Wall Wart outputs are wrong!

[hugoender]

So I have two random wall warts.

1) Plantropics
On the label it says:
-Input: 120VAC 60Hz
-Output: 9VDC 300mA

On both of my multimeters it says:
-Output: 14.95V !!!!!

2)Cyber Acoustics
On the label it says:
-Input: 120VAC 60Hz
-Output: 15VDC 400mA

On both of my multimeters it says:
-Ouput: 20.6V

So what is up with this!? Why are they so off? I was hoping to use the 9V wall wart to power my multimeter (and get rid of the 9V battery) but good thing I checked the voltage first!

 

[dknguyen]

That's just the way they are. These are unregulated wall-warts and will probably be closer to 15V under maximum load. Anything less, and they will be a higher output voltage since they are unregulated.

I tested about 30 wall warts at work and none of them were within 2V of the labelled voltage.

You can also buy regulated wall-warts.

 

[Mikebits]

Because they need a load.

Opps look DK beat me to it Which makes me think. It would be nice if the system could send a notice when another reply has been added while your in the middle of a post.

 

[mneary]

Your wall warts are not regulated. The 9V rating is only with a load. If you draw 300 or 400 mA the output will be closer to the nameplate voltage.

 

[hugoender]

WOW very prompt responses! lolol are you guys lying in wait for me to create a thread so you can pounce on it? Well thanks for the info

 

[hugoender]

Wait...one more thing... so can I use the 9V wall wart to power my DMM or is that a bad idea? I really want to make my DMM non-battery operated.

 

[dknguyen]

Because they need a load.

Opps look DK beat me to it Which makes me think. It would be nice if the system could send a notice when another reply has been added while your in the middle of a post.

It wouldn't be a problem if you just typed faster

Wait...one more thing... so can I use the 9V wall wart to power my DMM or is that a bad idea? I really want to make my DMM non-battery operated.

See how much your DMM draws, and what the voltage on your wall wart sags to under that same load. If you get a wall wart rated for a lower current, it maybe closer to the labelled voltage under no load.

 

[mneary]

If your DMM uses an ordinary "9V" battery then put the wall wart thru an LM317 hooked up to provide 9V. Don't forget the capacitors.

 

[mneary]

lying in wait for me to create a thread so you can pounce on it?

It's just before dinnertime. Either surf now or help in the kitchen.

 

[Torben]

I'd go with mneary's idea. Using a wall wart as the stepdown and rectification stages and just putting a voltage regulator (and necessary capacitors) on the output is a good, safe* way to make a first regulated linear power supply, without having to mess around with mains voltage since that's handled in the wall wart.


Torben

* - Well, safer than making your own from scratch. I take no responsibility if something catches on fire/explodes/zaps you/etc.

 

[Boncuk]

I have tested lots of wallwarts made in Germany and all of them have a safety margin of 5 to 10% of the nominal power.

E.g. rated for an output of 12V at 1A the stamped voltage decreases close to 12V at a load current of 1.05 to 1.1A.

However there is always enough room inside the enclosure to mount a low dropout voltage regulator - sometimes even enough for a small heatsink.

I think it is no good idea to operate a DMM from an unregulated power source. DMMs normally indicate low voltage, but don't care for voltages too high. The reference voltages will shift for a higher value and all you'll measure is nonsense, if the DMM hasn't been fried earlier.

There is a German common rule: "Wer viel mißt mißt viel Mist.", translated about "Who measures a lot measures a lot of garbage".

Couldn't translate "Mist" literally because of the risk for a ban.

Hans

 

[hugoender]

okay while time to create a votlage regulator

 

[Torben]

okay while time to create a votlage regulator

Probably simplest just to buy the voltage regulator. As mneary said, an LM317 is a good choice, as you can set it to output different voltages. An LM7809 will give you 9V regulated, an LM7812 will give 12V regulated (but you'll want more voltage going into it), etc.

Be sure to read the datasheets and use the correct capacitors to prevent it from oscillating and to help it deal with transients.


Torben

 

[hugoender]

Probably simplest just to buy the voltage regulator. As mneary said, an LM317 is a good choice, as you can set it to output different voltages. An LM7809 will give you 9V regulated, an LM7812 will give 12V regulated (but you'll want more voltage going into it), etc.

Be sure to read the datasheets and use the correct capacitors to prevent it from oscillating and to help it deal with transients.


Torben

Yeah.. I am making the dual voltage supply in the LM337 datasheet. I have the LM317 and LM337. The only thing is that I am not sure how to provide the negative voltage to the circuit as shown in the datasheet.

 

[Sceadwian]

Check any wall wart you're using under multiple load conditions. Simple power load resistors are a good way to check it, determine the voltage under the load conditions at and near where your device draws power at, a regulator may not be needed.

 

[Torben]

Yeah.. I am making the dual voltage supply in the LM337 datasheet. I have the LM317 and LM337. The only thing is that I am not sure how to provide the negative voltage to the circuit as shown in the datasheet.

How did you finally decide to generate the negative voltage?


Torben

 

[hugoender]

How did you finally decide to generate the negative voltage?


Torben

I just connected the -25V in the circuit schematic on the datasheet to the ground wire of the wall wart and connected the non-ground (live?) wire to the ground in the circuit.

And no I did not put that post in the wrong thread...its actually meant for both threads so I will link to it here: https://www.electro-tech-online.com/threads/road-to-success-in-hobby-electronics.40305/#post326810

That is my little power supply

 

[AllVol]

I have the LM317 and LM337. The only thing is that I am not sure how to provide the negative voltage to the circuit as shown in the datasheet.

You're not looking at the ground connection, are you? I've built several regulator circuits, and none have negative supply unless I use one of the negative regulators, such as LM7905, etc.

 

[Torben]

You're not looking at the ground connection, are you? I've built several regulator circuits, and none have negative supply unless I use one of the negative regulators, such as LM7905, etc.

The LM337 is the negative complement to the LM317.


Torben

 

[hugoender]

You're not looking at the ground connection, are you? I've built several regulator circuits, and none have negative supply unless I use one of the negative regulators, such as LM7905, etc.

As Torben already said, the LM337 is the negative voltage variant of the LM317.