Power adapter question

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This is presumably a very basic question for you guys, but I'd
appreciate if someone can confirm/deny my assumption.

I have an electric gadget that needs a power adapter which I've lost. It
states that it needs 12VDC @ 750mA but I can't find one that matches
that exactly.

My understanding from Google is that using something above is ok, under
isn't. So will a generic 12VDC @ 1A from DSE or Jaycar be ok to use?

Cheers

Re: Power adapter question
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Higher current rating is fine.

The only potential problem is if the product needs a closely regulated
12V DC. The voltage of "unregulated" plugpacks can vary a lot
depending upon the load. At no load it could be as high as 16V for
instance.
However, most commercial products are designed for use with
unregulated plugpacks, so you should be fine.

If you really want to be sure, get a 12V DC 1A+ regulated plugpack,
such as the Jaycar MP-3486

Dave.

Re: Power adapter question

"David L. Jerkoff"


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** Doesn't exist in stores yet.

DSE have this one:

http://www.dse.com.au/cgi-bin/dse.storefront/47f97ba20809d50a2740c0a87f9c06e5/Product/View/M9670




......  Phil




Re: Power adapter question

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That's how I understand it. If you use a power-pack which is below, it may
exceed its max-current. Higher power-paks are OK. In fact I use a 1.5A power
supply with switched output voltages which cover the couple of rechargeable
torches with no problems. I wouldnt use a different voltage however.

Most old mini B&W TVs, came with 12v 1A power-paks. Just make sure the
polarity is correct.  These days I notice the majority is +ve inner, -ve
outer.

Jason



Re: Power adapter question
Thank you guys for the replies, very helpful.

Cheers

Re: Power adapter question
Be carefull that the polarity is correct BEFORE you connect it.

John G.

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Re: Power adapter question

"John G"

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 **  Hey -  don't spoil his fun  ...



....  Phil




Re: Power adapter question
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I have a safety switch and fire extinguisher, will that cover me? ;)

Re: Power adapter question
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Ok so stupid question, how can I check that? All I can see is the plug
and the label telling me the required power.

I appreciate the help.

Cheers

Re: Power adapter question

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These days all plug-paks have a small diagram on the front showing the plug
as a symble: a circle (outer) with a dot in the middle,..meaning inner
conductor. The polarity is then shown. If the PP you have is of the older
type without the diagram, use a voltmeter, $15 from most Supercheap.Clints
etc, worthy investment.

Jason



Re: Power adapter question
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Thanks Jason. I'm sure the PP from DSE or wherever will have that, but
how do I then translate that in to the device I'm plugging it into? It
doesn't say, and I don't have the original PP to compare.

Cheers

Re: Power adapter question

"Han"

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**  Have you ever seen the film of  " Catch 22 "  ??

     It's a bit like that.




.......   Phil




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:)

Re: Power adapter question
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Do you have a digital multimeter?

Does the gadget have any other connectors with a common ground?
If it does then you could measure continuity between each pin of the
DC connector and the ground of the other connector to find which pin
is ground.

2nd option with a digital multimeter would be to simply measure the
resistance on the DC connector.If you get a reading in one direction
and no reading in the other, then the one with the reading shows the
correct polarity. This only works if the unit has a series protection
diode. Some products may show a value both ways.

Another option is to open up the unit and inspect the circuit board
and connector.
If the connector is wired to the circuit board and it uses red and
black leads, then red is likely the positive one.
For a circuit board mounted connector, the ground pin is most likely
the one connected to a big copper ground plane (if there is one).

Other options involve inspecting the circuitry, but that's a bit
harder to explain.

Otherwise your odds are better than 50/50 that the center pin is the
positive one.
Murphy's Law will however ensure that you get it wrong, and that the
unit won't have reverse polarity protection :->

Dave.

Re: Power adapter question
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Pretty much sums up the chances of most things I do being successful, so
worth a shot ;)

Thanks a lot for the very detailed reply, I'll buy a PP and just give it
a shot.

Cheers

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  Note that some equipment is not very well protected against reverse
polarity. So guessing is not a good idea the cheaper you go.

  If you're confident with taking things apart, have a purve inside the gear,
it sometimes gives you glaringly obvious clues as to which way around it goes.

  Things to look out for:

  Larger areas of copper tracks usually point to negative.

  Fuses, though technically could be placed anywhere, are generally placed on
the positive line.

  Polarised capacitors for filtering at the input are dead giveaways.

  Sometimes (but only where it's fused), you can find diodes placed 'wrong way
around' across the input to forcibly blow the fuse in that event.  That is
also a dead giveaway.

  Short of other obvious clues, you need to trace the lines to see what
circuitry it goes to and make a determination from there.  But you're likely
to see the more obvious clues before then anyway.
--
Linux Registered User # 302622
<http://counter.li.org

Re: Power adapter question

"Han"
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** OK  -  so must be familiar with Eastwood's philosophy on this point.

That is, his famous " punk probability " quotient.

" Was it 5 or was it 6  ??  How lucky do you feel ......   "

Completes Einstein's  " e=mc*2 "   &  Heisenberg's uncertainty triad.

Mind bending stuff........



......   Phil












Re: Power adapter question

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In that situation, I would remove the covers of the device you intend to
power,..look for any large electrolytic capacitors on its circuit board. Now
these are polarity sensitive, and if there is a largeone near the
plug-socket or elsewhere on the board, it's a fair bet they will indicate
what polarity the device is. eg electros often have an exposed case,..this
will be negative, and if it is very low resistance (equal to, or less than a
fraction of an ohm) to the outer plug-socket, you solved your problem. It
could be the device is positive earth, which in that case, the resistance
will be quiet high from the electro-cap case to the socket outer, but it's
positive lead will be a very low resistance to the outer plug-socket
connection.
It's common practice for circuit designers to employ some capacitive
filtering at the DC input socket, or close to it.

PS. if you are using a cheaper multi-meter as a resistance or
ohm-meter,..touch the 2 probes together on the range you intend to use (20
ohms fullscale, for example) and note the "probe short-circuit or zero ohms
residual reading. The cheaper digital m-meters usually have a short circuit
or zero resistance reading of less than ohm. You then subtract this reading
from any measurement you make.

Hope this helps,..Jason



Re: Power adapter question
Thanks again to everyone for the help. I'll take it apart tonight and
see if I can check the polarity from the obvious and what people have
advised.

Cheers

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