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Old Aug 14, 2013, 05:34 PM
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glenn2626's Avatar
United States, CO, Colorado Springs
Joined Jan 2008
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Help!
Simple LED on servo leads - could use help please

I have a simple circuit I'd like to design, but need some help.

I need to simply put an LED (and other necessary bits) between the + and - lines of a servo wire, between a receiver (or servo tester) and a servo itself (or ESC, etc). LED should light up if the voltage is 3.2V (from a well-depleted 1S lipo) to 6V, as an ESC or BEC might be set to. No adverse effects if reverse polarity is applied, and of course no light either.

Thing is that I can't seem to pick a resistor value that makes both ends work. If I have a generic/standard LED that has a 2.1V forward voltage, and 20mA working current, then I get this:

for 3.2V input: 3.2V-2.1V=1.1V / 20mA = 55 ohms
but:
for 6V input: 6V-2.1V=3.9V / 20mA = 195 ohms

Then if I try and use a 195 ohm resistor, then at 3.2V it will only draw 5ma (1.1V/195 ohm) and my LED won't light up (I assume). If I use 55 ohms with 6V then it will overdrive the LED at about 71mA.

Is there a simple (a few cheap parts) solution to this problem? Am I essentially looking for a current-limiting circuit (like a standard LED driver) ??

Thanks for your help in advance!

Glenn
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Old Aug 14, 2013, 07:55 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
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Quote:
Originally Posted by glenn2626 View Post
for 6V input: 6V-2.1V=3.9V / 20mA = 195 ohms

Then if I try and use a 195 ohm resistor, then at 3.2V it will only draw 5ma
That is just a limitation you have to accept when using a resistor. However it may not be as bad as you think.

An LED's forward voltage rises as current increases, so the current variation will be a bit less than you calculated. If you use a 'super-bright' LED then it should put out plenty of light even at 5mA. The human eye's response to light is logarithmic, so the apparent brightness change will be less than the numbers suggest.

If you must have constant brightness down to 3.2V then you need a current regulating circuit. This can be made from a bipolar transistor, 2 diodes, and 2 resistors. Alternatively you could use a low dropout voltage regulator.
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Old Aug 15, 2013, 11:21 AM
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Acetronics's Avatar
Le Treport, France
Joined Jun 2004
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for 20 ma Why not use a J-FET of C Grade ( BF 245 or 256 ) in series with gate connected to source ???

http://www.sonelec-musique.com/image...im_led_003.gif ... the 4148 is not compulsory !

that's the simpler way I know ...

Alain
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Old Aug 15, 2013, 06:45 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
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Originally Posted by Acetronics View Post
for 20 ma Why not use a J-FET of C Grade ( BF 245 or 256 ) in series with gate connected to source ?
That will work, though the current won't be as well regulated at low voltage (much worse if the reverse protection diode is included). Also JFET characteristics are not precisely controlled and may have a fairly wide spread between individuals, eg. the BF245C could have an Idss anywhere from 12mA to 25mA.

I estimate that without the diode, current would vary from (nominally) about 12mA at 3.2V to 16mA at 6V. This is less than half the variation that would occur with a simple resistor, and is probably good enough for an indicator light.
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Old Sep 11, 2013, 12:43 PM
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Thank you all for your responses. I've now caught up on orders from my recent vacation, so I'll have some time to test some of these theories.

I also saw these current limiting resistors from Hansen hobbies, and have some on order to test...

http://hansenhobbies.com/products/li...ics/led_cl_20/

Glenn
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Old Sep 11, 2013, 01:24 PM
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Le Treport, France
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Quote:
Originally Posted by glenn2626 View Post
Thank you all for your responses. I've now caught up on orders from my recent vacation, so I'll have some time to test some of these theories.

I also saw these current limiting resistors from Hansen hobbies, and have some on order to test...

http://hansenhobbies.com/products/li...ics/led_cl_20/

Glenn
does this ring you a bell ???
http://www.centralsemi.com/PDFs/prod...30-CMJH180.PDF

Alain
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Old Sep 11, 2013, 04:40 PM
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Alain,

Yes, looks like what it might be. I also found these that have ranges from 10mA all the way up to 350mA (for ~1W LED):

http://www.digikey.com/catalog/en/pa...=DC_Link_Table

I'm going to see how the Hansen product works compared to some of the choices above, then look into other chips like this, then get into the cost vs simplicity calculation for my needs...

Thanks
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