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wall wart basic question

L

LB

Jan 1, 1970
0
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.
 
L

Lord Garth

Jan 1, 1970
0
LB said:
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

Yes but you might have to add a regulator to the output. The output of
your wall-wart might be poorly filtered as well. Use a volt meter and
measure its unloaded output voltage. If it is higher than 5 volts, you
need a regulator.
 
H

Homer J Simpson

Jan 1, 1970
0
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

What sort of device?
 
H

Homer J Simpson

Jan 1, 1970
0
A wireless extra keypad for a wireless burglar alarm. (see spec sheet
link below)

In that case it's probably not very critical. I would use the other wall
wart. Or I'd get a cheap multi voltage one and use the 4.5 VDC setting.
 
E

ehsjr

Jan 1, 1970
0
LB said:
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

You might be able to use that, if you put a 20 ohm
5 watt resistor in parallel with the output so that
the wall wart feeds both the resistor and your keypad.

-------
| +|-----+-------+
| Wall | | |
| | [20R] [Keypad]
| Wart | | |
| -|-----+-------+
-------

The resistor needs to be at least 2 watts, but
5 or even 10 watts will let it run cooler.

But a regulated supply is called for, and the
above is only very crudely regulated, so you
may need to buy one. The output current
rating on a regulated 5V supply does not
matter, as long as it is at least 220 mA.
DCTX-512 for $3.50 from Allelectronics
http://www.allelectronics.com/
will do the job.

Ed
 
J

jasen

Jan 1, 1970
0
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

It it's 5v regulated, it's safe to use. most 5V supplies are regulated.

if it doesn't say you'll have to measure ...

Bye.
Jasen
 
R

Rich Grise

Jan 1, 1970
0
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

You could test your wall wart - put a load on it that's 220 mA at 5V -
that'd be - rats, I always forget if it's E/I or I/E - uhm, 22 ohms.

Like that other poster said, use a power resistor.

Then, if you have 5V, you're good to go. Otherwise, post back with what
you do measure, and we'll dream up something else. ;-)

Cheers!
Rich
 
R

roma

Jan 1, 1970
0
I have a electronic device that uses an OEM ($$$) wall wart rated at
5VDC and 220mA. In my collection of transformers I found one rated at
5VDC with the same plug/ polarity except it's rated at 500mA. Would it
be okay to use or should mA ratings match more closely?? Thanks.

Wall plug-in transformer adapters are a good economical source for
transformers. Beware: most are only halfwave rectified but that is no
problem if you output into a fullwave bridge. You can find them for
little cost at swapmeets, garage sales, surplus stores etc. Some things
to look for on the transformer case are voltage, current, and if it's
DC or AC, and if it's a positive or negative ground -- usually
indicated on the ouside ring of the plug symbol. AC is our first
choice.

When measuring the output voltage with no load applied the voltage will
always read as much as 40% higher than its rating. To determine the
true voltage and current rating of the unit apply a resistance load at
the ouput and measure the voltage.

For example, to find the resistance for a voltage rating of 12VDC @
300mA. Looking at Ohms Law with , E/I , E=12V, divided by I=.300A =
..040k ohms or 40 ohms and the wattage of the resistor should be E X I
or 12V X .300A = 3.6Watts. That's the wattage required for a permanent
application, but for a fast voltage measurement a one watt resistor
could be used.
roma
 
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