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Is it normal for the wall wart power supplies to be off of their rated voltage? I have one that is rated at 9vdc and it puts out 13.8ish and a 12vdc that puts out 19v.
If it's an unregulated wall wort, that's normal. Unregulated worts have very poor regulation. They output their rated voltage only at their rated current. Unloaded their voltage can go much higher.
That's why your DMM is the key indicator. Cheap chinese ones, especially, heats up and melts the casing with the transformer insulation.. I wouldn't expect 'em to be giving out rated values.
Is it normal for the wall wart power supplies to be off of their rated voltage? I have one that is rated at 9vdc and it puts out 13.8ish and a 12vdc that puts out 19v.
if there is enough space in the enclosure add a fixed 12V regulator (7812) with a 100nF cap and a small electrolytic cap (≥100µF) connected to the output pin.
It depends on what output voltage you're after and at what current.
Connecting an LM7812 to a 12V adaptor is no good, unless the current draw is a fraction of the adaptor rating because the regulator will start to dropout.
The trouble is the output voltage will fall when a load is connected - at full load the voltage will be near the rating.
The LM7812 needs at least 14V to regulate properly, so if you connect it to the output of the wallwart and connect the full load, the wallwart voltage will drop to 12V and the output of the LM7812 will drop below 12V and could be as low as 10V.
Yes, many DC wall warts are simply transformers and diodes and a single electrolytic capacitor. The problem is that the cap charges up to the peak voltage value if there is no load applied and so many wall warts put out around 1.4 times their stamped voltage value until a load is connected. This can be a problem when using a wall wart with something that it was not specifically designed for as the initial voltage may be too high for the device and blow it out. You really need to check your device before connecting to make sure it can handle the extra voltage for some time. Sometimes it can actually help a little however, when you need to power a light load with a wall wart but you need a little higher voltage than the wall wart is rated for. For one device i have i need at least 7v input and i can get away with using a 6v wall wart because it's unloaded peak output is a little higher like 10v or so.
There were also some wall warts like this that had no output capacitors, so you would only get the pulsating DC output, not a cleaner filtered DC output. This kind would work mainly for battery chargers but may not work with other devices that need a cleaner DC voltage.
Recently there has been a trend to ship regulated off line switching wall warts with various devices. These kind are regulated so they put out pretty close to the stamped voltage. These kind are not quite as reliable however.
Recently there has been a trend to ship regulated off line switching wall warts with various devices. These kind are regulated so they put out pretty close to the stamped voltage. These kind are not quite as reliable however.
Which I think is generally a good thing because an SMPS is much more efficient.
A word of warning: not all SMPSes are regulated, especially the smaller units. I have one which isn't regulated at all so I opened it up to check it wasn't faulty only to discover there was no feedback path so it couldn't be regulated.
The problem is that the cap charges up to the peak voltage value if there is no load applied and so many wall warts put out around 1.4 times their stamped voltage value until a load is connected.
That is normal on most power supplies and is not the cause of the problem.
The problem is that a cheap unregulated power supply has resistance in its transformer so the voltage drops when it is loaded.
Yes, the transformer resistance can play a part in the drop in voltage too, but that also depends on the output cap ESR and also
the leakage inductance causes a drop too. The diode drops have a part in it as well. Depends how cheap it really is i guess.
I agree however that the resistance might play a large part in it sometimes, but remember that the highest peak is due to the
sine peak and when the cap is charged to that value it is highest, and as load is added any ripple with always bring the
average output down even with less resistance. Maybe with extremely large capacitance and low ESR that wont be as bad.
Also, as the power supply ages the ESR goes up which makes the peak loading issue even worse.
Actually I think both the impedance and ripple are issues.
The high impedance (resistance and leakage inductance) means that the peaks will be flattened heavily when a a current is drawn and the capacitor is often cheap, undersized and high ESR so isn't much good.
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