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Re: [TCML] Calculating transformer output at lower voltages

Hi Gary, Jim, All,

I ran a test like that about 7-10 years ago using a pair of Triplett 630-A meters on the HV side (they have a top 6KV AC scale), and a Radio Shack 22-147B on the low voltage side to measure the respose in the "forward diection". I ran 0 to 48 V input using one 630-A then 45V to 95V input using a 630-A on each side of ground. These two plots were very colinear. I then tested it "backwards" using two of the RS 22-147Bs from 140V down to 0 V on the secondary and got a second plot which was fairly linear, but NOT Co-linear with the first two.
The 147Bs were calibrated before and after against a Fluke 77 series II and showed less than 1% deviation over the 0-140 V range.
I suspect that putting low voltages on the secondary may not create the same magnetizing effect as a low voltage on the primary due to the physical layout of coils and core especially with current limiting gaps. But if one is looking for only a "ballpark number", the reverse method should be OK.(~10-15%).

Matt D. 

-----Original Message-----
From: Gary Lau <glau1024@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Fri, Jun 8, 2012 4:40 pm
Subject: Re: [TCML] Calculating transformer output at lower voltages

Hi Jim,
I must disagree with your post.  The advice is often given to determine an
ST's secondary voltage by feeding 120VAC to the secondary and measuring
he resulting Vprimary, this makes sense.  However, having tried this, it
oesn't work. There's some very non-linear stuff going on with NST's, even
ell below saturation.  There was a thread on this topic several years ago,
 don't recall the resolution.
Regards, Gary Lau
On Thu, Jun 7, 2012 at 10:38 AM, Jim Lux <jimlux@xxxxxxxxxxxxx> wrote:
> On 6/7/12 6:29 AM, mrapol@xxxxxxxxxxxx wrote:

> I know the usual formula Vs/Vp = Ns/Np for calculating transformer
> voltages, but I have a problem in that I am dealing with a sealed NST
> and don't know the number of secondary or primary turns. What I am
> trying to do is calculate the output of a 12kV NST when it's getting
> less than 110 volts input. For example, if I feed the NST 30 volts AC,
> what does it put out? Is there any way to figure this out without a high
> voltage meter (which I don't have) and without knowing the number of
> primary or secondary turns?

 with no load, and with the voltage LESS than the nameplate input voltage,
 it scales linearly (the turns ratio isn't changing, after all)

 12kV out for 120V in
 3kV out for 30V in

 You can also put 120V *in* on the secondary, and measure the primary
 voltage.. 120V in gives you 1.2V out..

 For larger than nameplate voltages this won't necessarily work: core
 saturation and breakdown...

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