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how to determine a power transformers AMPerage rating?
Old 28th January 2013
  #1
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how to determine a power transformers AMPerage rating?

Hey
I found some wikkid cheap power trafos at a local surplus store, they are labelled as primary 120v secondary 17v, but give about 28v with no load. the sign said they were 2.5 amp, but also said they were 17v

is there any reasonable way to test and determine an approximate amperage tolerance? preferably without starting a fire
Old 28th January 2013
  #2
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Richard Crowley's Avatar
 
🎧 10 years
What exactly are these "wikkid cheap power trafos"? Are you talking about smallissh "wall warts"? Or some larger conventional chassis-mount unit? Is there no nameplate on the unit? Or make/model that you could reference, etc?

Wall warts are notoriously poor at line/load regulation. Their open-circuit voltage is usually significantly higher than under the rated load. They are designed for equipment with internal power regulation.

No, there really isn't a foolproof way of testing the current rating. Unless you want to conduct your own trials and monitor the core temperature, etc.
Old 28th January 2013
  #3
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🎧 5 years
How much current do you need?
Are they plain old transformers or more like "wall warts"?
Old 28th January 2013
  #4
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they are plain old transformers, about an inch square, not wall warts.

two wires on primary for mains
two wires on secondary that measure about 28v no load.

how could i go about measuring core temp to determine their safe amperage?
I have a "point and read" digital temperature reader, you point the laser at the thing and it gives you the temp. probably not scientific accurate but gives a good idea.

btw wikkid cheap = $1.50 !
Old 28th January 2013
  #5
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Richard Crowley's Avatar
 
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I wouldn't dream of expecting ~50W from a 50-60Hz power transformer of 1 inch square. That seems wildly optimistic and bordering on dangerous.

Don't quote me to your fire insurance carrier when your house burns down, but I would think that 25 degree temperature rise would be safe. That means 25 degrees (F) higher than the ambient (room) temperature.
Old 28th January 2013
  #6
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🎧 15 years
Hi
Look at some data sheets of similar physical dimension transformers which will give you a rough idea of the 'VA' rating. If they are supposed to be 17 volts then this would give a clue about the current rating. It would then be sensible to load it to this amount with a suitable resistor and leave it for an hour or more (monitoring every 5 minutes say) to see how hot the core gets.
If it is only about an inch cube then I would not expect much more than 3 or 5VA, possibly 10 at a push.
Matt S
Old 28th January 2013
  #7
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scottwilson's Avatar
 
🎧 10 years
You measuring peak to peak or RMS? 17vRMS is almost 28V peak to peak...

Also... might the sign have said 2.5VA instead of 2.5A?
Old 28th January 2013
  #8
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I just connected a DMM set to AC between the two secondaries... is this peak to peak?

2.5 VA would make a lot more sense.. if its 17v RMS and 2.5 VA, if my calculations are correct, it would be about 0.15 amps? that would make a lot more sense than 2.5 amps for sure!
Old 29th January 2013
  #9
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Whether it is PP or RMS depends on what meter you are using (not revealed). Of course, you are measuring it "open-circuit" with no load, so it isn't a Real-World test, anyway.
Old 29th January 2013
  #10
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its a cheap DMM, Mastercraft i believe.

for arguments sake, lets say its PP, and the actual voltage is 17v under load in the real world.

does V x C = VA apply?
17c=2.5
c=2.5/17
c=0.147

Assuming that all of the above is accurate, does that mean that the amps this trafo should safely operate under be 150 mA?
Old 29th January 2013 | Show parent
  #11
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Richard Crowley's Avatar
 
🎧 10 years
Quote:
Originally Posted by enginefire ➑️
Assuming that all of the above is accurate, does that mean that the amps this trafo should safely operate under be 150 mA?
150 mA sounds like a sensible rating.
Note that the symbol for current is "I" not "C".
You can use my handy calculators online here: EIRP Calculator
Old 29th January 2013
  #12
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Thanks RC for the link and correction, i will use I instead from now on

any suggestions for possible uses for a small trafo like this?
obviously for power and conversion of voltage!

at 150mA, I would imagine it could be used to power:
a phantom power box for 4 microphones
as a filament 6.3 or 12.6 power supply for a tube unit?
to power a small IC eq or gain stage? maybe something that uses a couple of 5532 or modern replacements.
(i know this has been discussed recently, regarding current needs and good design, and that no one here likes nebulous or try and see situations! please forgive me)
Old 30th January 2013
  #13
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Richard Crowley's Avatar
 
🎧 10 years
It appears to be made for powering a small solid state (discrete transistor or IC) circuit. The voltage isn't really suitable for powering tube/valve/firebottle filaments.
You would need to use a voltage quadrupler to get full 48V phantom power.
Old 31st January 2013
  #14
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🎧 10 years
Quote:
Originally Posted by enginefire ➑️
...they are labelled as primary 120v secondary 17v, but give about 28v with no load. the sign said they were 2.5 amp, but also said they were 17v
If the sign says they are 17V at 2,5A then that's probably what they are. 28V no-load is perfectly normal.
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