Taming a wallwart (plugin adaptor)

[Hero999]

out of curriosity, where are you getting this information? I'm not saying you are wrong but have heard differently.

My knowledge.

There are many types of adaptors:

• AC - just a transformer
• Pulsed DC - transformer & rectifier, (often only half wave) commonly used as a battery charger
• Smoothed unregulated DC - transformer rectifier and capacitor, there may be ripple on the output
• Regulated DC - transformer, rectifier, smoothing capacitor and IC regulator.
• Switching regulators and not all of these are regulated (the mobile phone chargers certainly aren't).
  

I have also looked at a number of WWs output with a scope. The pulsed DC one I looked at clearly had some sort of filter cap in it and looked like it used a bridge rectifier (ripple was 120 hz).

How do you know it had a filter?

Did you open it up?

Even if it did the capacitor might've been bad.

As I've mentioned in my list smoothed DC supplies will have a ripple on the output when a load's connected and even a regulated supply will have a small ripple but it'll be too small to see on a scope.

I think using an unregulated WW can work but you have to factor in the actual load (i.e. know the droop).

That's true.

Also, I wouldn't count on temperature stability of the voltage.

Apart from the diode voltage drop changing on the rectifiers and the copper losses in the transformer increasing the output is pretty temperature stable, the variation in the mains supply will have a far greater effect on the output thant the temperature.

I think a VReg is a very reasonable and cheap solution to gain certainty. Seems like good engineering practice to me...

I agree, regulators are very cheap and are certainly worth it, the only time I wouldn't recommend it is when you can't afford the voltage drop and you're not too bothered about the output voltage.

 

[philba]

My knowledge.

There are many types of adaptors:

• AC - just a transformer
• Pulsed DC - transformer & rectifier, (often only half wave) commonly used as a battery charger
• Smoothed unregulated DC - transformer rectifier and capacitor, there may be ripple on the output
• Regulated DC - transformer, rectifier, smoothing capacitor and IC regulator.
• Switching regulators and not all of these are regulated (the mobile phone chargers certainly aren't).
  

I was hoping for an authoratative site or reference. there seems to be lot of folk lore going around and I've not figured out the right keywords for google.

How do you know it had a filter?

Did you open it up?

Even if it did the capacitor might've been bad.

no, I did not open it up as I am using it in a gizmo I built.

The waveform difference between a transformer+diode and transformer+diode+filtercap is pretty obvious. You can see the RC decay going on with the filter in place. the pulsed DC wall wart showed the characteristic RC decay. I suppose, if i cared, I could estimate the size of cap based on where the decay curve intersected the rectified AC "pulse"