Regulated power supply

[GibsonGM]

Ok, I wanted to be sure you had some experience, because of course you will be connecting to mains, and I didn't want you to ruin equipment - or yourself! Ha ha.        To measure current, of course you need to be in series with the load and set to the proper meter setting....perhaps you have too much ripple and the meter can't resolve what's going on?  (?)   You should be able to get something, I'd think.

Yes, of course you can load test your transformer to see what it can do.  I prefer to do that with only the transformer and resistors.  I have various resistors with different power ratings I use to do this.   50 ohm, 10W, sometimes!  I use two DMM's, one set to measure current and one for voltage.    I watch the voltage drop as I decrease the resistance.  Current flow should match the resistance as expected.  There is a point where you are sagging down as far as you think you can get away with (is it 10% less than the rated voltage, or 20%?  I forget).   At this point you are starting to struggle to get the current you expect, and the voltage cannot keep up.   At that current draw, you note that this is the absolute maximum you want to ask the transformer to provide - ideally a little bit less, and not for any extended time periods.  This is where you're starting to beat on the trafo.   

I'm no expert either, he he...I do this by the seat of the pants.  PRR can tell us a more precise way to do this, with a more scientific foundation (please do!  This way is pretty primitive).   I've just found that this works for the things I do, which are all low-current.  I am always conservative and end up asking much less from trafo's than that 'beating it up' point.   One day I will want to put together something involving higher current, and this way probably won't work, ha ha. 

[Seljer]

If you don't have the transformer's name plate or a data sheet you can only get various subjective measurements.  If it has any markings look for one that is specified in VA (volt - amps), that's the power handling of the transformer (it's specified as apparent power -> VA, not real power -> watts). Divide that by the voltage and you'll have how much current it can supply (on either the primary or secondary side).

You can't actually measure the current what it's rated for directly. What GibsonGM's described about measuring the voltage sag as you gradually increase the load will get you some basic figures. You could probably also hook up some kind of temperature sensor and gradually increase the load until you decided its 'too hot'. Or you can measure the diameter of the winding's wire, calculate the surface of the cross section and assume the maximum current density inside is 3 amperes/mm^2 or so to be conservative.

One of the major thing that holds back a transformer's power handling capacity are losses: both in the copper windings and in the iron core. The more power you pull through it the more power is also wasted due to the resistance of the windings (the induced eddy currents in the core are mostly constant, regardless of load). Losses basically mean heat. You are stopped by bare thermal issues, with the wire's insulation melting and such as it gets too hot! The designer takes this into account and rates it for whatever current it can safely supply at expected operating environment temperatures without it becoming a fire hazard.


I found a transformer a bit bigger than yours in my junk box (6cm * 7cm * 3cm, weighs 0.96kg) and its rated for 55VA. If yours puts outs 20.6V DC recitified without any load, that means it probably does 21/sqrt(2) = 15V AC. 50VA / 15V = 3.3A, so yeah, it's in that area but the voltage may sag too much to get a good regulated 18V power supply at full current.

[JebemMajke]

Guys, sorry for misinformation.

I've found some numbers on it

573206-22

Above that there are letters, it says "AMC" in a rhombus

[Seljer]

Those numbers don't really seem to help, probably only some kind of internal part/serial number at some chinese factory or something 

[GibsonGM]

I'd go ahead and load it up, see what you get, since there doesn't seem to be too many other ways to get data.   Somewhere in an old ham radio book there was a chart of the trafo area indicating approximate VA, if I can find it I'll let you know....   

If you load it and it cannot handle what you're asking, you'll probably know it pretty quickly!   The voltage will peter out - you'll expect 300mA 14V, say, and will only get 50mA at 8V or something.    Just don't let anything get too hot or become hazardous.  I'm sure you know, just putting that out there - insulate ALL mains connections, use a power strip with a switch so you can kill it fast, and connect DMM with jumpers hands-free, all that stuff for safety.   

Start with high resistance and work your way down, make a list of your voltage and current. You can actually graph the results and get a good idea of what is going on! 

[JebemMajke]

How about using two transformers in one preamp?

I was thinking about Fet preamp running at 24 volts, and a sonic stomp after it, which runs on 9v. In one Box.

Are those transformers going to bother each other?

[Seljer]

Shouldn't be any problems.

But it's rather unnecessary. Because unless you're running a power amp or something like that that eats up a lot of current, for little preamps you have more than enough. Slap a 7809 on the 24V supply and be done with it.

[JebemMajke]

Well i've tried 18v regulatot, and it jumps from 18 to 19,3 v all the time 
Maybe to try lm as a regulator?