I am setting up control panels with bicolor (3 lead) LEDs for turnout indication. I selected a 160 ohm resistor based on a 5V power supply, an LED current draw of 20 mA and LED voltage of 1.8 [(5 -1.8) / 0.02 = 160]. However, when I put the resistor in series with the LED (on the cathode side) and measured the actual current draw, it was much higher than planned, around 30 mA. I moved up to a 430 ohm resistor and got the current down to around 11 mA, but it should have been much lower according to my calculation [(5 - 1.9) / 430 = 0.0074]. Can anyone explain the anomoly? Could it be due to how I am making my current measurements? Is there something going on with the current supply? It measures 5V, but the calculations suggest that it is putting out 7V. I am using a switching power supply (Cincon TR1505) that is supposed to hold the load to within 4%. Do these power supplies somehow compensate for the resistor voltage? Has Ohm’s Law somehow changed?
Not knowing how you measured the current, I’d suspect the probelm was there. A few things could be going on.
The resistor is probably a 10% tolerance device, so you have quite a range of possible actual values. You could measure it and wee what you really have. Then, calculate the current by measuring the voltage across the resistor. You can also check the voltage across the LED, while it is lit. I think there could be a bit of variation there, as well. If you are happy with the light with the larger resistor you can’t go wrong, since you are limiting the current enough, or more than enough.
Do both colors have the same problem? Some LEDs have different specs for each color. 150-160 ohm seems correct for most LEDs at 5 volts and 20ma. Measure the power supply output both DC and AC. On the AC scale the reading should close to or “0” volts. If you get a reading the power supply may be putting out ripple or AC voltage. Some switching supplies do this at low loads.
Also the 20 ma is typical , or another words +or- . Also 3 color Leds as most people use them run on +, _. and AC? Fred
Also, switching power supplies “like” to see a minimum load…didn’t see one specified on the spec sheet, though.
Do you have an LED part number?
All good answers. Check the supply voltage, Odds are it’s over 5V, especially if it’s not regulated. And if you are using an old computer power supply, the 5V side is almost ALWAYS high, even WITH the required minimum load. Without that load, the 5V can shoot way up. The 12V is usually low without proper load.
Does the spec sheet for the LED say 1.8v? It may have a different forward voltage than that. And common resistos are 10% tolerance. Combined with a power supply putting out a bit more than the 5v expected and you can hit the 30ma mark.
That’s the downside of a lower voltage supply, there’s not much room for error in the resistor value before you exceed the LED’s current limit. 20ma is usually the upper limit for LEDs - calculate using 10ma and use the next smallest standard value resistor, rather than try to calculate at the max.
–Randy
As Randy said, all good answers. Beware that your power supply output voltage may also decline as you add load. For all the reasons mentioned above, I use regulated voltage for any non-loco LED circuits. This is easily achieved by adding a voltage regulator. You can use a 7805 to keep your supply at 5VDC, or you can use an LM317T and adjust the voltage to match your LEDs. Both are widely available, the Radio Shack part numbers are 276-1770 and 276-1778 respectively. Most packages have wiring instructions. Just bear in mind that you may need more than one depending on the total amperage being supplied.
Thanks for all of your suggestions, but I am still at a loss to explain this. In answer to all of your questions: I have measured the power supply output and it is 5.14 V DC. I am measuring about 0.02 V AC when it settles down, but this may just be noise from my multimeter. I have the same problem with both colors of the LED. I don’t have a spec sheet for the LEDs (I bought them from Demar), but I am measuring 1.9 to 2 Volts across the LED depending on which color (red or green) is powered (the unpowered side show a minus 0.9 V at the same time. Does that make any sense?). The resistors are measuring very close to their ratings so that doesn’t seem to be the problem. The power supply is new and a switching type. Does this mean that it is regulated? Is this problem likely to go away as I add more LEDs in parallel and the supply voltage drops? I was planning to eventually power about 50 LEDs on this power supply (output is 2 A). If the problem is likely to persist, should I move to a voltage regulator (or just use a larger resistor)? Is a voltage regulator the same thing as a regulated power supply or are these different? Again, thanks.
How is the battery in your DMM? We have noticed at work that certain meters, notably the Fluke 87 will need a battery replacement even though the low bat icon or whatever indicator is used may still say you have useful power left. This will sometimes give you false readings on the lower scales.
Jim
What’s the voltage across the resistors. I think measuring mA with a meter is a recipe for misleading readings.
My appologies on this, it looks like I have led you all on a wild goose chase. After taking additional measurements, it looks like the high current flow was due to my inability to correctly use the multimeter on my circuit. I believe that I was bypassing the LED and the 2V voltage drop throught it, which accounts for why the measurements indicated a higher current than calculated using Ohm’s Law. The tip off to me should have been the fact that the LED went dark when I was taking my current measurements, but I am just learning how to use the multimeter and it obviously took me awhile to figure out what I was doing wrong. When I correctly measured the circuit current, it showed about 7 mA, which is just about what would be expected with a power supply voltage of 5.1 and resistor of 430 ohms [(5.1-2.0) / 430 = 0.0072). Sorry for putting you through this, but thanks for the input - it will defineitly help me in the future.
Hey, that’s how we learn–from our mistakes. I’ve been in the electronics business for decades and I still make silly mistakes. So, I must be continually learning. [:I]
Never appoligize for asking questions. It provides mental exercises which keep our brains in shape.