Some questions regarding AC-DC SMPS adapter OP etc

[khankll]

hi
i have few ac to dc smps adapters the light weight ones..
the spec on it says output 9volts 1 amps...
it has a round pin ... the outer connecter is negative adn the inner is positive..
when i measured the dc voltages it was 9.2 volts oc.. and when i measured its ac voltages it was like 25 volts.. y is this ? i am using a cheap DT-830B multimeter... also the 25 or so volts were shown when i connect the positve lead of mm to the inner connector and black lead to outer.. if i reverse the probes then it shows 0 volts... ? my wifi router is behaving frantakly dont know whats the issue . however after restarting it it works..

also this is aftermarket adapter for my router... the router needs 9volts 1 amp dc .. thats whats written on adapter adn in the manual ( TPlink wr740n) ..but the original adapter that came with this router was a big fat havy transfomer based adapter .. and the fck was that that was 12 volts output ... labeled and measured ... i dont know how it worked for a year in my device before stoping to work... and when it stoped working i opened it ... and to my surprise it was having just a transformer .. there were no capacitors...no diodes... no resistor... it was in other words just an AC adapter... how could it have worked and also for such a long time.. so when it stopped i bought the above mentioned smps adapter..

also another question regarding these adapter thingies...
i have a ZTE 831II adsl modem .. the input of the modem is labellled 9volts ac ( the ac symbol ~ is there instead of dc symbol = with the lower line dashed ) ...and the adapter is also ac .. its oc volts are 10.3 volts... can i replace it with some dc adapter.. ? or i must use ac adapter ? as i suspect that its adapter is causing problem ..

these two devices are connected together and i have some time to restart one sometime the other for the setup to work..

 

[khankll]

little bump

 

[CocaCola]

First off your diagnosis that the adapters are the cause of both units acting up is a little bit of a long stretch, if they are measuring out properly...

If you must replace the adapters get ones rated the same voltage and at or above (preferably above) the mA rating of the device... Don't go swaping an AC for DC adapter or vice versa...

As for why a different rated adapter worked or might work it's because the device has (might or should have is likely a better way to state it) internal components to compensate and operate at varying inputs within reason...

 

[khankll]

and what about

it has a round pin ... the outer connecter is negative adn the inner is positive..
when i measured the dc voltages it was 9.2 volts oc.. and when i measured its ac voltages it was like 25 volts.. y is this ? i am using a cheap DT-830B multimeter... also the 25 or so volts were shown when i connect the positve lead of mm to the inner connector and black lead to outer.. if i reverse the probes then it shows 0 volts... ?

 

[BobK]

Look more closely at the meter, are you sure it was not millivolts?

Bob

 

[CocaCola]

and what about

and when i measured its ac voltages it was like 25 volts.. y is this ? i am using a cheap DT-830B multimeter... also the 25 or so volts were shown when i connect the positve lead of mm to the inner connector and black lead to outer.. if i reverse the probes then it shows 0 volts... ?

You don't measure DC on the AC setting, and expect a real reading...

I'll dumb this down for you, if you want to get all the details you can research it further... This is also a 'generic' explanation not the entire concept, covering all multimeters or what not just the basics is something hopefully you an understand easily...

To measure AC voltage the meter likely rectifies half of it (this is why reversing the terminals killed the measurement) With that rectified AC the meter does some quick an dirty math and gets a reading of this rectified voltage... Remember AC is a sine wave so there is no 'constant' voltage to be measured when rectified unlike the DC that holds steady... The meter knows AC is supposed to be a sine wave and thus expects a high/low swing in the measurements it's taking works some math and gets a number... But since it's not getting any swing (since it's really DC) it's going to get inaccurate readings of all high none low (it probably doesn't care) it just takes those readings and goes with it plugging them into the next math formula... The meter also knows AC swings both ways (remember it only measured one rectified half) so it takes it's now incorrect readings and doubles it to give you your final voltage answer...

 

[khankll]

Look more closely at the meter, are you sure it was not millivolts?

Bob

yes its volts not mili volts..


and as i mentioned i get reading if i connect probes one way and no reading other way..

 

[khankll]

You don't measure DC on the AC setting, and expect a real reading...

I'll dumb this down for you, if you want to get all the details you can research it further... This is also a 'generic' explanation not the entire concept, covering all multimeters or what not just the basics is something hopefully you an understand easily...

To measure AC voltage the meter likely rectifies half of it (this is why reversing the terminals killed the measurement) With that rectified AC the meter does some quick an dirty math and gets a reading of this rectified voltage... Remember AC is a sine wave so there is no 'constant' voltage to be measured when rectified unlike the DC that holds steady... The meter knows AC is supposed to be a sine wave and thus expects a high/low swing in the measurements it's taking works some math and gets a number... But since it's not getting any swing (since it's really DC) it's going to get inaccurate readings of all high none low (it probably doesn't care) it just takes those readings and goes with it plugging them into the next math formula... The meter also knows AC swings both ways (remember it only measured one rectified half) so it takes it's now incorrect readings and doubles it to give you your final voltage answer...

'
in other words its normal for a dmm set to measure dc voltage using ac range to give more then the required dc voltage ?
i thought if i measure dc voltages with my meter set to ac i should get 0 volts..

 

[CocaCola]

'
in other words its normal for a dmm set to measure dc voltage using ac range to give more then the required dc voltage ?

Yes, somewhere in the neighborhood of say 2 - 2.5 times generally... It's not a real reading as it's being used improperly...

i thought if i measure dc voltages with my meter set to ac i should get 0 volts..

On a better multimeter that checks and detects the current as DC not AC you likely would get a zero reading or a complaint form the meter... On a cheap one that never bothers doing any checks and just goes with the flow taking it's measurements and spitting out a number, likely not...

 

[khankll]

hmm .. i see.. mine is not only cheaper but cheapest ..
now it akes sense.. thanks all