Using a higher rated DC transformer

Well, yes, sure. But that's a wonderful leadin to some related topics to ponder...

The reason (presumably) that they didn't put a regulator in the wall wart in the first place is that the equipment to which it's supposed to attach isn't bothered by voltages over some moderate range, and the expense of adding a regulator (taking more room, resulting in a larger case, ...dissipating more power, resulting in a larger case, ...) was not worth it. Remember, too, that we're talking an extreme range of loads here, and the equipment with which it is supposed to work could also provide a fairly constant load that results in a much more stable output voltage than indicated by the range from no load to full load. Also, very often the equipment connected to the supply has a regulator inside it. In fact, think of the reasons for using a wall wart in the first place: a key one is that you keep line voltage out of your equipment, meaning you don't have to worry about a passle of regulations concerning the safety of equipment connected to the line. You buy your wall warts from someone who has already taken care of that part of the equation for you. Doing the regulation is easy for you; taking care of world-wide power safety issues is not. You also avoid the hassles of dealing with the plethora of international power plugs and switching power voltages in your equipment; you use a cheap connector on your equipment and stock a range of wall wart types.

There are a lot more switching-type "wall wart" supplies out there now than there were twenty years ago or even ten, because the technology and volume has made them much cheaper and smaller than they used to be, because of a demand for supplies that run over a much broader range of input voltages, because of demand for or regulations for supplies that are more efficient (especially when lightly loaded), ... and the switching supplies pretty much always have much better output regulation than the transformer-driven unregulated ones. That's inherent in almost all the switchers.

Cheers, Tom

Well, yes, if you have any series resistance in the circuit, you'd expect at the very least that you'd have some variation from that; and certainly rectifier diodes have more drop at 300mA than at 1uA. When I was SPICing around with the choke-input filter circuit with a light load, I did notice that when you first fire it up, it can overshoot quite a bit, and charge the output cap to a voltage higher than the peak of the AC input to the rectifier. I suppose the limit on that, assuming a sinusoidal input and ideal diodes, is 4/pi times the sine peak.

Actually, some of the waveforms I've seen surprised me by not being flattened on the tops as I would have expected, but having a little peak there (and it was these strange ones I was thinking of, not flat-topped ones that would of course have better output regulation), slightly more "triangular" than a sine instead of being more "square". Don't ask me why. I just thought for a moment and realized that there was no way I was going to be able to figure out the cause easily, shrugged, and moved on to other things. Sometimes life's just too short to worry about such things.

Cheers, Tom

I look at it as more of a sawtooth at the peak while all the caps in the neighborhood get charged. Then the diodes cut off and the line voltage waveform continues on as a normal sine wave. So that might give the triangular peak you mentioned.

You are lucky. I find these little anomalies fascinating. When one catches my attention, I simply have to find out what's going on, and I'm not satisfied until then. Not knowing the answer bugs me.

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

I had responded to your other posting, but it never showed up. What I mentioned is that this is a real Bell System transformer. I believe it had to withstand a short circuit without self destructing. It was called reluctance protection, or something like that. My guess is that there is a lot of inductance involved, but it is all in the way the transformer is wound. This was probably not a good example for the OP.

I also measured a Nokia cellphone charger. Claims it is 3.6V, but measures

Tam

Hi Tam,

We are starting to get interested to find out how your transformer can behave in this fashion. Could you do us a favor and measure the output voltage with different resistors? For example, values from, say, 33 ohms to

The smaller values would dissipate some power. For example, 33 ohms might give (12^2) / 33 = 4.36 watts. You could probably get away with a 2 Watt if you measured the voltage quickly then disconnected the resistor.

Another way to do it is to get a bunch of resistors of the same value and add them in parallel one at a time. For example, 560 ohms might dissipate a little over 1/4 watt. Ten in parallel would give 56 ohms.

This would give a nice data set to work with, and it might be interesting for you to see if we can figure out what's inside.

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

Mike Monett wrote: ...

Well, I'm generally that way, too. As I recall, it was more that at the time I had too many other things on my mind that were more important, and I never got around to looking at it again. I should perhaps give it a try, though we're not in the same place we were then, and things may be entirely different.

Saw your posting to Tam asking for more info, and I'll second that. It will be interesting to try to figure it out. I'm not seeing how the transformer construction, probably with a magnetic shunt like in a neon tube transformer, would cause the 10mA voltage to be midway between the

Cheers, Tom

That's what caught my eye in the first place. I figure we might get a lot more success asking Tam to do some measurements instead of whacking his transformer with a hammer:)

I posted a query to Helmut regarding the strange behavior of LTspice. Here is the problem description from the note in the file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The purpose of this circuit is to examine the voltage regulation of a standard bridge rectifier. Diodes D1..D4 form a normal bridge circuit. The current source, ILoad, is a convenient way to change the load current. This circuit operates as expected when the second bridge is not present. Diodes D5..D8 form a second bridge to simulate the peak clamping effect when many TV's and HI-Fi's are turned on in the neighbourhood. C3 sets the slope of the peak ramp.

When the second bridge is added, resistor R4 no longer has any effect on the output voltage of the first bridge rectifier. It remains at about 14V as R4 is changed from 10 ohms to 10 megohms.

The first bridge operates normally when the current source is replaced by a resistor.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The file is

This might be of interest for people working with dc power supplies. I'll post the results of Helmut's investigation. Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

Mike, I hate to break it to you, but notice that you have the second bridge output referred to ground...

Cheers, Tom

The absolute best way is to break them open like this - or by dropping them on cement. Never saw them open - you lose plastic and may do more damage to the guts than a broken case would do.

you

Drop it on a concrete / hard floor, if it's a snap together one. The buttons ooze oil, and gum up / coat the carbon contacts with oil, causing them to stop working. A good washing with dish soap, cleaning the silicone rubber buttons and PCB usually fixes them. Make sure you rinse them well after the soap treatment, and putting them somewhere like on top of a radiator for few hours should insure they are nice and dry (but I doubt you will have much of a problem with that in Arizona!)

Jeff