AC Adapter problem.

[true_blue]

Hi guys, I noticed one of my external hard drives was going on and off throughout the day. So I switched the adapter with another same drive and it worked fine, but then the other drive had the same problem.

Western Digital MyBook 2.0 2tb
The adapters are 12v@1.5a

So I hooked it up to my multimeter and the supply gives 12.23v constantly. I fiddled with the wire at both ends and shook it to see if the voltage dropped and nothing happened. So I think the cable and the tip are OK.

My question is.. what could be wrong if the multimeter says its at the correct voltage yet it is powering my hard drive(s) on and off randomly?

Is it worth replacing the transformer?

I would take pics but I can't get the damn thing open as there are no screws and I think it must be glued shut. gah..

Cheers

[mariushm]

Re: AC Adapter problem.

Your multimeter measures the voltage when there's nothing connected to the adapter.

When you connect something, the adapter has to work harder to keep that voltage around 12v and it may just be the case that when the hard drive needs a lot of "juice", the adapter just can't handle it and fails.

You'd have to open up the adapter to see if there's any component showing signs of failure.

[true_blue]

Re: AC Adapter problem.

Thanks, I was thinking about testing it under load too. But a lot of youtubers seem to think its not necessary.
I will splice into the wire and connect croc clips then connect it back up and see how the voltage fairs.

I am really not sure how to get the thing open without drilling /cutting into it though.. is there a trick to it? There is no movement with the plastic and I can't get a knife inside anywhere.. it seems rock solid unlike the clip-lock types.

Cheers

Your multimeter measures the voltage when there's nothing connected to the adapter.

When you connect something, the adapter has to work harder to keep that voltage around 12v and it may just be the case that when the hard drive needs a lot of "juice", the adapter just can't handle it and fails.

You'd have to open up the adapter to see if there's any component showing signs of failure.

[stj]

Re: AC Adapter problem.

got a few pics of the psu casing?

[true_blue]

Re: AC Adapter problem.

OK I ended up cutting into it with the dremel. They had glued everything down.. the PCB as well. Ended up breaking a corner of the PCB. Pain in the arse. I'll just bin them in future unless they have screws!

[true_blue]

Re: AC Adapter problem.

Thanks for your help guys.. I'll atleast salvage a few bits from it but think the damage is too bad now.. I wish they didn't make things so shitty

[Per Hansson]

Re: AC Adapter problem.

For future reference a multimeter can not measure ripple.
You might have severe ripple and the multimeter says everything is fine and dandy
https://www.youtube.com/watch?v=JFZxx1phMvE

[true_blue]

Re: AC Adapter problem.

Thanks Per Hansson. That is interesting. I didn't know DC voltage is supposed to ripple.
So does bad ripple mean a cap is failing to keep the voltage steady?

Cheers

For future reference a multimeter can not measure ripple.
You might have severe ripple and the multimeter says everything is fine and dandy
https://www.youtube.com/watch?v=JFZxx1phMvE

[stj]

Re: AC Adapter problem.

DC is NOT supposed to ripple, that's what the caps are there to prevent!
btw, some meters can show ripple if you set them to AC setting.

[true_blue]

Re: AC Adapter problem.

Yeah that's what I thought.. the cap is meant to stop ripple from AC conversion I believe.
So when a PSU ripples then its most likely a bad cap?

I will try setting it to AC next time but I'm looking up buying a Oscilloscope as we speak!

My tool kit is slowly growing into an expensive addiction.. lol

I have learned some things from this thread.. Many thanks guys!

DC is NOT supposed to ripple, that's what the caps are there to prevent!
btw, some meters can show ripple if you set them to AC setting.

[eccerr0r]

Re: AC Adapter problem.

Actually if you have a small capacitor like a 0.01uF and hooked it in *series* with an AC voltmeter, that might be sufficient to detect ripple? If you get clean DC then you should see no voltage at all (except perhaps some transients you cant see upon normal use-too fast) but if it's really bad, you'll see voltage and that you have some ripple.

[budm]

Re: AC Adapter problem.

http://www.fluke.com/fluke/uses/comu.../true-rmsfacts
http://leddrivertesting.com/testing-...-power-supply/

[TELVM]

Re: AC Adapter problem.

... http://leddrivertesting.com/testing-...-power-supply/

"In above image you can see the AC Voltage present with DC.

The ripple voltage is 0.021 V . That is , 21mV.

This is the way ripple is measured. And according to us, this is the way ripple is measured."

Is this serious? Can we get a reasonably decent measure of ripple in SMPS's by just setting the frigging MM to AC voltage?

[stj]

Re: AC Adapter problem.

if the ripple frequency is not too high - yes.

[budm]

Re: AC Adapter problem.

Go by the Video at the bottom of the page, that DMM does not have the AC bandwidth (Fluke 115 has 50KHz bandwidth, also it does not indicate the minimum input Voltage required to read at 50KHz) to measure the ripple frequency of the SMPS which typical > 50KHz range.
For 50 (100Hz)/60 (120Hz) is no problem.

https://cdn.badcaps-static.com/pdfs/...dca89aff17.pdf

[Per Hansson]

Re: AC Adapter problem.

Also realize what the meter shows is a TRMS value, not peak to peak as a scope would show you!
And of course, the meter will only show as much as it's bandwidth allows...

[TELVM]

Re: AC Adapter problem.

For the scientific interest I've done some tests with a cheap multimeter on a couple PSUs while running comps.

1) With MM set on AC 200V, red probe on yellow wire (+12V) and black probe on black wire (ground), it displays ~26.1V AC idling, ~25.2V AC full load.

2) With MM set on AC 200V, red probe on black wire (ground) and black probe on yellow wire (+12V), it always displays 0.00V AC, no variation.

[modemhead]

Re: AC Adapter problem.

For the scientific interest I've done some tests with a cheap multimeter on a couple PSUs while running comps.

1) With MM set on AC 200V, red probe on yellow wire (+12V) and black probe on black wire (ground), it displays ~26.1V AC idling, ~25.2V AC full load.

2) With MM set on AC 200V, red probe on black wire (ground) and black probe on yellow wire (+12V), it always displays 0.00V AC, no variation.

Cheap multimeters are DC-coupled on the ACV ranges (ie. they have no DC-blocking capacitor). They also have a single diode rectifier for AC-to-DC conversion. The reading is scaled to display 2.2 times the average of this half-wave rectified signal, which for a sine wave, would approximate the RMS voltage.

Because of this design: 1) cheap multimeters are not useful for measuring very low AC voltage because the rectifier forward voltage drop begins to create significant error below say 10V or so. And 2) they definitely will not accurately measure AC voltage in the presence of a DC offset.

[budm]

Re: AC Adapter problem.

"2) With MM set on AC 200V, red probe on black wire (ground) and black probe on yellow wire (+12V), it always displays 0.00V AC, no variation." That is due to the single diode as mention above. Cheap meter, that is all.
I.E. D1 in the diagram of MASTECHS METER MAS830.

[TELVM]

Re: AC Adapter problem.

Thanks, there goes down in flames my "poor man's scope" . Was worth the try, you always learn from failures.