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TN 43: The Design

Posted: 5 years ago Quote #425
Transformers are the least understood of all components.  We thought we would give you some insight into the design.

How about the power unit first?  The first item in the design is the size of the core.  We must determine the output VA  (volts x amps) then add them together to get the total output VA (volt-amps). The next step to determine the type of steel to use  (for the core) and determine the core size that will support the VA requirement.  EDCOR only uses M6 grain oriented silicone steel so we can make a reasonable chart to determine core size vs. VA. Before all of this, we determined what flux density we will use on the transformer.  The flux on M6 is acceptable from 0 gauss to about 17500 gauss. We have chosen to use about 14000 gauss (1.4 Tesla). The higher the gauss the higher the operating temperature and frequency limitations (example 50 Hz or 60 Hz). We try to work with 50 Hz to satisfy most of the world.  If it works at 50 Hz it will work at 60 Hz. If it is designed at 60 Hz it may not work at 50 Hz.

The impedances are not taken into consideration, as the line delivery impedance is virtually zero until the fuse blows. The output impedance reflects the input impedance. The DC resistance is not taken into consideration. The considerations taken into the design at this point is the outputs and each of their VA. Example output #1 (300v-CT-300v  150mA) (90 VA). The first step is to find the turns for the primary (function of core size – low frequency – input voltage – flux density). Np has been determined with a formula based on core materials etc.  Np must carry the current required to deliver all of the output needs, so we must calculate each of the output requirements.  The number of turns is quite simple (Output E  / Input E  = turns ratio). We use a loss factor depending on the number in the total VA output  (typically 1.05 so the output voltage with no load is 1.05 times the desired).  Next is the wire size for each winding.  We use 500 cma (circular mill amperes) minimum to keep the unit cool and not stress the wire or levels of winding of the wire. Keep in mind this must be figured on continuous at full current flow. The build is calculated to determine the fit. About 70% is O.K. since insulation must be used between different voltage windings.

The choke is next.  Wow this is different. Since DC current is flowing the unit core must be gapped to limit the magnet. The gapping and the DC current flow affects everything. We test the unit at 120 Hz and with full DC flowing in the circuit. Many test the unit with 1000 Hz.  (Xl = 2piFL) We want Xl to be high at 120 Hz not at 1000 Hz. Put a bypass capacitor before and after the choke. It is virtually impossible to make a real choke above 10H at 300mA at 120 Hz at the choke frequency. It’s only a function of core weight. Take a few minutes to do the AC impedance math. The DC resistance on the choke is important as it is the (E=IR) DC voltage drop that is important to be as low as possible.  A 100 Ohm choke at 300mA drops 30volts, however, the higher the inductance, the higher the DC resistance.

The next is the audio transformer with no DC flowing.
  Keep all of the above in mind and add frequency and impedance into the equation. The turn’s ratio is a function of impedance now, not voltage output. The square root of Zout / Zin is the turns ratio. Example 600 Ohms to 10k Ohms is 4.08 so 1v input = 4.08v output and the T.R.=4.08.  The turn’s ratio is a direct function of E ratio and sees how it relates to Z.  We do not consider any correction factor when audio T.R. is considered except when the super power amp is driving a distribution transformer  25v ,70v ,100v, 140v, etc. lines with multiple loads. The turn’s ratio will also be the voltage ratio. Most of these type transformers will be 2 or 4 ohms input and the high distribution line level voltage outputs. Exactly the way power lines work.  The reverse is the speaker transformer. Bring
Brian Weston
Tech Support
EDCOR Electronics Corporation
Posted: 5 years ago Quote #431
What is  "cma - circular milliampere"? I use current density: amperes per conductor cross section area, usually millimeter^2, standard used in power transformers is 2.5A/mm^2
Posted: 5 years ago Quote #432
CMA is the current rating of the copper wire. I is how much amps the gauge of wire can handle.
Brian Weston
Tech Support
EDCOR Electronics Corporation
Posted: 3 months ago Quote #927
how to claculate input and output transformers for microphone preamplifier?
Posted: 3 months ago Quote #928
The input and output transformers are determined on your preamp design. The transformers are really just matching transformers. You need to know the impedance of the microphones you will be using. Most work well at 150 Ohms. You then need to figure out what impedance your preamp wants to see on the input. That gives you the matching ratio for the transformer. The size of the transformer is based on the signal level. Because the microphone level is low, a small transformer, like the MX series, is good.

The output transformer is figured out the same way. You first figure out what the output impedance of the preamp has and what the equipment you are going into wants to see. Again the size is based on the signal level. If it is line level, you would need a bigger transformer then for a mic. level signal.
Brian Weston
Tech Support
EDCOR Electronics Corporation