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.

 

[ZipZapOuch]

post a datasheet

 

[Diver300]

I assume that there are two secondaries, both labelled "15V 12V 0V"

If so, you should disconnect all the wires and and check that there is no connection between the secondaries. If there isn't a connection, join the 12V on one to the 0V on the other. There should be 24 V between the 0V that hasn't got the join wire and the 12 V that hasn't got the join wire.

If not, measure the voltage of every point relative to the join wire and post the results here.

 

[ChrisP58]

When You connect the two secondaries in series, you need to make sure that they're in phase with each other.

Remember that AC changes direction 50/60 times per second.

If one secondary swings positive when the other swings negative (out of phase) the net voltage will be zero.

If that's what's happening, just reverse the connection to one of the secondaries.

 

[Tex260Z]

yes two secondaries, are both labelled "15V 12V 0V".
No data sheet.
Thanks guys, I will check it tomorrow.
Is this something like I need?: https://www.jaycar.com.au/400v-35a-bridge-rectifier/p/ZR1324

 

[For The Popcorn]

Is this something like I need?:

That's way overkill. The transformer is rated for 8 amps maximum, so any current rating greater than that will be fine.

You're feeding the bridge rectifier 24 VAC. The peak-peak voltage is 24 volts × root(2) =

24 × 1.414 = 33.9 volts pk-pk. A bridge rectifier rated for 40 or 50 volts will provide plenty of margin.

 

[Nigel Goodwin]

The requirements are particularly vague, but depending what they actually are it's most likely he also needs a substantially large reservoir capacitor to smooth the output from the rectifier.

As for the bridge rectifier, I'd be happier with a 100V one (giving margin for spikes), at least 10A, and bolted down on a heatsink/chassis. As for the capacitor, using a crude 'rule of thumb' at least 1000uF per amp, so an 8000uF (probably 10,000uF) electrolytic at 50V or 63V - more if a smoother supply is required.

 

[Diver300]

so an 8000uF (probably 1000uF)

I think that you mean "probably 10,000µF"

 

[Nigel Goodwin]

I think that you mean "probably 10,000µF"

Oops!!! - yes, I'll edit it, thank you

 

[crutschow]

It looks like the secondary has two taps, one 12V, and one 15V, so it doesn't appear that you can get 24V.
The full-wave (Schottky diode bridge) rectified 15V tap would give about 20Vdc.
How much DC voltage and current do you need?

 

[JimB]

It looks like the secondary has two taps, one 12V, and one 15V, so it doesn't appear that you can get 24V.

But there are TWO secondaries.
See post #5

JimB

 

[ZipZapOuch]

Connect one "0V" on one secondary to the "12v" on the other secondary. Then measure between the unconnected "0" and "12" with the AC volt mode on your meter. You should have 24v.
Edit:sorry, Diver300 already said that.

 

[Tex260Z]

Hi guys, Ok. tried both ways around, getting 26 volts AC one way, all good
So I am resurrecting a power supply I made donkeys years ago based on an LM338, (see attachment), just needs a transformer and rectifier, everything else has been tested and works. I am going to use it to charge numerous Lipo battery packs I have built for many appliances (cordless vacuum, drills & cordless screwdrivers etc. I have quite a few different configurations and cell charging spec variations. Off to buy a rectifier this morning.

I was wondering if it is possible to run the 12 volt fan I have added and power lamp (connections to be moved) off the 12 volt tapping at the same time (marked "F" in pic). this thing originally ran off a 12 volt transformer.

At the end of the day I mostly need app 14.5V. output, most packs are about 5 amps max charging.

 

[crutschow]

if it is possible to run the 12 volt fan I have added and power lamp (connections to be moved) off the 12 volt tapping

Sure. if the fan is designed to be powered from AC.

At the end of the day I mostly need app 14.5V. output, most packs are about 5 amps max charging.

For that you might consider adding a SPDT switch to go from the 24Vac to the 15Vac tap to the bridge, which will reduce the dissipation in the LM338 @ 5A by about 45W.

 

[Tex260Z]

Thanks Crutschow, bit beyond my comprehension I am afraid, I would need to see a drawing I think.
Thanks for the heads up on the AC one the fan drive, completely slipped my mind.
Anyways, went to Jaycar, picked their brains? a bit, and they sold me two diodes, a GW212 MB354 for the LM338 driver, and a a GW142 BR106 for the fan. I will cobble it together tomorrow and see what i get.

 

[Tex260Z]

Whoops forgot to ask, that would need to be a On-Off-On switch, correct?

 

[crutschow]

I would need to see a drawing I think.

Here's how to wire the transformer and connect a SPDT switch to go between 15Vac and 24Vac to the bridge rectifier:
You don't need an On-Off-On switch, just a standard, 2-position, SPDT.

I did a similar thing on a bench supply I build many years ago, to minimize the dissipation in the regulator when powering lower output-voltage loads.

 

[ZipZapOuch]

Here's how to wire the transformer and connect a SPDT switch to go between 15Vac and 24Vac to the bridge rectifier:
You don't need an On-Off-On switch, just a standard, 2-position, SPDT.

I did a similar thing on a bench supply I build many years ago, to minimize the dissipation in the regulator when powering lower output-voltage loads.

View attachment 141784

Automatic switching with a comparitor and relay would be doable if one would be motivated to put the effort in.

 

[Nigel Goodwin]

Automatic switching with a comparitor and relay would be doable if one would be motivated to put the effort in.

There's not much effort involved, and you can use multiple taps on the transformer as well. Many commercial PSU's use that method - in the one I use at work you hear the relays clicking in and out as you alter the voltage.

 

[Tex260Z]

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.