rrinker Yes, many meters in the 'ohm' setting are actually reversed compared to what you'd expect by the color of the leads, or even the labels on the instrument. The easy one to remember about LEDs is the flat - it's flat, like the minus sign, because it's the negative lead. --Randy
Yes, many meters in the 'ohm' setting are actually reversed compared to what you'd expect by the color of the leads, or even the labels on the instrument.
The easy one to remember about LEDs is the flat - it's flat, like the minus sign, because it's the negative lead.
--Randy
Good mnemonic. I'll remember that one. Flat is minus 'cause it looks like a minus sign.
David Starr www.newsnorthwoods.blogspot.com
I have some P1K C-liners, and they came with LEDs. A DH165LO (for Life-Like engines) should work just fine, and keeps the 1.5 volt bulbs or LEDs from blowing. Remove the factory diode board, and plug in.
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
dstarr I do the ohmmeter thing 'cause I can never remember whether the long lead or the lead near the flat spot should be plus or minus. Or the flat spot is so tiny I can't see it. Or the leads are the same length. Or it's a surface mount LED without any leads. The ohmmeter trick always works. One caution. Most (maybe all) ohmmeters make the red lead plus. But I usually check a new meter against a flash light battery. Measure the resistance of the battery. The resistance will read HIGHER, when the battery voltage opposes the ohmmeter's internal battery. The resistance should read higher when the plus end of the battery is connected to the red lead of the meter. If it doesn't something is wrong.
Since not all meters have the positive voltage on the red lead for measuring resistance, it's a good idea to determine which lead does and remember it or mark the meter. What I do is measure the resistance of a rectifier diode, like a 1N4001 or similar. Whichever way you connect the meter's leads that shows a low resistance, the positive meter lead is touching the UNBANDED end of the diode. The meter lead on the end of the diode with the white band then has the negative voltage.
..... Bob
Beam me up, Scotty, there's no intelligent life down here. (Captain Kirk)
I reject your reality and substitute my own. (Adam Savage)
Resistance is not futile--it is voltage divided by current.
LEDs don't care which lead the resistor is in. Works the same in either lead. LEDs DO care about polarity, they have to be forward biased or they don't light. I use an ohm meter to figure out which way to hook a LED up. Measure the LED resistance, first with the red lead of the ohmmeter on one LED lead and then switch the LED around and ohm it out in the other direction. One direction will read low resistance and the other direction a higher resistance. When the LED shows a low resistance, it is forward biased and in fact you can often see it glowing faintly from the ohmmeter current.
You want to install the LED with the plus lead from the decoder going to the same pin that the red lead of the ohmmeter was on when the LED was forward biased. The lead from the decoder is most likely plus, but you can use the meter to check that, just in case.
I do the ohmmeter thing 'cause I can never remember whether the long lead or the lead near the flat spot should be plus or minus. Or the flat spot is so tiny I can't see it. Or the leads are the same length. Or it's a surface mount LED without any leads. The ohmmeter trick always works. One caution. Most (maybe all) ohmmeters make the red lead plus. But I usually check a new meter against a flash light battery. Measure the resistance of the battery. The resistance will read HIGHER, when the battery voltage opposes the ohmmeter's internal battery. The resistance should read higher when the plus end of the battery is connected to the red lead of the meter. If it doesn't something is wrong.
LEDs don't like reverse bias. Many of them will destroy themselves in milliseconds (too fast for the eye to see) if subjected to reverse bias of 5-6 volts. If you make a mistake and wire a LED in backwards, you will convert it to a Darkness Emitting Diode (DED) the instant power is first applied.
Manufacturer's name of LED's doesn't matter much, electrically all LED's are the same. They need a resistor to limit forward current to 20 mA or less. In fact 10 mA makes 'em glow plenty bright enough for model railroad work. When forward biased they all show about a 2 volt drop across the LED. Reverse bias of 5-6 volts destroys them.
Reistor goes to either lead, does not matter. Long lead of the LED goes to the BLUE wire on the decoder, which is the plus side and common for all functions. If you've already cut the leads, there is usually a flat on the LED case which indicates the NEGATIVE wire, which goes to the white or yellow or whatever function wire. Also, if the LED has a clear case, the bigger chunk of metal is the negative lead, the smaller piece is the plus.
Actually it doesn't matter at all which lead of the LED gets the resistor as long as it is series with the LED.
It does matter which decoder wires go where.
The mind is like a parachute. It works better when it's open. www.stremy.net
I have bought some LED's that folks here said would work well from LHS, but lost the tag off the bag, and I do not remember the name brand.They came with there own resistor. I know the resistor goes on the long end of the LED. Using a Digitrax hard wire harness say a DH 123 white wire goes to the end with of the LED with the resistor and blue wire goes to the short end, right? In this case I am wiring a Athearn F7 A so there is no back up light so blue wire is not being used as a common so to speak.
Next project will be a PK 1000 E-3's, wires are all black. In upgrading these engines stock PC board are removed. I used a DH 123 with a plug in harness. How do I tell which side gets the resistor with all the wires being black. On that same note one of the F3's still has it stock LED (I blew the other one with a DH 163) I like to save the stock LED using a resistor that came with the LEDs I bought again which side. Or What Digitrax Decoder Will Work With It's Stock LED? Assume buy one and save it stock LED.
Next engine will be Erie Built PK 1000, is it a LED or bulb? It stock headlight sucks, so I am hoping it is a bulb. Again it has all black wires, but it will keep it stock PC board.
Cuda Ken
I hate Rust