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Power adapter question

H

Han

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
 
D

David L. Jones

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?
Click to expand...

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.
 
J

Jason James

Han said:
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?
Click to expand...

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
 
J

John G

Be carefull that the polarity is correct BEFORE you connect it.

John G.
 
H

Han

Be carefull that the polarity is correct BEFORE you connect it.

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
 
H

Han

** Hey - don't spoil his fun ...

I have a safety switch and fire extinguisher, will that cover me? ;)
 
J

Jason James

Han said:
Ok so stupid question, how can I check that? All I can see is the plug
and the label telling me the required power.
Click to expand...

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
 
H

Han

Be carefull that the polarity is correct BEFORE you connect it.
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
Click to expand...

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
 
P

Phil Allison

"Han"
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.
Click to expand...


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

It's a bit like that.




........ Phil
 
D

David L. Jones

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
Click to expand...

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.
 
H

Han

Otherwise your odds are better than 50/50 that the center pin is the
positive one.
Click to expand...

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
 
J

John Tserkezis

Pretty much sums up the chances of most things I do being successful, so
worth a shot ;)
Click to expand...
Thanks a lot for the very detailed reply, I'll buy a PP and just give it
a shot.
Click to expand...

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.
 
P

Phil Allison

"Han"
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.
Click to expand...


** 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
 
J

Jason James

Han said:
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.
Click to expand...

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
 
H

Han

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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