I read somewhere that it was necessary to use a resistor when connecting a led to J2 but in another post I read that it is not necessary because the mastercard sends a pulse to the led. I can't find the post now but which is correct?
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I read somewhere that it was necessary to use a resistor when connecting a led to J2 but in another post I read that it is not necessary because the mastercard sends a pulse to the led. I can't find the post now but which is correct?
Hi,
If you are referring to the OpenCockpits Mastercard then you definitely need the resistors. The other post you are remembering may have been about the DisplayII daughter card; that one multiplexes its 7-segment displays so that resistors are not necessary.
However,
it all depends on the led types. If you use normal leds (not the high efficiency) which draw around 10mA and have a voltage drop of about 2,3Volt then you can connect 2 leds in series to the mastercard without the use of a resistor.
The reason is the following. A Master card output will give about 4,8 volt when no current is drawn from it. But when you connect a led in series with an appropriate resistor and you draw 10mA out of the output you will see that the voltage will
drop from 4,8 volt to a lower voltage. This shows that a mastercard output has some kind of internal resistor too.
And so empirically I have connected 2 leds in series without resistor and I had (if I remember correctly) some 8mA going
through the leds. Try it out with your leds , measure the current drawn with the two leds in series and see if not too much is drawn out. Then its sometimes better to have two leds burning then one only with the rest of the power lost over a resistor.
Greetz Peter
I have 13 5mm leds, 35mA 2.0Vf each, each connected to one output of my mastercard J2 for my MCP with no resistor and they are working fine. I just wondered if not using resistors could cause any other problems .
Hi,
I think this is not OK... you should disconnect one led, connect an ampere meter in between and measure the current going
through when the led is switched on. But as the drop voltage is only 2V I doubt the current is limited to 35mA.
I would make that current measurement and connect a resistor in between to keep it under 20mA (I think that is the max current put foreward by opencockpits) . Otherwise failure of the output buffers of the mastercard might occur faster then expected.
Greetz Peter