Hypothetical Question

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K8MHZ said:
Pulsating DC works like AC on transformers. That's how automobile coils work. Make and break produces the required magnetic field movement in this case.

So hypothetically, you could use DC pulsating at 60hz with about a 70 percent duty cycle and the doorbell would hypothetically work just fine.
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The DC component on the primary could still cause a problem unless you make the pulses bi-directional.
 
I assume this setup would not be an issue with resistance heat.....

So, how is an ac trany different from a dc trany? What is in one that allows it to work on one and not the other. DC has a steady current but I don't understand why that would hurt a transformer. Does an ac trany need to have the pulse so as not to heat up???
 
No such thing as a DC transformer.. If there were, Westinghouse/Tesla would never have beat out Edison/Steinmez.

Transformers work following Faraday's Law. Paraphrased: The AC (changing voltage) on the transformer primary coil induces a changing magnetic field in an iron core. The changing magnetic field in the core induces a voltage in the secondary coil.

The key is the magnetic field has to be constantly changing to induce a voltage in the secondary. If the magnetic field does not change - as in DC on the primary coil, then there is no voltage induced into the secondary coil.

ice
 
A DC coil opposes current flow with pure resistance, so they often show a high value on an ohmmeter.

An AC coil opposes current flow with resistance plus reactance imposed by the alternations, and the reactance is often the much larger component of the two, so they often show a very low value on an ohmmeter.

If you're applying straight DC to an AC component, without the reactance there's a much lower opposition to current flow and you'll likely burn up the coil.

Conversely, if you're applying AC to a DC coil suddenly you have all that high wire resistance plus the reactance created by the AC, and this creates an even higher opposition to current flow than the coil is normally designed for.

This is the reason why if you're unsure of whether a coil is AC or DC, you always test with AC first. If the coil is supposed to be DC it will simply be weaker but likely won't be damaged.
 
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There are DC - DC converters that change from one voltage to another - fairly high efficiency, 80% - 90%.

They work on the principle of using the input DC tor drive a high frequency oscillator - 20kHz +. The HF AC is run through a transformer to get the needed voltage, then rectified back to DC. The reason for the high frequency is the transformers are much smaller than for 60Hz.

Even the big cross country DC 500kV lines use this principle. At the terminal, the DC drives an oscillator (probably called an inverter or converter - can't use the word "oscillator" when the transistors are bigger than a hockey puck). But they use 60hz. Then they send the current through a transformer to get to useable voltage levels.

ice
 
John120/240 said:
Some Diesel Electric locomotives generate AC then change it to DC. Then do they change it back to AC ?
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I would expect that they use it to drive DC motors. They may control the current by chopping it (pulse width modulation) and they may do without commutators and brushes by steering the DC to different poles as the motor spins. (Similar but not identical to what a VFD does with an AC motor.)
In the earliest electric and diesel-electric locomotives they just switched winding taps and used enormous resistors to get less than full speed from the motors.
 
Stratovarious said:
I think we should do it , I doubt your typical track home door bell draws much over 500mA:

Forget car batteries ;

(13) 9 volt batteries , just plug one into the other series , and
tap the ends.

Do it at night in case it ain't pretty.


Hypothetically speaking...........
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Not a fair test because the internal resistance of the 9V batteries will likely be more than the DC resistance of the transformer.
 
iceworm said:
No such thing as a DC transformer.. If there were, Westinghouse/Tesla would never have beat out Edison/Steinmez.


ice
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I beg to differ with you.

Ignition coils are DC transformers. The primary and secondary both have polarity. They can also tolerate the rated DC voltage indefinitely. Like when someone leaves the key on an older vehicle while the points are closed. The secondary feeding the spark plugs is DC and the plugs have anodes and cathodes. If the polarity is reversed, the plugs will wear out faster. One reason for switching to - ground was to prolong spark plug life.

Take a car coil, a 12 volt battery, a spark plug and a plug wire. Bond the threaded part of the plug to the case of the coil. Connect the positive lead of the battery to the + terminal of the coil. Touch the negative lead of the battery to the - terminal of the coil for a fraction of a second and then take it off. A spark will occur. About 10kV or more.
 
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One day while working at my auto electrical shop, I got nailed by the ignition on a 1980 Citation. I had been nailed many times, but this was like getting hit by a truck.

Most GM engines at the time used an HEI (High Energy Ignition) distributor with the coil on the top of the distributor. That version did not come in a four cylinder configuration so they used the HEI electronics and a separate outboard coil that looked like a standard AC transformer.

One day I had the parts from a 4 cyl. Chevy laying around so I made up an ignition system using the coil, distributor, a plug and a wire powered by a car battery. When I spun the distributor the spark plug fired VERY strong. So I decided to see how far I could make the spark jump using pieces of steel. It turned out to be nearly an inch and very audible. I estimated 50 or more kV to bridge the 1 inch gap.

By comparison, a regular point style set up would be lucky to get 1/4 inch spark.
 
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