Wall Wart outputs are wrong!

[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.

I'm a little confused. Do you have the LM317 circuit with +Vin hooked to the wall wart +, and gnd connected to the wall wart -, at the same time you have the LM337 circuit's -Vin connected to the wall wart - and gnd connected to the wall wart's +?

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

Again, nice work. I must have been confused by the fact that it's traditional to continue a discussion in the same thread in which it was started.


Torben

 

[hugoender]

I'm a little confused. Do you have the LM317 circuit with +Vin hooked to the wall wart +, and gnd connected to the wall wart -, at the same time you have the LM337 circuit's -Vin connected to the wall wart - and gnd connected to the wall wart's +?

I only have one wall wart with cut cables (the others one still have the cylindrical plug) and so what I did to make sure everything worked was connect the LM317 first (with ground to ground and 5VDC to +Vin). Then I plugged that same wall wart to the other side (with ground to -Vin and 5VDC to ground). If I were to make it for real, I would need to connect two wall warts to the circuit... one to power the positive and one to power the negative, unless you know of some trick to get around this and power the whole thing with one wall wart.

Again, nice work. I must have been confused by the fact that it's traditional to continue a discussion in the same thread in which it was started.


Torben

Yes, it just so happened that you pouncing on all my posts before I can even finish them so you read that other post before I was able to come here and post it here. The only reason I posted it on the other thread was because I post all of the stuff I do on that other thread. After all, all these circuits I am making are my Road to Success!

 

[Hero999]

Use a 6V wall wart or the 9V wall wart with an LM7808.

 

[Speakerguy]

What you're seeing is the effect of poor transformer regulation. Small transformers have very poor voltage regulation - 30% or more is very common. Big toroids like you might see in an audio amp have great regulation - 5% or less with the >300VA units.

Look at some specs for AC/AC toroid power transformers. You'll see that they might spec 15VAC @ 1A with 20% regulation. It will actually output 18VAC with no load, and 25.5VDC minus 1.4V diode losses = 24.1V after rectification and filtering caps.

If you load it down so it is actually drawing its full rated current, it will drop down to its rated 15VAC and you'll get 21VDC minus 1.4V diode losses = 19.6VDC.

Your wall-wart, like all unregulated wall-warts, has a really poorly regulated AC/AC transformer inside which gets rectified and filtered. Because it's poorly regulated the no-load DC votlage is higher than it should be, like in the example above. It's common for 12VDC unregulated supplies to be 15-16VDC at no load.

It's also good to keep this in mind when designing power supplies. If you size your filter capacitors in a power supply so that they will handle the nominal output of the transformer after rectification, the actual (no-load) voltage after rectification and before regulation can exceed the voltage rating of your electrolytic caps.

For example, 15VAC rectified should be 21V minus 1.4V diode losses = 19.6V when rectified and filtered, and I might choose 25V electrolytics for filtering caps. Poor transformer regulation means that 19.6V could easily be 25V or more - and poof go the caps. 35V or 50V would be a better choice in this example.

But in your case, I would just find a LM7809 voltage regulator to knock it down to the 9V you need. There's no need for the slightly more complicated LM317 here if you're just replacing a battery.

 

[Speakerguy]

If you want both + and - supplies with a single wall wart it is very easy to do.

You will need an AC to AC wall wart (NOT DC!) and you will hook it up as a voltage doubler. All you need is the wall wart(12VAC or more is good), two diodes (1N4002 are good) , two filter capacitors (1000uF @ 35V is good), four bypass capacitors (0.1UF ceramic) and two voltage regulators (LM317/337 or LM7809/7909).

This is how you would wire it:

**broken link removed**

D3,D4,D5,D6 are optional if you are doing <25V out.