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Old 06-18-2015, 10:54 AM   #1
ivtec
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Default Adapters Voltages?

Hi guys; can you tell me what is the correct load Voltage on a Power Adapter,
for example i have here an DC 3v 200 ma power adapter when i test it on my Fluke it reads about 7v,without load, is this bad?
i rarely find a Power adapter that reeds the exact voltage that is in the adapter specs always have 2 to 3 volts or more more,what is too much voltage?

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Old 06-18-2015, 12:07 PM   #2
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Default Re: Adapters Voltages?

The linear non regulated power supply will show higher Voltage without the load, it should be 3V with 200mA load, without the load the output can as much as 50% higher, but in your case, your meter may have bad batteries or way out of calibration.
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Old 06-18-2015, 04:19 PM   #3
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Default Re: Adapters Voltages?

Quote:
Originally Posted by budm View Post
The linear non regulated power supply will show higher Voltage without the load, it should be 3V with 200mA load, without the load the output can as much as 50% higher, but in your case, your meter may have bad batteries or way out of calibration.
Thanks budm, but i measured with another meter and it shows same v,
if you say without load it can be 50% more, then it's only 1 volt over .
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Old 06-18-2015, 04:25 PM   #4
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Default Re: Adapters Voltages?

"DC 3v 200 ma power adapter when i test it on my Fluke it reads about 7v" 3V to 7V that is way more than 50%.
3V, so 50% higher will be 4.5V, you are getting 7V.
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Old 06-18-2015, 08:34 PM   #5
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Default Re: Adapters Voltages?

Once I had a 12V 500mA wall wart and unloaded it was reading around 28V (but with a 24 ohm resistor - 500 mA load), it was 10% of the labeled 12 volt rating.
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Old 06-18-2015, 10:25 PM   #6
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Default Re: Adapters Voltages?

Quote:
Originally Posted by pc7fan View Post
Once I had a 12V 500mA wall wart and unloaded it was reading around 28V (but with a 24 ohm resistor - 500 mA load), it was 10% of the labeled 12 volt rating.
If that is the case I hope it has 50VDC rating caps in that wall wart.
To get 28V the cap is charged up to the peak of the transformer Voltage, so in this case the transformer AC is around 28V *0.707 = 19.796V!
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Old 06-19-2015, 03:48 AM   #7
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Default Re: Adapters Voltages?

That is pretty normal situation for wall transformers and usually the smaller the transistor, the higher the no-load voltage which than drops drastically with load. Not sure where you took that 50 % from, that is no general rule…often it is that way, often not.
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Old 06-19-2015, 09:35 AM   #8
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Default Re: Adapters Voltages?

"the higher the no-load voltage which than drops drastically with load. Not sure where you took that 50 % from" from taking apart 100's of 100's of wall warts for failure analysis, beside if they design the wall wart with such high output impedance they will have to use higher Voltage cap rating.
Also if the device being power by this wall wart and only drawing 80% of rated current for head room purpose, the supply Voltage will be way too high for the device.

"Once I had a 12V 500mA wall wart and unloaded it was reading around 28V (but with a 24 ohm resistor - 500 mA load), it was 10% of the labeled 12 volt rating." Just think about this, what Voltage will be feeding the device if it is only drawing 100mA or less in standby mode? Will you want to use this wall wart?

Last edited by budm; 06-19-2015 at 09:45 AM..
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Old 06-19-2015, 12:41 PM   #9
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Default Re: Adapters Voltages?

Most devices have DC-DC inside today and for battery chargers, it is actually intention as the voltage slowly rises above nominal (charged batteries always have more than nominal).

BTW I am talking about 50/60Hz transformers, are we on the same subject here?
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Old 06-19-2015, 12:44 PM   #10
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Default Re: Adapters Voltages?

"BTW I am talking about 50/60Hz transformers, are we on the same subject here?" Yep, simple linear power supply with iron core transformers, with simple bridge and filter cap, not regulated output.
28V no load and 12V full load, that is the worst transformer regulation factor I have ever heard of, no load of 1.8 factor (no load is 80% higher than full load) spec is about the worst I ever run into.
http://carroll-meynell.com/technical-transformer

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Old 06-19-2015, 01:15 PM   #11
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Default Re: Adapters Voltages?

Small 3-5V ones often have 6-9 V…
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Old 06-19-2015, 02:27 PM   #12
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Default Re: Adapters Voltages?

Worst case factor of 1.8:
3V * 1.8 = 5.4V
5V * 1.8 = 9V

12V:28V = 2.33 factor, very bad transformer.

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