Will this wall wart fry my LEDs?

[MillenniumFalsehood]

I have a wall wart for my Klingon D-7 cruiser. The cruiser's LEDs consist of two white LEDs and two yellow LEDs. The stats of the wall wart are:

UNIDEN AC Adapter

Model: AD-420
Input: AC 120V 60HZ 7W
Output: DC 9V 350mA

The circuit was designed to run on a 9v battery, but now that I have the wall wart I am curious: will the 350 milliamps fry the LEDs and if so, how can I remedy that? I'd love to use this, but I certainly don't want to fry the LEDs because I have already sealed up the model and I don't want to tear it up again.

[macfrank]

The circuit was designed to run on a 9v battery, but now that I have the wall wart I am curious: will the 350 milliamps fry the LEDs and if so, how can I remedy that? I'd love to use this, but I certainly don't want to fry the LEDs because I have already sealed up the model and I don't want to tear it up again.

Did you use a resistor on the LEDs? If you designed the circuit properly and the LEDs each has a current limiting resistor to limit their current to a safe value at 9V, then it shouldn't matter how much current your wall wart has, as long as it's enough to light the LEDs.

[MillenniumFalsehood]

To be clear, the circuit is a series, with a resistor and four LEDs in the same line. Will it still be okay to use the wall wart?

[jwrjr]

If it will work with a 9 volt battery, then it will work with a 9 volt AC adapter. The current rating doesn't enter into the matter. But make sure that the resistor is there, and not depending on the internal resistance of the battery (like the keychain led flashlights).

[tetsujin]

UNIDEN AC Adapter

Model: AD-420
Input: AC 120V 60HZ 7W
Output: DC 9V 350mA

Consider this:

Often, these stats lie. Specifically, with an unregulated wall wart, the voltage is usually intended to mean "you could connect this wall wart to a voltage regulator to get this voltage". So that 9V wall wart could be giving you 10 or 11 volts (or any higher amount, really), and the voltage put out by the wall wart may vary depending on what's connected to it. You'll want to test that: design a simple LED circuit made to operate between 9-12 volts, hook it up to the wall wart, and use a voltmeter to see how much voltage the wart is putting out. The LED and resistor in series are important 'cause you want to know how much the thing puts out under load - not how much it puts out when nothing is connected.

So if you have a circuit made for 9V input, it's safe to connect it to a regulated 9V supply. An unregulated supply, however, will give you more than 9V so you can regulate it down to 9.

In the case of your circuit - you've got four LEDs in series on a 9V supply? That would lead me to believe each LED drops around 2V, probably less, and so the resistor is supposed to drop 1-2V. So if your input changes from 9V to 10-11V, you're likely doubling the voltage drop across that resistor, and thus the current through your circuit. If the circuit was designed to give the LEDs their maximum rated current at the 9V input (that is, with no margin for error) then this change in input voltage could be fatal. If the circuit was designed to operate the LEDs somewhere in the middle of their operating range, then it may be OK - I would try some tests with a meter to find out more specific information about the circuit (plot its current vs. input voltage at a couple different points close to 9V, maybe, and compare that against the specs of the LEDs, if known) and, assuming the circuit can't easily be replaced if burnt out (that is, if it's built into a model already) then I just wouldn't risk it: I'd build or buy a regulator circuit and not hook up any input that doesn't go through that circuit.

[MillenniumFalsehood]

I made sure the LEDs are only getting the minimum required to function(I took a varistor and turned it all the way down, then hooked up the circuit and turned it up slowly until the LEDs were at the desired brightness. Then I read the resistance of the varistor and found a resistor that was similar to the voltage readout.)

[Navboyry]

Am I correct in gathering from this topic that mA dosn't really matter? As long as the volts arn't too high nothing will burn out?

I bought 2.5 Amp Regulated 1.5 -7 V wall wart. I have tested it with my light and nothing blew up, but then again I only ran it for a few seconds.

[macfrank]

Am I correct in gathering from this topic that mA dosn't really matter? As long as the volts arn't too high nothing will burn out?

It depends on what you're talking about. An LED (like all diodes) has a voltage to current curve. Below a certain voltage (around 3V for red LEDs)
an LED will draw only so much current. Increase the voltage and you'll draw a little more.

However, when you reach that "knee" at around 3V, a small increase in voltage will lead to the LED drawing a huge amount of current - eventually frying the LED due to the generated heat.
Typically you'll add a "current limiting resistor" , with a value based on the voltage that you're applying to the LED. The resistor acts as a voltage divider, making sure that the LED never sees a voltage past it's "knee", thereby limiting the current that goes through the LED to a safe value.

If you know the voltage that your wall wart is putting out (and it's well regulated) it's total current capacity is unimportant, as long as it's more than the maximum amount of current that your LEDs will draw.

The LED center is probably one of the best LED sites around, especially since they have easy to use calculators and an excellent FAQ.

Frank

[Sparky]

If the wall wart is not putting out the right voltage the 'current limiting' resistor will not save your LEDs. Careful reading of the LED manufacture’s literature will reveal this.

A resistor cannot compensate for changes in the power supply voltage. It will drop only the voltage defined by how much current the string is drawing, ohms * current. If the voltage on the power supply rises, this voltage will not be consumed by the resistor it will be passed on to the LED.

Also in the future be careful about mixing colors. Most of the time the LEDs of different colors want different currents. A series of circuit will have to be calculated with one current going through all elements. Inline current regulators could change that, but simple daisy chained LEDs all get the same current. If they all want the same current and different voltages you are fine, the resistor will absorb/drop the extra voltage at the given current and all will be super-doper.

Also be careful about measuring the wall-wart free standing. Most power supplies require some load to show the correct voltage. hook it up and connect it to a mockup of the LED chain you have in the sealed model. and check read the voltage across the power connector. If it is still to high , you can add another resistor right there to drop the extra voltage.

Use the same current rating you used to calculate the resistor in the ship.

Extra voltage/current
If you measure 9.8 volts. . .and your LED loop wants 20mA
0.8/20mA
40 ohms
Will be your extra resistor. Round up tot he nearest available resistor.

[Navboyry]

Sadly I am very mathmaticly stupid (and I'm going for a degree in Aerospace...). I'll just hook it up and run it at a lower volt setting and pray it wont melt or blow up my model.

[Sparky]

The math is very straightforward ohms law any IC men (I think thats the spelling, dad was one when he was in the navy) should be able to help you with this.

The best way to test it before hooking it up to the model is to make a duplicate chain of LEDs made with the same resistor used in themodel. Get ready to hook it up to the wall-wort and then we'll tell you were to take the measurement then we can go from there. If you take some life of the LEDs in your test loop it's no big deal.

[macfrank]

Sadly I am very mathmaticly stupid (and I'm going for a degree in Aerospace...). I'll just hook it up and run it at a lower volt setting and pray it wont melt or blow up my model.

Try the the LED calculator - no math required.

[Scott Hasty]

Bottom line: WAL WARTS SUCK, period. I have yet to find a regulated one and have yet to find one that comes close to their listed output voltage.

Bottomest line: I see a lot of guessing and whatnot here. BUY A DMM, PERIOD. You can get them super cheap just about anywhere. Then there is no doubt.

Scottie

[jwrjr]

I use AC adapters most of the time. There are two ways to beat the unregulated voltage problem (if not more). Use a voltage regulator to power the leds. Or choose led resistors so that a power supply voltage 25% higher than rated does not over-power the leds. I use both methods.