High transformer input current

[Variac200]

I built a 125W transformer with input voltage of 50V and an output voltage in taps (6). When I energised the transformer and tapped the primary input voltage to a Cathode Ray Oscilloscope, and output left opened, A HIGH CURRENT WAS OBSERVED IN THE PRIMARY. I disconnected the connection to the oscilloscope and there was no such high current.

Is this an EMI problem?

 

[jghrist]

Sounds like you built an autotransformer. What output voltage? A diagram would be useful.

 

[ScottyUK]

Where did you connect the scope ground?

 

[Variac200]

this is a schematic of the transformer ...(attached)

 

[Variac200]

the scope ground was connected to the neutral lead of the transformer

 

[Overvoltage]

Aren't oscilloscope inputs low impedance? Or am I remembering incorrectly (been a few years since I've been around one). If they are, you probably effectively shorted the input voltage terminals.

 

[DRWeig]

Actually, oscilloscopes should have a very high input impedance. It sounds like possibly Variac200's scope input is shorted -

EMI from the oscilloscope should have no effect.

What are you measuring the current with?

An actual photo of your setup would be helpful to us.

Best to you,

Goober Dave

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[jghrist]

As I now understand it, you connected the scope input across the primary of the transformer, with nothing connected to the transformer secondary. Did you measure the high current before or after where you connected the scope? Was the current going through the scope or the transformer?

 

[Variac200]

the transformer was connected to a transformer trainer (TT179) with analogue measuring instruments. The current was measured by the trainer as well as the voltage. I only used the oscilloscope to view the waveform since the input current gave a high value.

 

[ScottyUK]

Transformers are generally well-understood and well-documented, and quite predictable in behaviour in the smaller sizes and at moderate frequencies.

As you designed and built this thing, how about some design parameters?

Core material?
Operating flux density?
Core cross section?
Primary turns?
Frequency?
Primary voltage?

 

[jghrist]

I don't know what a transformer trainer is, but I assume that it was measuring current ahead of where you connected the scope. If that is the case, it was measuring the current in the transformer and the current in the scope. If the high current went away when you disconnected the scope, I would assume that the scope was faulty and was drawing a high current, not the transformer. What happens if you connect the scope to the transformer trainer and disconnect the transformer?

 

[pwrtran]

Your transformer has 6 taps on secondary, would is be possible that another tap switch was accidentally closed so you had a short circuit between taps?

 

[waross]

My thought also, pwrtran.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Variac200]

Thanks for the replies. The transformer trainer is a lab equipment for studies on small power transformers. On the issue of shorting of taps, the taps were properly isolated from each other.