9V DC power supplies putting out 13 - 16Volts?

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Robindale Station

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Aug 5, 2007
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Hello. I am planning to wire up some LED's for my layout, and have been taking advantage of the helpful current and resistor calculations on ledcalc.com. I've taken an old 9V DC power supply from a phone as my power source. When I measure the voltage from the supply, however, it reads 13.7 VDC. Much higher than the stamp on the power supply says. I tried a different 9V power supply, and that one read 16VDC!! I'm glad I checked (and yes, the voltmeter was set to VDC). Does anyone know why this might be happening? Seems like I'll have to find a 5 Volt DC power supply to get 9V, but that seems wrong, too. Do the power supplies need some kind of a load?

Thanks for any help,

Chris
 
Robindale Station said:
Hello. I am planning to wire up some LED's for my layout, and have been taking advantage of the helpful current and resistor calculations on ledcalc.com. I've taken an old 9V DC power supply from a phone as my power source. When I measure the voltage from the supply, however, it reads 13.7 VDC. Much higher than the stamp on the power supply says. I tried a different 9V power supply, and that one read 16VDC!! I'm glad I checked (and yes, the voltmeter was set to VDC). Does anyone know why this might be happening? Seems like I'll have to find a 5 Volt DC power supply to get 9V, but that seems wrong, too. Do the power supplies need some kind of a load?

Thanks for any help,

Chris
Click to expand...

That's correct, the power supplies need a load. I got a 12v/1.5Amp power supply from Radio shack that was supplied with a $8000 micro controller, and totally freaked when I measured the voltage as 15.8 volts. I checked with several sources and confirmed that is normal for the simple types of AC-DC power converters that come with the household devices/ battery eliminators.


Make sure you put a resistor inline for your led's, and all will be well. I got from RS a few 12v/20mA LED's for a project, so following the formula V=IR,

12v = (.02A)I I=12/(.02A) = 600 ohm resistor, I put a 580 or a 680 ohm resistor (whatever was available) inline, and all was well.
 
Are you sure the LEDs were 12 volts? Typically, LEDs are 2 volts and 10 - 20 mA. Depending on what you are using the LEDs for, you may have to vary the resistors for the different colours of LEDs. Typically, a red LED will require a higher valued resistor (1,000 ohms on 6 volts) to tone down the brightness of the LED.

Bob M.
 
Most power supplies have a min-max rating on them. I have been working with mA power supplies lately. They usually read something like: "5VDC 200mA~500mA." I like to test them at both settings. Usually I use a resistor and series of LEDs, along with the multimeter. The LEDs are just for fun, and serve as a visual indication of voltage spikes/fluxes. Sometimes my digital multimeter misses them due to the timing of its cyclic counter.


Also be sure to check the circuit a few times before final assembly, if you are using more than about 4 LEDs.

Once LEDs warm up, they usually require slightly less power. If you're running more than 4 LEDs, you run the risk of over-driving them. Once that happens, you have a DSF (Darkness Sucking Friode).

And.... Important!
When hooking multiple LEDs up in PARALLEL:
Every LED needs its own resistor. Even so....
The weakest LED will have more current pushed to it by the stronger ones. This runs the risk of over-driving it, and turning it into the dreaded Friode. Because over-driven diodes no longer conduct (99.9% of the time), you have a break in the circuit. The problem becomes compounded at that point. Power for the Friode gets diverted to the remaining LEDs. The next weakest link goes pop (pretty funny when it's audible). Now you have a logarhythmic increase in power with each newly created DSF, and begin a cascading effect that will kill the entire LED chain.
This is the worst-case scenario, and rarelly happens to people that run the calculations. In your case, you should be fine.

When hooking LEDs up in SERIES:
One resistor will suffice for the whole chain. (Usually)
If one pops for some reason, the chain goes dead. This creates some irritating troubleshooting, at times. However, you only have to find and replace the one DSF; instead of the entire chain.

With the LEDs you've chosen, I dont think you have much of a choice. Parallel is probably your best bet. Otherwise, you have to remove the resistors from some (or most) of them.
 
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