BDL168 LED Spec

[halkuff]

Hi, looking for the LED spec - the Digitrax write-up is hard to translate into a off-shelf part

[rrinker]

Do you mean for the 10 pin connectors for external LEDs? Just hook up LEDs - no resistors needed, they are already on the board.

 Key to understanding the manual is know that LEDs are current driven devices. You can think of it this way: a regualr incandescent light bulb is a voltage device. it has a set voltage rating. Apply a higher voltage than the rating, and it will burn out. Apply a lower voltage, and it will still light, though be dimmer, until you get too low for it to even be visible. An LED is a current device. It has a specified maximum current rating. Give it more current than than, and it will effectively burn out. Apply lower than that current, and it will still light up - LEDs are pretty efficient and so work at well under the max current, until you get too low for it to work.

 The BDL-168 manual says the outputs are set for 3ma, while most any LED you will find will be rated 20-25ma. So, the supplied current is less than the limit of the LED, and thus it will work fine with no additional consideration. The only voltage consideration for an LED is the supply voltage needs to be at or above the voltage drop of the LED. Most red/green LEDs are around 2.8V, and the BDL-168 manual says the outputs are 5V, so again, no problem.

 Voltage for an LED acan be thought of like the current for an incandescent bulb. Say you have a 1.5V 15ma bulb. You need to supply no more than 1.5V (see the voltage part above), and you need a power supply that provides at least 15ma. And less and the power supply is overloaded. If the power supply produces more than 15ma, it won't hurt, the lamp only draws 15ma, even if the power supply can handle a load of 500ma. So with an LED, it has a voltage rating, say 2.8V. So your power supply needs to supply at least 2.8V, but if you have a 5V power supply, it still works - the key is making sure the LED gets no more than the maximum current rating.

 More than you wanted to know, but understanding a little of the background will help understand why the answer is: just connect an LED as shown and don't worry about it

                      --Randy

[halkuff]

[quote user="rrinker"]

Do you mean for the 10 pin connectors for external LEDs? Just hook up LEDs - no resistors needed, they are already on the board.

 Key to understanding the manual is know that LEDs are current driven devices. You can think of it this way: a regualr incandescent light bulb is a voltage device. it has a set voltage rating. Apply a higher voltage than the rating, and it will burn out. Apply a lower voltage, and it will still light, though be dimmer, until you get too low for it to even be visible. An LED is a current device. It has a specified maximum current rating. Give it more current than than, and it will effectively burn out. Apply lower than that current, and it will still light up - LEDs are pretty efficient and so work at well under the max current, until you get too low for it to work.

 The BDL-168 manual says the outputs are set for 3ma, while most any LED you will find will be rated 20-25ma. So, the supplied current is less than the limit of the LED, and thus it will work fine with no additional consideration. The only voltage consideration for an LED is the supply voltage needs to be at or above the voltage drop of the LED. Most red/green LEDs are around 2.8V, and the BDL-168 manual says the outputs are 5V, so again, no problem.

 Voltage for an LED acan be thought of like the current for an incandescent bulb. Say you have a 1.5V 15ma bulb. You need to supply no more than 1.5V (see the voltage part above), and you need a power supply that provides at least 15ma. And less and the power supply is overloaded. If the power supply produces more than 15ma, it won't hurt, the lamp only draws 15ma, even if the power supply can handle a load of 500ma. So with an LED, it has a voltage rating, say 2.8V. So your power supply needs to supply at least 2.8V, but if you have a 5V power supply, it still works - the key is making sure the LED gets no more than the maximum current rating.

 More than you wanted to know, but understanding a little of the background will help understand why the answer is: just connect an LED as shown and don't worry about it

WOW! Thanks