AC/DC adaptors: voltage over normal?

[autir]

Over the years I have collected several AC/DC transformers/adaptors. These adaptors provide higher voltage than their nominal value. Below follow some calculations I have made with one of them.
The theoretical voltage of the adaptor is 3V DC. I connected several resistances in the adaptor's jack and measured the voltage in the resistance's ends.

Resistance (Ohms) Real Voltage (Volts DC)
no resistance 6.58
470 K 6.51
100 K 6.39
12 K 6.21
5.6 K 6.18
9.6 3.15
shorted (no resistance) 0,16

We can clearly see a pattern: the voltage lowers when the resistance lowers. But it is nowhere near 3V! What is the matter?

[Kevin Weddle]

You should get the correct voltage if the current is what is listed on the device. I think the DC adaptors are unregulated to boot. The device you plug it into will have a regulator. Regulators are designed for maximum benefit when the current is just right. That is why you should not double up on regulators. Although I have done it for purposes of general expermentation.

[autir]

What do you mean by the phrases "unregulated to boot" and "double up on" ? ::)

I thought that the current listed on the device is the maximum allowable, and not the proposed value. What is the relationship between these two? I have dismantled the adaptor, it consists of an AC/AC transformer and a bridge rectifying circuit (4 diodes etc.). What could cause this alteration in voltage when changing external resistance? Could it be the internal resistance of the adaptor?

One more thing: what kind of damage does a transformer receive if the current it provides is higher (say, double) than the nominal value? I suspect that the wires are heated up and they eventually melt; In any case, can we deal with this by providing extra cooling to the transformer?

[ante]

Hi autir,

Most of these

[Kevin Weddle]

No I don't think the current rating is maximum. This is the amount of current expected. The thing also has a capacitor I believe to get rid of the ripple. You have an RC circuit when the AC from the transformer drops. The ripple is created from the discharging capacitor and the charging capacitor when the AC increases to it's peak. The average voltage you get should be the peak voltage. But with a changing load you create more ripple which changes the DC voltage you measure. The more the ripple the lower your average.

[ante]

No Kevin, I did not say it was the maximum current just the nominal. Nominal and maximum is not the same. But I said that the transformer should not be overloaded, and it will be if you double the nominal current output, which was autirs question. To complete the answer to autirs question; yes you can to some degree compensate by cooling of the transformer (never on double the nominal) but this is not recommended. Overloading a transformer is not only about overheating, it

[Kevin Weddle]

By "unregulated to boot" I meant unregulated "also". By doubling up on regulators I was talking about the situation where you could put two regulators in series to try and get better regulation without setting the current right. A way of compenasating for not having the correct current is to try and overdo the regulation. One might try this if you have a 1A regulator and are only using 100mA. You never know but doing this might prove beneficial. But I will stick to one regulator at the correct current for the best small signal conditions.

[audioguru]

Kevin,
You don't have to put regulators in series to get "better" regulation, a regulator operating by itself within its ratings provides excellent regulaion.

Look at the datasheet for a 7805 regulator. Its line (input voltage change) and load (output current change) regulation and ripple rejection are almost perfect. Change the input voltage or load current over a vast range and their output voltage changes only millivolts or less, like they are designed to do.

There is no such thing as the "correct" load current for an IC regulator. They work perfectly well from no load to their max rated load. Also, you can feed them any voltage from 3V more than their output all the way up to their max input voltage.