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I have a device (an ADSL modem) that when plugged in to an ac adapter does not seem to do anything (power light stays off).

I would like to know how to measure voltage for an ac adapter to make sure it is working. I have multimeter, but when I connect the inner and outer rings of the supply, it does not register any voltage (I assume it probably needs some load in order to give a voltage but I am not sure).

Output rating is 12 V / 1.4 A.

I tried with a different ac adapter and it also did not register voltage. Yet while measuring a AA battery it shows correct values, which leads me to believe that I am not doing something right.

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    \$\begingroup\$ All the AC adapters that I have ever tested have will show an output voltage with no load attached. The voltage level may change some when a load is applied but none have simply zero volts in open/no load. (Note that the variation of output will be greatest for the older style adapters that are bulky with heavy transformers inside). Note that some AC adapters have an AC output and thus need to be measured using an AC voltage scale on your multi-meter. Also, although may be too obvious to mention, the adapter needs to be plugged in when you check for the output voltage. \$\endgroup\$ – Michael Karas Feb 9 '13 at 13:35
  • \$\begingroup\$ @MichaelKaras: I thought so too. And I did plug the adapter :). But seems that I cannot measure adapters with DC output with multimeter set to DC measurement (only ones with AC output and Multimeter set to AC measurement show me reading). What I am doing wrong? \$\endgroup\$ – Bo. Feb 9 '13 at 14:31
  • \$\begingroup\$ I am not sure what could be wrong. The set of measurement combinations that are working / not working do not lead to a deterministic behavior. In the past I have had some strange behaviors with multi-meter measurements that were caused by wire breaks inside the meter leads. Some measurements would work whilst others would fail. It all came down to how the leads were positioned and held. It is possible that you have to hold the leads in a quite different position when measuring a battery versus the DC adapter. Maybe - just maybe the AC signal can couple through an intermittent lead. \$\endgroup\$ – Michael Karas Feb 9 '13 at 14:45
  • \$\begingroup\$ @MichaelKaras: I understand what you are saying. The multimeter has not been used that much and I doubt it is malfunctioning. It is probably that I am doing something wrong than that there is a damage to the multimeter. \$\endgroup\$ – Bo. Feb 9 '13 at 14:54
  • \$\begingroup\$ Just checking for another data point...is your multimeter a digital one or one with a scale and needle? Some of these styles with the "analog meter" scale have a small peg by the needle that keeps it from moving down beyond the lowest end of the scale. This means that the meter leads are polarity sensitive (i.e. ++ input must go to positive and -- must go to negative). Virtually all digital type multimeters will simply show a negative reading if the meter leads are connected to the voltage source on the reversed polarity. \$\endgroup\$ – Michael Karas Feb 9 '13 at 15:43
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Since you're not getting any reading for two different adapters there's indeed a chance that you're doing the measurement wrong.

Is the AC adapter also AC out? If it is, then you'll have a zero reading if your multimeter is set to DC. Setting the multimeter to read AC voltages you should get a reading different from zero, even without a load. The voltage will be higher than the rated voltage, like a 12 V AC adapter may give 14 or 15 V without load. Likewise, if the adapter has an unregulated DC output the unloaded voltage will be higher than the rated voltage.

So, check that you're measuring AC voltages with the multimeter in an AC position.

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  • \$\begingroup\$ Steven, your post was helpful. I just checked few other adapters and I can confirm following: I can measure adapters with AC output but I cannot measure any adapters with DC output (DC measurement is working for plain AA batteries but not for adapters). What could be wrong? \$\endgroup\$ – Bo. Feb 9 '13 at 14:29
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I can only think of 2 possibilities: 1) Are you absolutely sure you are making contact to the inner and outer rings of the adapter connector? 2) Are you using the same range scales on your multimeter when you measure the adapter output vs an AA battery? It is possible that your multimeter works on a low voltage range for the AA battery but not on a higher range for the adapter. If you are using different ranges, I suggest leaving the multimeter at the range for the AA battery and then try to measure the adapter voltage. At worst, the multimeter will indicate overrange. If it does, then your multimeter is faulty. If it doesn't, then you may have multiple faulty adapters.

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    \$\begingroup\$ I am really not sure what is wrong. Well I know it is me but do not know the reason. I have put the voltage range to 20 V for both battery and adapter (original adapter is rated to 12V; other adapters are even lower voltage). And I have used similar procedure for AC output adapter as well as DC output adapter with respect to 1). I have tried higher and lower ranges to no avail. Other adapters are working since I plugged them in their respective devices and they work. But I am not sure why my multimeter would be faulty if it can measure DC for other cases... \$\endgroup\$ – Bo. Feb 9 '13 at 20:37
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You may need to check the connector type on the end of the adapter. There are two general kinds of power supply adapter plugs in common use. The picture below shows the two types - both of which are referred to as "barrel type" or "coaxial power type".

enter image description here

Note that the type at the top of the picture has a full metal inner sleeve that is easy to contact with the multimeter probe. The alternate type as shown in the lower section of the picture only has a pair of contact fingers on the inside. Connection to a multimeter probe can take some finesse to make sure the probe hits one of those contacts.

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An AC power adapter is only a transformer with secondary AC rated at 12VAC. These are usually used to power multimedia amplifiers - the other parts converting AC to DC voltages are already inside the circuit of the amplifier. It consists of 4 diodes connected as a bridge recifier and a filter capacitor.

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    \$\begingroup\$ Welcome to EE.SE, Paulinz. I don't know if you noticed but this question is over three years old. Your answer may be useful to someone in the future but is let down by messy capitalisation ('ac' for AC, 'v' for V, first letter of sentence, etc.), spelling and punctuation and unnecessary abbreviations ('p' for power). All of these give an impression of the author which is not in his/her favour. You may wish to edit. \$\endgroup\$ – Transistor Aug 28 '16 at 8:38

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