Transformer current coupling

[lowell]

I'm curious as to how a bad power tube can blow a mains fuse... or any bad component, maybe a filter cap or something. I guess I don't see how current demand on the secondary affects current on the primary of a PT. I understand somewhat how a transformer couples AC and can transform voltages higher/lower, and how an OT transforms a power amp to low impedance to drive a speaker. I just don't get how it works backwards in regards to the mains fuse. This question sounds weird to me now... I appologize if it is .. Can someone shed some light on this?

[Steve Conner]

This is one of these tricky things in electronics, like explaining to your kid how Santa Claus actually delivers 750 million gifts in one evening.

If the electronics in your amp consume, say, 100 watts, then at least 100 watts must pass through the PT. However the PT itself consumes practically no power. Therefore, conservation of energy demands that loads on the PT's outputs must affect the current drawn at its input.

A practical example: If you have an (almost ideal) transformer with a turns ratio of 10:1, and you put 100 volts AC on the primary, then 10 volts will appear on the secondary. It will draw practically no current from the 100 volt source. If you now draw 1 amp from the 10V secondary, say by connecting a 10 ohm resistor to it, then 0.1A will be drawn from the 100V side. 10 watts either way, which is what I mean by conservation of energy.

A real transformer will not obey this exactly, you always get a little less power out than you put in. That's why they get warm in use.

[lowell]

This is one of these tricky things in electronics, like explaining to your kid how Santa Claus actually delivers 750 million gifts in one evening.

ha!

Ok so the current ratio is the same as the voltage ratio. If your mains fuse is a 4amp and you have a PT with a 10:1 ratio, and the load on the secondary is drawing 400ma (4/10), goodbye fuse. Is that correct?

[Enzo]

No, If you have 100v in and 10v out, fusing the primary at 4 amps means 400 watts. To blow that fuse, your secondary 10v would have to load down to 40 amps. The current is inverse to the voltage.

ANd by the way, transformers have no direction. You put some AC voltage on the primary, you get AC voltages on the secondary proportional to he turns ratios. But you can also apply AC voltage to the secondary and have it come out the primary the same way. If I take a 120 to 12v transformer and connect 12v to its secondary, 120v will come out its primary.

Think of a transformer as a set of gears. You can drive either gear and it makes the other turn. Neither gear has a specific speed, it only has a ratio to the other gear. Same with transformers. If I took Steve's 100v to 10v transformer and only fed 50v into it, only 5v would come out the secondary.

The core of the transformer doesn;t have voltage or current, it has a magnetic field.

The Santa Claus part is simple, he doesn't use union workers.

[Steve Conner]

Ha! Next you'll be telling me that Santa Claus's elves are actually Mexicans.

Ok so the current ratio is the same as the voltage ratio

Close, but not quite. The current ratio is the inverse of the voltage ratio, because that's what's needed to make conservation of energy hold. If you step the voltage down 10 times with a transformer, you step the current up 10 times. Like I said, watts= volts*amps, and watts in = watts out of the transformer.

This by the way is the original reason that AC was chosen for house current. It allowed the power company to distribute the power at a high voltage (12kV is common in the USA) and install a transformer close to your home to step it down to 120V. They can deliver a 200 amp service to you, but it would only draw 2 amps from their 12kV lines, so they can make them out of much thinner cable. It takes miles and miles of cables to electrify a town, so the saving in copper more than pays for the transformers.

And sure enough they are bidirectional. If you somehow get hold of one of their polemount transformers, you can plug it into your dryer outlet and 12kV will come out. Why on earth you would want to do this, let's not even go there...
http://www.youtube.com/results?searc...=pole+pig&aq=f

[lowell]

Ok thanks guys. Got it. thanks!

I believe I read somewhere that 1 of Santa's Elves weigh the inverse of the weight of his beer gut. How the elves maintain that ratio is the real mystery and magic.

[Enzo]

If you somehow get hold of one of their polemount transformers, you can plug it into your dryer outlet and 12kV will come out. Why on earth you would want to do this,

Two words:

Jacob's Ladder.

My Climbing Arc (Jacobs Ladder) - MySpace - Truveo Video Search

[lowell]

If I have an amp w/ 4 dual triodes (say 12ax7) and 2 6v6 and 2 6L6 power tubes what would the mains fuse value be? I think those tubes' heaters in total would draw 2.4amps... right? How do I determine the amp's wattage consumption? Finally, is the fuse value really wattage consumption divided by mains voltage?

[Enzo]

Your fuse would be something like twice the normal current flow. You don;t want a 5 amp fuse protecting a circuit that draws 5 amps.

Watts are watts. 120v at 1 amp is the same power as 12v at 10 amps - 120 watts. That is how you translate secondary loads to primary needs - power.

Where did you look up tube heater draw?

6L6 - 0.9A each
6V6 - 0.45A each
12AX7 wired as 6v - 0.3A each

So two 6L6 (1.8A), two 6V6 (0.9A), and four 12AX7 (1.2A), totals 3.9 amps I think.

3.9A at 6.3VAC means 24.6 watts of heater.

Power = volts x amps.

SO at the primary, 24.6W = 120v x ?


Solving for current. P = VxA becomes A = P/V

So: A = 24.6/120 = 0.2A

0.2A of 120v mains is for the heaters.

But that is not the whole story. Your B+ draws current. If you have 480v B+ - I picked the number becuase it works with 120v simply - and your power tubes are biased to 40ma at idle, that means 19.2 watts each - AT IDLE. When you are playing, all those 50 or 100 or whatever watts have to come through the transformer. SO add that. Plus the 12AX7 circuits are all class A so their current draw is steady, but it is another 3-4 ma per tube, so another 12-16ma from the B+ for them, so another 7.8 watts for them.

The bias circuit draws about nothing. But even the voltage divider resistors draw a milliamp or two, so another tenth of a watt for that.

Got a pilot light bulb? Add another watt or two.

Your transformer is not 100% efficient, its wires have resistance if nothing else. SO a few watts are lost to that.

SO you add all those things up for your amp, get the total power used, and find out what current that represents at the mains voltage. That is your basis for selecting a fuse. Add some headroom for the fuse so it doesn;t blow every time you hit a powr chord.

That is why your 100 watt amp might have a 300 watt power rating and a 4-5 amp fuse.

[lowell]

sweet. thanks Enzo, that helps alot. yeah it was late and I was totally making up the heater draw for the 6L6... oops.