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I've been doing some reading about transformers and have gathered that they are only able to increase or decrease voltage when using AC. I was wondering whether a switching transistor circuit (the circuit that turns on and off really fast), would be enough to induce another voltage in the second coil of the transformer. I've heard that it's the switching property of AC that actually moves the electrons around. Will this constantly switching DC work?

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Yes. In fact most MODERN power supplies use that principle. That is called "Switch-Mode Power Supply" (SMPS). It is almost required by law in new electronic equipment (including phone chargers, etc.) for efficiency greatly exceeding the old-style big "heavy-iron" transformers that ran at electric mains frequencies (50Hz and 60Hz). SMPS typically operate at 10s or 100s of KHz and use a much smaller ferrite transformer for equivalent power as the old-school "heavy iron".

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  • \$\begingroup\$ Do they use this switching effect to use transformers with DC? \$\endgroup\$ – Scrapper Dec 18 '16 at 10:02
  • \$\begingroup\$ Yes, certainly. The technique is used in DC-to-DC converters, "boost converters", "buck-regulators" etc. etc. It is also used with mains power AC. There probably doesn't exist any computer that doesn't have a SMPS powering it. \$\endgroup\$ – Richard Crowley Dec 18 '16 at 10:04
  • \$\begingroup\$ What would such a circuit look like? I was thinking of trying to use it to boost the audio signal of a song from my iPad and effectively make a really loud portable speaker. I've tried just wiring the audio jack straight to the speaker but that was too quiet. Any ideas on how to make it louder? \$\endgroup\$ – Scrapper Dec 18 '16 at 10:20
  • \$\begingroup\$ Spend a few milliseconds with Google and you will find more information that you can read in the rest of your life. There are dozens of commercial products that will take the audio out of your phone, iPad, tablet, computer, etc. and have a speaker(s) and amplifiers to boost the sound. \$\endgroup\$ – Richard Crowley Dec 18 '16 at 10:24
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You can use a couple of transistors to create an AC signal to feed the primary of a transformer and this works in many different applications: -

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What this circuit attempts to avoid is a continuous DC current flowing in the primary because this can saturate the transformer core rendering the transformer inefficient and fairly useless.

A single transistor can be used and a typical example is a flyback transformer: -

enter image description here

Here, the energy stored in the primary coil's inductance (when the transistor is switched on) is released into the secondary when the transistor switches off. The diode and the 0.47uF capacitor are there to quench any unused energy and prevent the core gradually slipping into saturation.

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Yes, this is done regularly. Look up something called a flyback converter for lots of information.

In fact, here is a example of where I did exactly what you're talking about in a recent design:

The IPULSE signal is straight from a spare PWM output of a microcontroller. When this line goes high, Q8 is turned on, which builds up current in the primary of TR1. When Q8 is shut off, the energy stored in TR1 is transferred to its secondaries. That causes a squirt of current thru D13.

This example is a classic flyback configuration in that the primary and secondary don't conduct at the same time. This allows wide latitude in the output to input voltage ratio. Basically, it transfers Watts thru the transformer, and the output load largely determines the V x A combination those Watts come out as.

Not shown in this schematic snippet, the resulting power voltage on the right side is threshold-detected, and the result of that fed back thru a opto-isolator to the microcontroller. That input of the micro is configured as a shutdown for the PWM generator. The net result is a isolated regulated power supply that runs itself without any firmware envolvement after initialization.

I have found that transformers intended for POE (power over ethernet) use are handy for applications like this up to a few Watts. In this case the isolated power required is well under a Watt. It runs a isolated serial interface that can do RS-232, RS-485, or RS-422, depending on configuration.

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  • \$\begingroup\$ Why didn't you use a simple isolated DC/DC? \$\endgroup\$ – Simon May 5 '17 at 20:42
  • \$\begingroup\$ @Simon: What I show is a simple isolated DC/DC converter. I'm not sure what your point is. \$\endgroup\$ – Olin Lathrop May 5 '17 at 20:43
  • \$\begingroup\$ @Oling Lathrop: I am just interested in your design decisions because I am facing this decision quite regularly. \$\endgroup\$ – Simon May 5 '17 at 20:46

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