"variable" (switch) ac to dc diagnostic help please

[taiga]

Hello ladies and gents,
 - not sure if this is correct sub forum -

I am just starting this wonderful journey into electronics but before I can invest in a good variable power supply I was planning on using my switchable ac-dc adapter. it's got 1.5v 3.0v 4.5v 6v 7.5v 9.0v and 12v, so not too bad.
Now the issue is that, the power it's putting out on each of those settings is a far amount more....

stated out -- actual out
1.5v -- 3.4v
3.0v -- 5.6v
4.5v -- 7.5v
6.0v -- 9.5v
7.5v -- 11.5v
9.0v -- 14.0v
12.0v -- 18.0v

Any ideas on a fix. After looking inside I'd say its a simple device but I also don't really understand how the resistor switch is working

please let me know anymore info that may help

Cheers,
B

[SeanB]

The simple ones are a basic multitap transformer with the switch selecting the tap used to connect to the bridge rectifier. The output voltages will always be higher with no load, and with a load of 500mA ( at a guess on the rated current) the voltage will be closer to the indicated.

There is no real way to fix this short of using a proper regulated supply. The other alternative is to add an external regulator using something like a LM317 as a regulator and powering it with a voltage of about 3V more than the desired output.

[taiga]

Thanks for the reply I took another look and that makes more sense how the whole thing works. It's a 300mA max adapter. I tried with a 0.5A 12vdc computer fan and the voltage didn't seem to drop too much. Regardless I'll just have to start saving my pennies for an entry level power supply.

[birrbert]

I'll just have to start saving my pennies for an entry level power supply.

Or try to build one. It's an excellent opportunity to learn and in the end you'll have a great power supply.

Here's my topic: http://mjlorton.com/forum/index.php?topic=268.0
Of course, you don't need to go in the direction I went. You can follow Martin's project too; just search for the term "diy bench power supply" on his channel.

[taiga]

I'll just have to start saving my pennies for an entry level power supply.

Or try to build one. It's an excellent opportunity to learn and in the end you'll have a great power supply.

Here's my topic: http://mjlorton.com/forum/index.php?topic=268.0
Of course, you don't need to go in the direction I went. You can follow Martin's project too; just search for the term "diy bench power supply" on his channel.

I would like to build something like this but i am really just starting and feel like i don't know enough. maybe trying to do it anyways would be good for learning, or maybe i'll fry all the components :S

[steve30]

Hello ladies and gents,
 - not sure if this is correct sub forum -

I am just starting this wonderful journey into electronics but before I can invest in a good variable power supply I was planning on using my switchable ac-dc adapter. it's got 1.5v 3.0v 4.5v 6v 7.5v 9.0v and 12v, so not too bad.
Now the issue is that, the power it's putting out on each of those settings is a far amount more....

stated out -- actual out
1.5v -- 3.4v
3.0v -- 5.6v
4.5v -- 7.5v
6.0v -- 9.5v
7.5v -- 11.5v
9.0v -- 14.0v
12.0v -- 18.0v

Any ideas on a fix. After looking inside I'd say its a simple device but I also don't really understand how the resistor switch is working

please let me know anymore info that may help

Cheers,
B

I'd guess that it is an unregulated supply. Depending on the internal components, you might be able to stick a regulator like an LM317 in easily enough, and have either a potentiometer to control the voltage, or use the existing switch and a set of resistors (to give a switched fixed output, like it does now).

Could you supply a photograph of the inside so we can see how it currently works?

[TechJunkie]

Hello ladies and gents,
 - not sure if this is correct sub forum -

I am just starting this wonderful journey into electronics but before I can invest in a good variable power supply I was planning on using my switchable ac-dc adapter. it's got 1.5v 3.0v 4.5v 6v 7.5v 9.0v and 12v, so not too bad.
Now the issue is that, the power it's putting out on each of those settings is a far amount more....

stated out -- actual out
1.5v -- 3.4v
3.0v -- 5.6v
4.5v -- 7.5v
6.0v -- 9.5v
7.5v -- 11.5v
9.0v -- 14.0v
12.0v -- 18.0v

Any ideas on a fix. After looking inside I'd say its a simple device but I also don't really understand how the resistor switch is working

please let me know anymore info that may help

Cheers,
B

As SeanB stated, you need to place a load on the output to get a valid reading. Just don't place a load which draws more current than the rated output. Use Ohms laws and calculate a resistance which will give you about 75% of the max rated current load.

[taiga]

Thanks guys! I will try to upload a photo and also test with a load.
That's one of the things I find hard to understand, the voltage being a potential and changing with load. Lots to learn!!