winding shoestring transformers

[kepeb]

so, more thoughts on cheap transformer projects.
just finished my preamp build and want to learn/talk about something

so, i had a dead wall wart/ac-dc adapter (240 - 9vdc) i have partially disassembled whilst reading some old papers and pages on transformer re-wiring.
removed the rectifier/filter part, took out all the iron (EI) and seems the primary was open.
after partially unwinding i found what i believe to be a fuse taped in there before the wiretap out.
checking the dcr on this winding the other side of the fuse gives 900ohm, so its still good?

the secondary measures only 1.5ohm dcr,
is this the measure inductance? do i need the iron in and the other winding next to it before taking readings?

the core is 1/2" x 1/2"
so would my core size be reffered to as 1/4" core?? (0.5 x 0.5 = 0.25?)

i thought i'd post now and get some chit chat on possible experiments like rewindings/removing only so many, what uses i could adapt this for etc. before i do anything stupid like start unwinding and counting

 

[lernith]

Since it was buried in the windings the fuse was probably a thermal fuse.  These'll blow for a wide variety of reasons-- if the transformer was under-rated for the gear it was supplying, if the gear had its own short, or if the wall wort was poorly ventilated (e.g. under a pile of laundry).  It might seem strange to place a fuse that effectively destroys the transformer; but the alternative might be letting the coatings of the primary and secondary windings melt and short to each other.

Removing the core will change the inductance of the windings, but you would need to have an inductance meter to read it.  DCR is direct current, inductance is AC.  Check the DC resistance on a handy 8 Ohm speaker-- it'll be something like 6.5 Ohms.  No, there aren't 1.5 little orphaned Ohms on the garage floor somewhere; that's just how coils and cores treat AC and DC differently. 
Oh, and I don't mean to say there is a fixed ratio between DCR and inductance.  DCR is largely controlled by wire gauge and inductance by core type and winding count.

There could be a short within one of the windings, this wouldn't stop you from getting a DCR reading, but it sure would be sweet if that thermal fuse (if thermal fuse it is) did its job and kept the windings intact.

Replacing the fuse should fix it, but I guess this is an academic discussion.  I'm useless there.

 

[kepeb]

yea, thats good stuff

i dont think i would reuse it as a mains transformer again as i'm far to inexperienced to trust its safety afterwards. besides i cut open the molded plastic to get it out too.

so i can clarify
inductance is the ac resistance when the unit is assembled with the core in place.
the dcr i can understand is different and when looking at speaker impedances, usually just round it up to the the nearest standard but there is not a sure way to calculate one from the other? the inductance is the same as impedance?

and from what PRR has said in another thread, there is no 'real impedance' inherent in the transformer only its reflection?

from a guess based on another posters suggestion (dcr + 'a little bit more' x 10) this particular transformer could be seen as having maybe 10kohm : 20ohm  or 500:1??

 

[mad.ax]

You already knew the voltage ratio. (240 - 9vdc) = 26.66
Turn ratio = voltage ratio.
Impedance ratio = turn ratio squared = 26.66 * 26.66 = 711

Axel

Edit: the symbol for squared doesn't work either... Isn't that a U.S. character ?

 

[Speedskater]

Edit: the symbol for squared doesn't work either... Isn't that a U.S. character ?

NO. It's the forums software that's unhappy with the character.

 

[jdbakker]

Edit: the symbol for squared doesn't work either... Isn't that a U.S. character ?

NO. It's the forums software that's unhappy with the character.

Somewhat OT: There's always superscript and subscript (as in 26.662 = 711)

JD 'H2O' B.

 

[kepeb]

thats good, ok
so, assuming its rated correctly its 18 watts also if 200ma

i'm sure i remember reading a method of determining a wires gauge by wrapping it round a pen and measuring...
anyone know this?

i'm thinking maybe i could work out the amount of turns from this and then remove some from either side to adjust the ratios. (and adjusting this on the primary would reduce total power right?)

 

[PRR]

> Isn't that a U.S. character?

The lowest common character-set is ASCII. One small step above 6-bit Teletype (actually a lot of TeleType was 5-bit Baudot). No squared sign.


Why do you want the inductance of a power transformer?

It matters, but not in any simple way.

And the calculations have ALREADY BEEN DONE.

You still have the secondary. Rated 9V. Energize it. Lower is safer. Get a 6V 1A AC transformer AND a 1 ohm 1W resistor to feed the 9V winding. (If the resistor smokes, something's wrong.) Slip one or 10 turns of thin wire around the windings. Measure its AC Volts. Do math.

You now know how many turns for 9V, and can figure how many turns for 120V.

 

[kepeb]

i get it!!!

ok so should i reassemble the whole thing and put the iron back to get that reading?

and either way, anyone have any tips for when i do put it back together? after i slot it all into place i cant see it staying still, what kind of sealant/varnish do you use to hold it together?

i wax potted a couple of guitar pickups once to reduce microphonic effect, is there anything DIY like that which carries over to transformers too?