Multi tap transformer

[Tex260Z]

Hi, little help required please (newbie).
Firstly on this new Multi tap transformer I bought, it says I can get 24 Volts by using two taps, joining the two 12's didn't work, correct config please?
Secondly recommendation for a rectifier to match, specs, good brand etc.?
I need DC for a variable voltage controller device.
Thanks in advance.

 

[Tex260Z]

The only other transformer I have is a toroidal out of a Subwoofer, but I have no idea what the outputs might be as all the cable are cut (mains is marked but not orientered as to active and neutral). I cannot find any info on this online.

 

[Tony Stewart]

not such a reliable source of info

 

[Diver300]

A quick update, I follow what you guys are proposing, unfortunately it isn't going to happen for the time being.
It seems I have fallen victim to an "expert", at time of purchase I inquired as to the orientation of the four pins just to make sure, I was shown the + sign on one corner and told that was the AC positive, and the diagonally opposite was the negative, a ten second test was all it took for all the "Special Smoke" to be released, transformer is now dead on all pins, (so much for "Primary thermal fuse fitted"!
Revisiting the schematic it appears that the + is for the DC output side?

Just shows even at 76 you can still learn lessons the hard way.

View attachment 141794

The normal markings on a bridge rectifier are "~" and "~" for the two AC input pins and "+" and "-" for the DC output pins.

On large square rectifiers it's most likely that the + and - are on opposite corners. The +ve is nearest the chamfered corner. If the terminals are flat tags, the one that points in a different direction to the others its the +ve.

 

[crutschow]

I was shown the + sign on one corner and told that was the AC positive, and the diagonally opposite was the negative

The clue that he was less than an "expert" would have been that AC does not have positive and negative polarity, it only has phase, or hot and neutral.

 

[crutschow]

(mains is marked but not orientered as to active and neutral

There is no difference in hot (active) and neutral on a transformer, unless you are somehow concerned about the phase (which is hardly ever the case).
So just connect it either way and measure all the output voltages.

 

[Tex260Z]

Thanks Diver300, yes I should have just trusted my gut, but I always try to get confirmation from a good source.
In fact now that I check I see the small rectifier they gave me to run the fan is marked exactly like that, I just checked the large rectifier and it appears to be OK, unlike the original trannie.

So would I be right in presuming the heavy black is the output neutral and the two orange active outputs for a start?

 

[Diver300]

I don't know what wires do what on the toroidal transformer.

Measure all the resistances of the windings and post them here.

When powering the transformer for the first time, add an incandescent bulb in series with the transformer. If you do that and try to power the wrong windings of the transformer, the bulb will illuminate and there won't be damage. If the connection is correct, the bulb will not illuminate, or will be very dim. You can then measure all the voltages.

 

[Tex260Z]

Got it, thanks

 

[Tex260Z]

Ok, finally got a minute to run the meter over it so I went straight for the voltages with the following results:
Orange + orange = 30 volts
Orange + black = 15 volts.
Grey + grey = 3.5 volts.
Blue + blue = 23 volts.
Green + green = 18 volts
Green + black = 9 volts
All figures slightly rounded up or down for even numbers.
I'm liking the 30 volts, connected the large rectifier and got fractionally lower DC output, happy days!

Next is to decide whether the 18V or 8V is best for the indicator lamp, I think 9 should be enough.

So when I connect it up to the LM338 rig and apply a load (maybe charging a small 12V gel cell?) I believe I should be able to determine the Amps capability by looking for voltage sag? (and heat).

There is also a small board which carries the AC input fuse, this has a couple of blue capacitors? on it, I presume for some sort of filtering/ noise rejection?

 

[schmitt trigger]

The way to estimate the maximum safe power that you can draw from a transformer, is to load it and measure the temperature rise above the ambient. As a rule of thumb I would not expect more than 40C rise.

 

[sagor1]

The blue capacitor-like items on the AC input are probably transient surge protectors (MOVs)

 

[Nigel Goodwin]

The blue capacitor-like items on the AC input are probably transient surge protectors (MOVs)

They look more like simple filter capacitors, and appear to be labelled C002 (and presumably C001) - one is across the output (to the transformer) and the other across the mains switch.

 

[ChrisP58]

Ok, finally got a minute to run the meter over it so I went straight for the voltages with the following results:
Orange + orange = 30 volts
Orange + black = 15 volts.
Grey + grey = 3.5 volts.
Blue + blue = 23 volts.
Green + green = 18 volts
Green + black = 9 volts
All figures slightly rounded up or down for even numbers.
I'm liking the 30 volts, connected the large rectifier and got fractionally lower DC output, happy days!

Next is to decide whether the 18V or 8V is best for the indicator lamp, I think 9 should be enough.

So when I connect it up to the LM338 rig and apply a load (maybe charging a small 12V gel cell?) I believe I should be able to determine the Amps capability by looking for voltage sag? (and heat).

There is also a small board which carries the AC input fuse, this has a couple of blue capacitors? on it, I presume for some sort of filtering/ noise rejection?

The 2nd picture in post 21 indicates that the transformer might be rated at 80 Watts. But, without knowing if that number is really a power rating, or just part of the part number, it's only a guess.
I suggest comparing it's physical dimensions to transformers of known power rating. Here is one place to start.

Be aware though, that that will be the total power of the transformer, not what you can expect from any single winding.

Also, remember that the DC voltage after rectification and filtering will be about 1.4 times the AC voltage.

 

[Diver300]

The resistances of the secondary windings will be difficult to measure accurately as they are so low, but the resistance of the primary winding will be quite easy to measure.

The link that ChrisP58 shares has the efficiencies as well as the copper and iron losses for various sizes of transformer. If you estimate that about half the copper loss will be in the primary, you can estimate the rated power from the resistance.

It's probably easiest to work out the resistances for various power ratings, and see which is closest to yours. For instance, the 100 VA size has a copper loss of 9.8 W, so assuming half of that is in the primary, that's 4.9 W. 100 VA is 0.42 A at 240 V, so that would be about 28 Ohms.

The 80 VA comes out at 42 Ohms, so the primary resistance changes quite a lot between sizes of transformers, so a quick measurement will give you a reasonable idea of the VA rating.

 

[sagor1]

They look more like simple filter capacitors, and appear to be labelled C002 (and presumably C001) - one is across the output (to the transformer) and the other across the mains switch.

Looking at the pcb traces, the only way power can go between the white plug and red wires is if the blue thing is a NTC thermistor of some sort. They are usually black in colour, but who knows…
But the second one does not make sense, it is across the white device pins.
Without knowing the part numbers, we are just guessing….

 

[Nigel Goodwin]

Looking at the pcb traces, the only way power can go between the white plug and red wires is if the blue thing is a NTC thermistor of some sort. They are usually black in colour, but who knows…
But the second one does not make sense, it is across the white device pins.
Without knowing the part numbers, we are just guessing….

It makes sense if they are capacitors, and the white two pin socket is where the ON/OFF switch plugs in - which is what I presumed is what the white plug is for? (it's common to have the on/off switch pluggable in that way, and with that type of connector).

 

[Tex260Z]

Ok, thanks guys, mounted the (input) board on stand offs, good to go.
I will get my digital Infrared Thermometer on the job.
Green & Black 9V lights the LED indicator fine.

 

[Tex260Z]

Here ya go, cap details.
Just two more wires to connect to rectifier at other end and I'm done.