Re: gEDA-user: Transformer as voltage transducer?

[Vaughn Treude <vltreude@deru.com>]

Randall,
I just happened to do some research on this.  The following part:
NEC PS2505L-4-A
is a quad optocoupler.  On the input side, each optocoupler has two 
diodes connected in opposite directions.  These stimulate a 
phototranistor output.  I was using them mainly to see if an 24 VAC 
control voltage was on or off, so I set up the circuit to go into 
saturation.  I also put a large cap on the output to smooth the voltage.
But you can limit the current on the input so you get an output that's 
proportional to the input.  One possible downside (for your application) 
is that this circuit will full-wave rectify your signal.  There's 
another part, the 2501, which has only a single diode on the input.  You 
could use two 2501 couplers to measure the positive and negative parts 
of the waveform.
Vaughn

John Doty wrote:
> On May 25, 2007, at 7:21 PM, Randall Nortman wrote:
> 
>> I would like to measure mains voltage (110-240V, 60Hz), but I want my
>> measurement circuit to be isolated from the voltage being measured.  I
>> am going to be sampling the waveform at a high sample rate (relative
>> to the 60Hz waveform being measured) and comparing that with the
>> current on the same lines with (near-)simultaneous measurement.  So it
>> is important to me that the voltage waveform not be distorted or
>> phase-shifted, and the voltage I see should be related to the source
>> voltage by a simple linear ratio.
>>
>> Seems like a simple transformer will do it, in theory.  But I'm not
>> sure about how they will work in the real world.  Do transformers
>> distort the voltage waveform or phase?
> 
> Some. Cores are nonlinear, there are ohmic losses, eddy current  
> losses, magnetization inductance, leakage inductance, capacitance, ...
> 
> For a stiff voltage drive and a light load, you'll probably find the  
> distortion minimal. But you haven't stated a quantitative requirement  
> and transformer manufacturers generally don't write specs in a way  
> you can relate to here, so you'll want to to get a transformer and  
> make some measurements, I suppose.
> 
>>   What happens if I put a load
>> on the secondary of more than a few nA?
> 
> For a small power transformer you'll probably find the difference  
> between no load and 1 mA difficult to measure.
> 
>>   What non-linearities are
>> there?
> 
> Mainly the nonlinear hysteresis curve of the core.
> 
>>   What sort of transformers are going to give me the best
>> response?
> 
> Hard to say. Again, you haven't stated quantitative requirements, and  
> the usual specs won't tell you anyway. So, I suggest just getting a  
> cheap power transformer, trying it (look at the AC in and out on a  
> scope, say), and if it isn't good enough come back here with details  
> of the problem.
> 
>> Or if the transformer idea is awful, any other ideas for isolated
>> voltage transducers?
> 
> A transformer is probably fine. Alternatively, google "isolation  
> amplifier". These are good for sensing current (through a small  
> resistor) too.
> 
> John Doty              Noqsi Aerospace, Ltd.
> http://www.noqsi.com/
> jpd@noqsi.com
> 
> 
> 
> 
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