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Follow up to Which Transformers

Blog Post created by dmercer Employee on Oct 21, 2014

Follow up to Which Transformers.

In a previous blog posted on July 11, 2014 I looked at a number of possible transformers to use with the Analog Discovery module that can be adapted to solder-less breadboards. The choices that were looked at previously had a lowest usable frequency limit of 10 KHz or greater. As a follow up to that, here is another possible “transformer” that will work down to much lower frequencies.

Pulse Engineering makes a range of current sense inductors that are a toroidal coil with a hole in the center that the user passes a wire ( or loops of wire ) through to sense the AC current. The PE-51718 center tapped 100 turn 20 mH version is shown in figure 1. The size, excluding the leads, is 20mm tall x 11mm wide x 10mm deep which is still small enough to fit on a solder-less breadboard.

PE-51718 .jpg

Figure 1, Center tapped 100 turn 20 mH current sense inductor

The 20mH winding inductance along with a source resistance of 100 Ω should support input frequencies down to around 500 Hz. To test this I tightly wound 14 turns of magnet wire ( approximately 2 feet in length ) through the center hole to use as the secondary. The nice thing about using this coil as a transformer is that you can choose any number of turns you want for the secondary. Up to when the center hole is filled based on the gauge of the wire used. I used some tape to hold the wire in neatly in place.

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Figure 2, Bandwidth test circuit

pulse_trans_14-100_100.jpeg

Figure 3, Frequency response plot PE-51718 with 14 turn secondary, 100 Ω source resistance

The lower frequency limit, based on the roll off seen at the primary, seems to be around 500 Hz as we expected based on the primary winding inductance. The upper frequency limit, based on the roll off seen at the secondary, is about 300 KHz to 500 KHz.

To try and lower the usable frequency more I added a second 100 Ω resistor across the primary winding to result in a net 50 Ω impedance. The extra resistor lowers the signal strength across the primary by another 6 dB, however. The result is plotted in figure 4.

pulse_trans_14-100.jpeg

Figure 4, Frequency response plot PE-51718 with 14 turn secondary, 50 Ω net resistance

The lower frequency limit seems to be around 300 Hz now, with the same upper frequency limit.

Lower 50 turn and higher 200 turn 5 mH, 80 mH inductance versions are also available which will have correspondingly higher and lower minimum frequency limits.

Pulse Engineering current sense inductors are available from:

Mouser: http://www.mouser.com/

Part #: 673-PE-51718NL, Manufacturer Part #: PE-51718NL, $3.39 20 mH 100 turns CT

Part #: 673-PE-51719NL, Manufacturer Part #: PE-51719NL, $3.39 80 mH 200 turns CT

Part #: 673-PE-51687NL, Manufacturer Part #: PE-51687NL, $3.39 20 mH 100 turns

Part #: 673-PE-51688NL, Manufacturer Part #: PE-51688NL, $3.39 80 mH 200 turns

Part #: 673-PE-51686NL, Manufacturer Part #: PE-51686NL, $3.09 5 mH 50 turns

DigiKey: http://www.digikey.com/

Part #: 553-1547-ND, Manufacturer Part #: PE-51718NL, $3.39 20 mH 100 turns CT

Part #: 553-2109-ND, Manufacturer Part #: PE-51719NL, 80 mH 200 turns CT

Part #: 553-1543-ND, Manufacturer Part #: PE-51687NL, $3.39 20 mH 100 turns

Part #: 553-1546-ND, Manufacturer Part #: PE-51688NL, $3.39 80 mH 200 turns

Part #: 553-1544-ND, Manufacturer Part #: PE-51686NL, $3.09 5 mH 50 turns

Electronic Goldmine: http://www.goldmine-elec-products.com/

Part #: G16976, PE-51718, $3.95 20 mH 100 turns CT

These transformers could be used as injection transformers to measure loop response at lower frequencies than was done in this lab activity write-up:

Activity: Measuring Loop Gain [Analog Devices Wiki]

As always I welcome comments and suggestions from the user community out there.

Doug

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