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My DC adapter has AC output is that right? Answered

Hey guys i obviously need more sleep.

I brought a wall adapter so i could make a huge amount of LED flash to music. Now that i have it home I released i have a different one than i intended to pick up. Its 12VDC 2.5A i only wanted the 1A mostly cos it was cheaper. Will the extra 1.5A make a huge difference especially to the TIP31?

The adapter actually has AC Adapter written on it and the packaging says Switchmode Regulated Plugback. I'm gonna need a multimeter before i plug the LED's to the power or will it be safe to just go by the numbers on the adapter?




7 years ago

A DC adapter means DC output !!

A TIP31 is a  3A  40V  low-gain  NPN  transistor.

Can't  answer your  "huge difference"  question  until  I see  your circuit.

You should all ways make sure with a multimeter on parts you get.


My circuit is essentially this https://www.instructables.com/id/Music-LED-Light-Box/step6/Building-the-circuit/ but i will put 6 LEDs in series and 10 parallel so its 60 all up.

The LED series parallel array wizard told me that they will only draw 350mA if i have the LED forward current at 20mA and 500mA if they have 50mA forward current it doesn't seem right to me.

Drawing such little current compared to what the plug is rated at will mean the voltage coming through will be significantly greater?

Thanlks for the adapter help its so ridicuously obvious i couldn't make sense of it, silly me.

I  saw your reference ible
And saving the kicker problem for last,   let us work the current question first.
I hope your LEDs are all the same ( otherwise this is probably not gonna work ).
If the LED current is 20ma the 6x10 will need a total 200ma
If the LED current is 50ma the 6x10 will need a total 500ma
The TIP31 can easily handle 500ma.

I do not believe a single plastic LED has a forward current of half an ampere.

Now the Kicker ;
Your target circuit does not use resistors
Fact ;
LEDs also have a characteristic forward voltage which varies a little even in the same batch
of one color and manufacturer.
When you stack 6 in series these Voltage tolerances can add up badly
( one string of 6 could be almost a volt lower then another string of 6 )
This could make some strings dimmer and some brighter which I'm sure you do not want

Solution ;
Add 10 resistors just like the picture
Now you can have a balanced set of lights
and if you are willing to have different resistors you can mix colors.
I think Resistors are just so Great.

There are other engineering improvements I would do but not in this text box.
When you have the actual supply voltage and the actual spec LED current and voltage drop,
then it is possible to calculate resistor values.

Good Luck


I haven't been able to purchase a multimeter yet but i tried the circuit anyway. It worked but very in consistently the trouble with the LED's is i brought them from dealextreme and they don't provide a data sheet.

I assumed they 2.0v so i put 6in series they light up very dull so i removed the 6th one in each row and they a little bit brighter but they don't all flash at once now it will be the 2centre rows or the outside rows alternating.

Possible bescause of the LED's being very different or would solder affect the conductivity? I twisted the wires then i soldered.

When i touch the bottom of my board where all the joins are the ones i touch light up slightly even when not plugged in, is that my body supplying power to the circuit?

Also if they draw 20mA each why will 6 in series also draw 20mA? I don't understand.

Thank you you've been a wonderful help

1)  Sounds like loose connections or a mistake wiring. 
      Did you wire per my diagram with a resistor ?
      Did you test the LEDs without the TIP31 or was it in the test ?
      If the LEDs are dim ( dull ) try changing the resistor to one with less resistance.
      It could be your wall wart is not supplying enough voltage.
      Now it would help if we had a measure of the voltage.
2)   A good solder joint helps guarantee reliable connections.
       A poorly soldered joint can mess up almost any circuit.

3)  You must report to Dr. Who immediately,  your amazing electro body
      can power the Tartis :-D
      It could be static, or your body is completing part of the circuit
      if your circuit still plugged into the electric outlet.

4)  That is the basic property of electric topology.
      ( current is like water flowing through a pipe )
      Components in series share the same current.
      Components in parallel share the same voltage (each may have a different current ).



7 years ago

If your description is accurate, it's a switching power supply. That's regulated, so the output will be 12V regardless of the load. One LED or sixty LEDs, the output voltage should remain stable.

A power supply with greater current capacity isn't a problem. Current draw is a function of the circuit itself, not the PS.

"AC Adapter" surely means that it adapts mains AC, to low voltage DC?


Yes that makes perfect sense. I haven't been having a good day in regards to thinking things through.

A power supply, unless specified otherwise, is rated by the voltage it produces and the maximum current you can draw from it without that voltage dropping or the power supply disabling or cooking itself. Having extra amps available has no effect; your circuit will draw what it needs. Not having _enough_ amps available would be a problem.

if it says 12VDC, that means 12VDC output. When they say "AC Adapter", they mean an adapter which plugs into your AC wall outlet.

I have personally bought a Radio Shack 12VDC wall plug supply
that measured 17VDC open circuit and
only measured 12VDC at rated current.
Obviously unregulated,
Just a Bridge and Capacitor supply.

I'm pretty sure Scotty has an unregulated wall wart.
What if Scotty's circuit does not use the rated current
and the voltage is high enough to damage something ?

Measuring my wall plug helped me make a more tolerant circuit.


Interesting. Yes, I've seen unregulated power supplies... but I haven't seen them often. Apparently Radio Schlock assumed that particular brick was going to be used with something that either didn't care or had its own regulator.

Putting a meter on it is always a good idea, in any case. Sometimes they're defective.

all ac adaptors give out dc unless specified.

if you want to be sure, check and see if there is a ground wire, its often the third additional wire.

onlly devices with a high ac output will have the grounding wire, especially if its a stepdown transformer, (converts high voltage ac to low voltage ac), and 12v 2.5A is rather high.
though usually on ac adaptors with ac output, they dont specify the current.

anyways, ac or dc should realy matter with leds, as they only allow one way.
but if you cant prove it isnt dc, just add a diode onto one of the wires. diodes only allow current one way, hence, you will convert it into dc, though the led's with do that too, im not sure as to wether ac damages them.