Gestalt said...
a transformer can be used as a dc to dc converter. as can an inductor.

You got in over your head right here

Gestalt said...
anyways, the point of using a transformer other than step up or step down properties is the isolation it provides.

You cannot use a transformer to step up/down or isolate DC. An inductor (or a transformer primary or secondary) is a short circuit to DC, just as a capacitor is to AC.

Gestalt said...
now i am in over my head.

Now you're well and truly in over your head.

Ian K said...

FormulaNova said...


Hate to scare you Ian, but they still use three-terminal regulators when the price matters. Who cares about efficiency when the user only cares about price.

Switchmodes can have variable frequency, variable pulse width, or a combination.

Silicon Chip (the magazine) has a few DC motor controllers that use switching designs to reduce motor speeds. Not quite the same as a regulated switching power supply, but they do use the same IC in the same fashion.

What! not the LM309?

No, not the LM309. I grew up with 7805's myself.

Anyway, no, I was referring to a TL494 which seems to have been used in switchmode PC power supplies for a long time.

Ian K said...
From what I can gather the switch mode regulators work by tapping current into one end of an inductor at high frequency by switching the supply. Then I suppose the tapping frequency is regulated to make sure the load at the other end of the inductor has the specified voltage across it? Anybody found a simplified circuit diagram of one? Must be a diode or two involved - they still use diodes don't they?

Ian, a switchmode power supply simply chops up DC into AC and uses a transformer for the conversion. The transformed AC is then rectified back into DC.

A buck converter, on the other hand, uses just an inductor which is charged at a certain duty cycle:

upload.wikimedia.org/wikipedia/commons/5/52/Buck_operating.svg

It can only step down, and is very efficient, especially compared to the linear (LM78xx-series) regulators. In order to provide accurate output voltage, there is a feedback loop (not shown in the above picture) which regulates the frequency and duty cycle of the supply side. A very typical regulator IC would be the LM2576, which requires just the inductor, diode and a few external capacitors, and has a typical conversion efficiency of over 80% (there are even more efficient versions). Despite it not being linear, the output voltage is quite clean, and with minimum ripple.

A car inverter is a switchmode supply. It takes the input 12V, chops it into high-frequency (as in, higher than 50 Hz) AC, pushes it through a transformer, and integrates (smoothes) the output. The chopping circuit is modulated appropriately, in order to provide an approximation of a 50 Hz sine-wave after the integration step. There is a "flyback" control loop to provide accuracy. The additional step up in voltage, as well as the DC offset removal, are done by a secondary transformer, operating at 50 Hz.

Thanks D1, the Buck converter looks like what I was imagining. Wouldn't they be the best for reducing 12 volts to the lower voltage of most portable computer gadgets? But then again with a bit of synchronised switching at the load end more than 12 volts should be possible.

There's an analogy to the water hammer pump, the slow water swings a hammer that belts a smaller volume of water to a higher level.




I like it that electronics always has a plumbing analogy

1. Input voltage (water head) running into the pipe (inductor)
2. Switching on/off 2N3055 (auto tap turning on/off)
5. diode (one way valve)
6. Capacitor (reservoir with compressible air on top)
3. Output voltage (water with higher head)

d1 said...

Gestalt said...
yes i understand that, however i always thought, and could well be wrong, that pulsing dc could be stepped up or down via a transformer.

That's correct. The transformer has to be designed with the square wave input in mind (the fast rise time is a challenge). And yes, classic transformers do provide galvanic isolation, and are used for this purpose. There are transformer types, such as the autotransformer, which do not possess this attribute though.

cheers.