Center Tapped and Regular Transformer

On 27-08-2012 23:50, Anand P. Paralkar wrote: > Hi, >

Hi Everybody,

Firstly, thanks a lot for your detailed replies. Although this started-off as a thread on my doubts on the transformer, this thread got (unintentionally) drawn to another topic. One that I was planning to post here anyway.

The reason I said that getting both - the positive and negative source from a transformer without a center tap is difficult is that I tried what a lot of people have recommended here. I built a circuit that had the two diodes connected to the transformer secondary. One diode had its anode while the other had its cathode connected to the transformer secondary. The other end of these diodes were connected to a capacitor each. These capacitors had a common point which we could call the ground.

I was surprised to find that this ground actually drifted! When I measured the voltage across the ends of the capacitors (the end connected to the diodes), the voltage measured was constant. However, when I measured the voltage across the ground and the other end of the capacitors, I saw that this voltage changed. So the V+ source and V- source with respect to the ground was not constant!

I don't know the exact reason what causes this drift. But as a remedy, I put a resistor in parallel to each of the capacitors. (P. E. Schoen has posted this).

This stopped the ground from drifting but I don't think one could use this solution in a practical circuit. A resistor in parallel at the output of a voltage source will not hold up in case of a heavy load (low load resistance).

That's why it seemed difficult.

There are ofcourse many other circuits that have been suggested here. Thanks a ton gentlemen.

Thanks, Anand

For some reason that circuit just does not look appealing..

Not knowing the application it is kind of hard to come up with the proper solution however....

Transformer Bridge +-+-++---+--------+ -. ,+--------+ A A + | | )|( +--++ | --- | | )|( +--+(-+ --- | | -' '+--------+ A A + | | +-+-++ + +------------------+ | .-. | | | | | | | + | R1, R2 100K | | | | |\| Ilimit | | '-' +-+|-\ ___ | | | | >--|___|-+---+ Commom | +-----+|+/ GND | .-. |/+ | | | | | | | | | '+' | +---+--------+

Jamie

You did connect the midpoint of the capacitor string to the other end of the transformer secondary, right? Otherwise the whole thing won't do much at all

Putting a bleed resistor on the caps reduces the supply impedance by making the diodes conduct harder on each peak.

Cheers

Phil Hobbs

Adding a resistor across each the caps is just loading down the circuit enough so that stray fluctuations disappear.

Back in the Tube Amplifier days a heavy ballast resistor was often placed across the B+ to ground (eg. 5K @ 25W - 50W) to stabilize the voltage when there wasn't a loud signal to process - kept the B+ within the capacitors' and other components tolerances...

John :-#)#

It also discharged the capacitor, to protect the 'Darwin types'.

Yeouch....charged caps can be 'fun'.

And from rec.humour.funny quoting - From *Orbit*, the Journal of the Rutherford High Energy Laboratory,Didcot, England (31 January 1965) p.12

Ten Commandments of Electrical Safety

(1) Beware of the lightning that lurks in an undischarged capacitor lest it cause thee to be bounced upon thy backside in a most ungainly manner.

John ;-#)#

That's why no broadcast engineer worth his pay ever trusted bleeder resistors. They had a nasty habit of opening up, with no physical signs of the failure. A shorting stick inside every cabinet with HV.

Is that how "Fly Backs" got their name ? :)

Jamie

unbelievable.

Jamie

(8) Verily, verily I say unto thee, never service high-voltage equipment alone, for electric cooking is a slothful process, and thou might sizzle in thy own fat for hours on end before thy Maker sees fit to end thy misery and drag thee into His fold.

I think you got that backwards, bud!

Jamie

Please:

E = -L di/dt

Opposing the change that produces it.

This is .basics, so we need a little pedantry.

'Lame-nard'.

Maynard A Philbrook JR KA1LPA Willimantic, CT 06226 We are the original "Brand Rex" company

Ground one end of an inductor. Apply a ramp of increasingly-positive current into the free end. Measure the voltage at that inductor terminal. It will be positive. So, with that sign convention,

That's the way most people do it.