impedance

Hi all,

I have somewhere a chart which lists transformer current and voltage in with the impeadance of the winding. For example "Z" will draw 1amp at 100V at

50Hz, but how is Z worked out ?

I would like to design a high frequency transformer which will draw say 1amp at about 700khz, I need to work out what the inductance would be for the coil.

Anyone help ?

Thanks, Chris

Reply to
exxos
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with

1amp

I think you need to be a bit more specific on what you want. A transformer is, as the name implies, something that transforms. To say that you need a transformer that draws 1A at 700kHz is actually meaningless. There are two components of a transformer's primary (input) current:

(1) Current needed to overcome the losses in the transformer. This will be there irrespective of whether there is any secondaty (output) current; and (2) Current needed to drive the output of the transformer. This depends on the load connected to the secondary winding.

Perhaps you are thinking of an inductor, the values for which you can determine using the information provided. The impedance is calculated by dividing the voltage across the inductor 100V by the current that is flowing in the inductor (1A) giving an impedance of 100 ohms at 700kHz. From there, you can work out the inducatnce by using the formula for inductive reactance (impedance) Z = 2 * PI * Freq * L. Of course this is a theoretical calculation and any practical inductor would have to account for magnetic and resistive losses.

Hope this helps, Mark

Reply to
Mark Little

at

flowing

there,

reactance

Hi Mark,

I just quoted the primary rating here, didnt think of the secondary at this point. Though say the primary will pull a max of 1amp at 100 volts at

700khz, the secondary will be setup up by about 10 times to give 10kv 0.1a. the value for the primary would be around 40mH, though when secondary is added it will be a lot higher which gets to complex for me to work out. As long as the whole thing ( assume a secondary short ) does not pull more than 1amp from the primary then this will probably be what im looking for.

Thanks, Chris

Reply to
exxos

this

0.1a.

than

Chris, In a theoretical transformer, the primary current when the secondary is open circuit is zero, but in reality, current must flow to generate the magnetic field that cuts the secondary winding. The "magnetising current" is governed by the "natural" impedance of the primary winding. In theory, the inductance can be made very high, reducing the magnetising current. The inductance of the primary depends on the permeability of the core and the precision with which the coil is wound. What sort of inductance you can achieve will depend on the construction method of the transformer. you can't simply keep adding turns, since the resistance of the wire will limit the current that can be provided and increases the heat in the transformer. It also reduce the coupling efficiency with the secondary winding.

If we assume that the open circuit current of the transformer will be 5% of the full load, then the primary current, with open circuit secondary, will be 50mA. This means that the inductance of the primary (with no secondary load) should be in the order of 2000 ohms at 700kHz which equates to an inductance of 0.45mH. Depending on the type of construction, the transformer may require more or less more standing drive to achieve the 1A secondary current at the desired voltage.

You will also need to take into account that the high voltage secondary winding will be close the low voltage primary winding, so insulation breakdown amy be an issue.

regards, Mark

Reply to
Mark Little

In article , snipped-for-privacy@home.co.uk mentioned...

Without the voltage, the impedance cannot be determined.

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Reply to
Watson A.Name - "Watt Sun

Note that 700Hz is radio frequency transformer design so normal line or even switching transformer design is inadequate due to conductor, skin effect , core, and insulation characteristics and losses. If loss is not too important and a quick experimental start with easily available components is needed at that power range look at a switching power supply transformer in a (e.g). pc. Otherwise, low frequency radio transmitter design..

Reply to
H. Dziardziel

As

open

magnetic

governed

inductance

depend

adding

of

transformer

Mark,

The transformer will not have a core, I know the voltage stress will be a issue though for the moment I need to work out the inductance values. Something alone the lines of 1T primary and 100T secondary. I have a setup wich has 12T pri and a large coil with indcutance of about 100mH, this setup draws about 4amps at 50V input tot he primary, though the actual inductance what the driver circuit sees is somewhat a mystery :-\

chris

Reply to
exxos

setup

inductance

Chris, The inductance of the secondary is essentially unimportant in determining the primary current when there is no secondary load (assuming that there is only minor losses in the secondary coil.

The formula for an air core inductor with a single layer has the simplified formula (from ARRL handbook) where the length is >= 0.4diameter

L = (d^2 * n^2)/(18d + 40l) Where L is inductance in microhenries, d is the coil diameter in inches, n is the number of turns and l is the length of the coil in inches.

I would suggest that a 12T air coil would be in the order of microhenries. This appears to be supported by your measurements, since a 50V drive giving

4A is an impedance of 15 ohms at 700kHz. This equates to an inductance of about 3.4 microhenries which is probably in the right ballpark.

regards, Mark

Reply to
Mark Little

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