# Power transformer impedance

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#### hirafu_boarder

Hi,
I am going to use a 6X5GT tube rectifier in my power supply.
From the data sheets it needs at least 150ohms of limiting resistance on each plate (for capacitor input).

By measuring the power transformer I get
- 18 ohms across primaries (100V) whilst shorting a secondary
- 275 ohms across secondary (320 to CT) whilst shorting primary

So I think I have Rt=Rs + (n^2)*Rp
=275 + (3.2 ^ 2) * 18
= 454 ohms on each plate.
(PSUDII gives the same result).

Does this calculation look correct?

Does my measurement of the secondary look correct?
(it didn't seem to change whether I shorted the primary or not).

Thanks!
Jeff

#### Tom Bavis

Yes, you're correct. Inductance is reduced by shorting primary, but resistance is not.

#### hirafu_boarder

Thanks for the confirmation Tom!

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