Using a higher rated DC transformer

Using a higher rated DC transformer

Post by Conna » Tue, 07 Nov 2006 23:32:50


Good morning, all.

I have an electronic device that requires a DC Wall transformer rated
for 12VDC at 600 mA. Unfortunately, I have lost the power supply and I
cannot find another one with that exact rating. My friendly Radio Shack
representative tells me that the device only "pulls" what it needs from
the transformer, the transformer does not "push" that much to the
device. He tells me I can use a 12vdc at 1A for the device and it will
work.

Is this true? I do not want to fry my device by "pushing" 400mA more to
it than it needs. If he is right, may I use a transformer rated for
12vdc at 2.5 amps protected by the same principle? You can tell I'm not
an electrical engineer. :) I would appreciate any thoughts. Thank you!

Matthew
 
 
 

Using a higher rated DC transformer

Post by nobod » Wed, 08 Nov 2006 00:03:29


You could even use one rated for 25 or more amps (should you be able to find
one) as long as you do not exceed 12VDC on the voltage. Your RS rep knows
whereof he speaks.


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Using a higher rated DC transformer

Post by eschuylerT » Wed, 08 Nov 2006 00:08:01


Matthew,

This time (surprisingly!), the Radio Shack guy is correct. As long as the
transformer is rated for the same voltage and *at least as much* current,
you'll be fine. The connected device will draw only the current it
requires, regardless of the capacity of the source.

Regards,
Eric
 
 
 

Using a higher rated DC transformer

Post by Conna » Wed, 08 Nov 2006 00:24:38

Hi guys! Thank you very much for your reply! On another newsgroup, a
gentleman mentioned that if the higher rated wall transformer is
UNregulated, then it may put out higher voltage but if it IS regulated,
I should have no problems. So with that in mind, 2 questions.

1) Is this accurate?
2) The power supply I intend to use reads:
12VDC 2.5A MAX

Does the word "MAX" there imply that devices drawing a lower current
can indeed make use of this transformer. Does that word imply it is
"regulated"?

Again, thank you!
 
 
 

Using a higher rated DC transformer

Post by nobod » Wed, 08 Nov 2006 00:37:02


Yes, that's accurate to some extent. It really depends on the equipment it
is meant to power. If the device uses an unregulated 12VDC wall transformer
then it will have an internal regulator. If the internal regulator is rated
for the higher swings (maybe 18V) of the unregulated wall transformer you're
OK.

Unfortunately, it's the missing wall transformer that you need to examine to
determine whether it was regulated or unregulated. The only alternative is
to open up the device and trace the power leads to see what regulator it
uses.

MAX means MAX. IOW, all it means is that it cannot supply more than 2.5A.

How heavy is the new 12VDC/2.5A transformer? If very light it is most likely
a reugulated switched-mode power supply. If relatively heavy, it's probabvly
a magnetic transformer type and may be either regulated or unregulated.
Regulated 12VDC @ 2.5A is somewhat rare but not unheard of.

What is the device? Maybe someone who also has it can tell you whether it
needs 12VDC regulated or unregulated.

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Using a higher rated DC transformer

Post by Conna » Wed, 08 Nov 2006 00:45:28

> Yes, that's accurate to some extent. It really depends on the equipment it

The device is a LinkSys BEFCMU10 Cable Modem.


Actually, I was so desperate to get it working, I bought another
identicle cable modem and I'm using ITS transformer temporarily until I
can get one that will work. Then I will return the new one to the
store. So, I do infact have THE right transformer at this time. What
markings should I look for?


It weighs 5.6 ounces and is 2" x 3.5" x 1.25" in dimension.

Thank you!
 
 
 

Using a higher rated DC transformer

Post by nobod » Wed, 08 Nov 2006 01:25:44


My guess (and it is just that - a semi-educated GUESS) is that it's a
switched mode power suppy (SMPS) which are almost all regulated. I've a
15VDC/1.0A regulated SMPS which weighs about 4 oz. while all of my 12V
magnetic transformer style power supplies weigh 12 oz or more. But the
heavier transformer type could still be regulated.

The only way to be certain is to contact Linksys Support and ask them,
telling them you lost the original power supply.

Otherwise, if you have a friend with a voltmeter, you can measure it. Or,
assuming you are going to buy the replacement at RS, ask your friendly RS
rep to measure the one from Linksys. If regulated it will measure 12V while,
if unregulated, it will measure more, up to 17-18V.

One final clue is that California, Europe, East Asia and other regions now
have efficiency standards for wall transformers that can only met by SMPS
types so my guess is that Linksys will be using them - I know that D-Link
(which I use) does so.


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Using a higher rated DC transformer

Post by nobod » Wed, 08 Nov 2006 01:35:01

One added caution. The measurement needs to be made with a cheap multimeter
rather than a more sophisticated True RMS Voltmeter. The latter will show
the equivalent DC voltage which will be close to 12V even for the
unregulated supply.


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Using a higher rated DC transformer

Post by Conna » Wed, 08 Nov 2006 02:16:58

>The measurement needs to be made with a cheap multimeter

Good thing I'm cheap, eh? I have one available. Again I would like to
say thank you to everyone for your helpful comments! I really
appreciate your taking the time!

Matthew
 
 
 

Using a higher rated DC transformer

Post by nobod » Wed, 08 Nov 2006 02:41:53


A final point - using a regulated 12VDC supply will likely work even if the
device expects an unregulated supply. In the latter case, the internal
regulator will output an even lower voltage and as long as there's adequate
headroom (i.e. the regulator needs a couple of volts to do its thing), it
will be OK.


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