Author Topic: How accurate should a transformer voltage be?  (Read 4635 times)

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How accurate should a transformer voltage be?
« on: June 17, 2012, 07:27:36 am »
Hi

I picked up a transformer today that says "6-0-6" on it. When I power it up, it puts out 12V p-p, 8.6V RMS (centre tap to one secondary - double the above across both secondaries, of course). Actually, first question - is the "6-0-6" referring to p-p or RMS voltage? If the former, then it's out by 100%, if the latter, then still out by 43%. It's only rated for 250mA, and the voltage didn't drop at all at 100mA or so.

What's a reasonable expectation for accuracy with these things?

Cheers
john
 

Offline IanB

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Re: How accurate should a transformer voltage be?
« Reply #1 on: June 17, 2012, 07:58:58 am »
We never talk about p-p voltages with AC supplies (how do you even measure that anyway?). Standard AC voltages are always quoted and measured as RMS (or average voltages assuming a sine wave for meters which do not have a true RMS function).

A 12 V transformer is 12 V RMS nominally, but it will always deliver more than that in reality. The voltage will drop closer to an actual 12 V if you apply the rated load, but there is a conservative de-rating factor in there as well.

Extra voltage should not matter for most uses, since you will always regulate it down to the required value if the voltage is critical for the application.
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Re: How accurate should a transformer voltage be?
« Reply #2 on: June 17, 2012, 08:52:01 am »
We never talk about p-p voltages with AC supplies (how do you even measure that anyway?). Standard AC voltages are always quoted and measured as RMS (or average voltages assuming a sine wave for meters which do not have a true RMS function).
Note that an average-responding meter would still show the RMS value for a transformer. The RMS value is just approximated by measuring the average rectified value and converting it to RMS by assuming the signal is sinusoidal (multiply by sqrt(2)).

Try loading both windings with 250 mA, the voltage will probably be closer to 2x 6V rms. In most cases the only effect of a higher voltage from the transformer is slightly more dissipation (in the case of a linear regulator). If it would exceed the max voltage for some of your regulator, you might considering switching to other regulators with a higher max. input voltage, since line voltage can also be expected to fluctuate a bit. In the US, +/- 5% from nominal is allowed, in the UK and AU -6% +10% from 230 V (so 240 V is comfortably within spec and they don't have to change any equipment to switch to '230 V').
 

Offline IanB

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Re: How accurate should a transformer voltage be?
« Reply #3 on: June 17, 2012, 11:37:44 am »
To illustrate transformer voltages, here is a graph I made of a transformer labelled 12 V 2 A. As you can see the output voltage is above 13 V even at a load of 2 A. Unfortunately I didn't record the mains input voltage at the time but I think it was very close to the expected 120 V. Note that many consumers in the USA get a mains voltage of 110 V or lower due to being a long distance from the electricity supply. These consumers would be disappointed if their "12 V" transformer delivered less than 12 V when the mains voltage was low.

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Offline ejeffrey

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Re: How accurate should a transformer voltage be?
« Reply #4 on: June 17, 2012, 07:27:29 pm »
Hi

I picked up a transformer today that says "6-0-6" on it. When I power it up, it puts out 12V p-p, 8.6V RMS

Your notation is wrong here.  An 8.6 V RMS source is 24 volt peak-to-peak.  It is 12 V pk.or 12 volt amplitude.


Quote
What's a reasonable expectation for accuracy with these things?

It depends quite a bit on the transformer.  Low VA transformers generally have poor regulation: the voltage drops a lot between open circuit and the rated load.  Add on to that the normal supply variation and your result is quite believable.  Larger transformers have better regulation out of necessity.  Poor regulation is the result of low efficiency and larger transformers would overheat if they had such poor efficiency. 
 

Offline Zero999

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Re: How accurate should a transformer voltage be?
« Reply #5 on: June 18, 2012, 02:55:06 am »
how do you even measure that anyway?
With a precision rectifier and peak detector or an oscilloscope of course.
 

Offline IanB

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Re: How accurate should a transformer voltage be?
« Reply #6 on: June 18, 2012, 06:14:11 am »
how do you even measure that anyway?
With a precision rectifier and peak detector or an oscilloscope of course.

Well sure, but that's not a standard readout on a typical multimeter.
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Offline vk6zgo

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Re: How accurate should a transformer voltage be?
« Reply #7 on: June 19, 2012, 03:20:02 pm »
how do you even measure that anyway?
With a precision rectifier and peak detector or an oscilloscope of course.

Well sure, but that's not a standard readout on a typical multimeter.

The point is,of course,that peak-to-peak is really an imaginary value,as both half cycles are not present at the same instant.
It is a convenient fiction if you are looking at an AC signal with an Oscilloscope,but that's all!
 

Offline T4P

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Re: How accurate should a transformer voltage be?
« Reply #8 on: June 19, 2012, 03:26:14 pm »
Vpp is pointless only Vpeak is (Or also called Vmax)
 

Offline demat95

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Re: How accurate should a transformer voltage be?
« Reply #9 on: June 19, 2012, 04:16:12 pm »
Note that many consumers in the USA get a mains voltage of 110 V or lower due to being a long distance from the electricity supply.

The power transformers that get high voltage from, lets say 13,2KV to 120V, have compensation windings so that they ensure no matter how far from the central you are, you always get 120V.


Where is your transformer made? Usually chineese crappy ones have horrible specs. The ones I purchase are usuallly chineese crappy ones lol
 

Offline T4P

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Re: How accurate should a transformer voltage be?
« Reply #10 on: June 19, 2012, 04:36:30 pm »
Note that many consumers in the USA get a mains voltage of 110 V or lower due to being a long distance from the electricity supply.

The power transformers that get high voltage from, lets say 13,2KV to 120V, have compensation windings so that they ensure no matter how far from the central you are, you always get 120V.


Where is your transformer made? Usually chineese crappy ones have horrible specs. The ones I purchase are usuallly chineese crappy ones lol

They can also fluctuate due to well, power plants peaking generators kicking in and also going offline all these can affect the voltage ever so slightly
 


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