Tour of my DIY Launch Controller

[jrap330]

Very nice Josh...love the black case..any reason for using screw terminals on base unit versus using banana plugs and jacks across the board?

 

[jqavins]

Someone stated a lot of LEDs have an internal 1K resistor..whether true or not I do not know. Look at LED specs and determine if it states a resistance value...if not buy it and slap an ohmmeter across it..one why you will get a value...change leads you will get infinity. I hate to buy resistors just to leave them in my garage. Radio shack come back....

Noooo!!!

Some may have an internal resistor. Others may have an internal current limiter that does not have a V=IR linear characteristic. Such a current limit may or may not be low enough to prevent ignition. All have a nonlinear characteristic, no matter what current limiter they have or don't have, because that is intrinsic to all diodes.

There will be a voltage drop when the LED is conducting, and an ohmmeter will report a resistance of that voltage divided by the current, but that is not a true resistance. What the ohmmeter tells you means basically nothing except which lead is which (and it won't always tell you that.) If there's an internal current limiting feature, it may well not protect the igniter.

If you want the simple answer, us a resistor. If you want a more sophisticated answer, read the LED's spec sheet to see what it has for current limiting, if anything, and use that to determine if you need the resistor. But you should never rely on what an ohmmeter tells you about a diode.

 

[Joshua F Thomas]

Someone stated a lot of LEDs have an internal 1K resistor..whether true or not I do not know. Look at LED specs and determine if it states a resistance value...if not buy it and slap an ohmmeter across it..one why you will get a value...change leads you will get infinity. I hate to buy resistors just to leave them in my garage. Radio shack come back....

I have not run into any of these mythical LEDs with internal resistor, and I've built a lot of electronic stuff.

 

[Joshua F Thomas]

Very nice Josh...love the black case..any reason for using screw terminals on base unit versus using banana plugs and jacks across the board?

The screw terminals are a more solid connection. Can't come as easily loose. Those terminal strips are also less expensive and more easily available than banana plugs and jacks, which I had a surprisingly difficult time sourcing. Apparently everyone wants them. You can certainly use the banana plugs, it should still work electrically.

 

[Joshua F Thomas]

But you should never rely on what an ohmmeter tells you about a diode.

Quoted for truth. Do not depend on a diode for current limiting. Diodes are not linear response devices.

 

[jrap330]

Noooo!!!

Some may have an internal resistor. Others may have an internal current limiter that does not have a V=IR linear characteristic. Such a current limit may or may not be low enough to prevent ignition. All have a nonlinear characteristic, no matter what current limiter they have or don't have, because that is intrinsic to all diodes.

There will be a voltage drop when the LED is conducting, and an ohmmeter will report a resistance of that voltage divided by the current, but that is not a true resistance. What the ohmmeter tells you means basically nothing except which lead is which (and it won't always tell you that.) If there's an internal current limiting feature, it may well not protect the igniter.

If you want the simple answer, us a resistor. If you want a more sophisticated answer, read the LED's spec sheet to see what it has for current limiting, if anything, and use that to determine if you need the resistor. But you should never rely on what an ohmmeter tells you about a diode.

Thanks for correcting my error.

 

[jqavins]

I have not run into any of these mythical LEDs with internal resistor, and I've built a lot of electronic stuff.

As much as 40 years ago I used LEDs with a built in clamp, but not a resistor. You could hook them directly to a 5 or 6 volt source and they were perfectly happy. I don't know where to find them today. And in any case, I wouldn't trust them to protect the igniter.

 

[neil_w]

A built-in resistor is such a trivial convenience when you're already going to the effort of assembling a device with many components. Resistors cost nothing and are easy to solder in, and I would so much prefer to choose exactly the LED I want, and set the continuity current to the desired value.

 

[jqavins]

Hear hear. If you've got a built in resistor, you still need to determine whether or not it's enough and you are likely to add a resistor of your own anyway. With a built in clamp like I used to use (not in a launch controller) if that's too much current then adding a resistor complicates matters in unknown ways.

Use a resistor. Use a simple, uncomplicated LED and a resistor.

Horse dead. Beating terminated.