety's sake i'd like to fuse for that
possibility, Ian

As Ali said, its a matter of adding up the current draw of the LED's on each circuit, I've seen LED's rated for max continuous current anywhere between 10 and 100 mA.
Maplin list a range of glass fuses both in mA and Amp values, so you should have no difficulty getting the fuse you require. The LED's will typically withstand a higher momentary current then their quoted maximum, so I would calculate the total current draw, add around 5% and use the next closest higher value you can obtain.

[POPEYE]

As Ali said, its a matter of adding up the current draw of the LED's on each circuit, I've seen LED's rated for max continuous current anywhere between 10 and 100 mA.
Maplin list a range of glass fuses both in mA and Amp values, so you should have no difficulty getting the fuse you require. The LED's will typically withstand a higher momentary current then their quoted maximum, so I would calculate the total current draw, add around 5% and use the next closest higher value you can obtain.

Thanks Maurice, Will do

[MattF]

I have only ever fitted 1/4 w resistors to the positive leg (1 K ohm for red and 470 ohm for green amber yellow) to limit the current they will draw.
As your average 12v LED will draw some where in the region of 20-25mA - you would need to install and run between 40 and 50 LED's to draw 1amp current!

Nothing like dredging up an old thread 1.5K is a more suitable resistance for a 12V supply. Worthwhile using metal film rather than carbon for reliability, as well.


Matt

[Apache]

Nothing like dredging up an old thread 1.5K is a more suitable resistance for a 12V supply. Worthwhile using metal film rather than carbon for reliability, as well.


Matt

Quite!

By my reckoning though, that only gives you a LED current of 8ma. If using cheapo 'standard' LEDs (which will probably be more suitable for dash lighting given their wider viewing angle and more diffuse light) rather than ultrabright, I expect this will give a rather uneven, dull illumination. I've always worked on around 15ma for standard LEDs, whilst 10ma or so is fine for ultrabright types (though pulsed for 'strobe' type lighting for bike lights etc, 100ma or more depending on the length of the pulse, gives an incredibly bright flash).

Reminds me of the time I built a set of nav lights for a bungee launched RC glider and flew it in pitch darkness with some success for a little while!

[MattF]

Quite!

By my reckoning though, that only gives you a LED current of 8ma. If using cheapo 'standard' LEDs (which will probably be more suitable for dash lighting given their wider viewing angle and more diffuse light) rather than ultrabright, I expect this will give a rather uneven, dull illumination. I've always worked on around 15ma for standard LEDs, whilst 10ma or so is fine for ultrabright types (though pulsed for 'strobe' type lighting for bike lights etc, 100ma or more depending on the length of the pulse, gives an incredibly bright flash).

You are indeed correct on the current. It may have been better if I had used the words:

A more suitable and safer resistance rating.

General LED's vary between the 10-50mA range as a rule, so the 1.5K is a safe option.
( Old habits die hard ). Obviously, if the actual current rating of the LED is known, a
specific resistance can be calculated for it. I just try to use safe values that prevent the
yellow/orange/red/aaargh syndrome

Reminds me of the time I built a set of nav lights for a bungee launched RC glider and flew it in pitch darkness with some success for a little while!

Was the short flight due to a sudden, I never saw THAT, scenario?


Matt

[Apache]

We actually lost it on one flight. You needed to keep 100% focussed on the lights as if you looked away you lost it immediately, and we did! We heard it land a few hundred yards away on a bit of tarmac, completely undamaged! Thats the nice thing about positive stability!