Transformer help

Thread Starter

jmh474

Joined Jun 26, 2014
38
I have a 12vac 3 amp transformer and I need to get 24v from the transformer for a project - variable power supply.

There's a cheap kit from yes you guessed it China but it requires 24vac.

Now I know your going to say why not just buy the appropriate transformer but iv not got the money, I have been tempted to use an old laptop charger and ?hack? to increase the voltage to 24v and bypass the bridge rectifier on the power supply kit.

This is the kit I'm working with

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/283003936123

I know it's cheap and cheerful but Im making do with what I can afford. And there's a lot of mods people have made to this kit for better results.

Iv got a shed full of salvaged pcb full of components which I reuse.

Any help would be much appreciated
 

be80be

Joined Jul 5, 2008
2,049
You can find a 24 volt AC transformer for free if you know where to look ask your local HVAC installer most throw away there old units when
they install a new one I got 2 nice ones for free.
These where from old gas furnace. 3 amp some are even 5 amps but most are 3

That one is 1.6 amps
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,249
That Chinese power supply kit is a copy of a 15 years old Greek kit that does not work because most of its parts are over-loaded. I helped fix it years ago but this Chinese kit does not use my fixes. Many of my fixes will not fit on the little Chinese circuit board.
Did you notice that the text says the kit has an LED display? They lied, it doesn't have one.
 

Thread Starter

jmh474

Joined Jun 26, 2014
38
I'm not to worried about the led display as I have a led voltage and current display I want to use, but this is an added bonus as I'll open up a case to get money off lol, sorry to say but that's there fualt.

I do have another 12v transformer BUT there not matching amp rating my other one is only 1.5 amps would it be OK to run them in series to get 24v??

I'll have a look over you fixes and see what I can use
 

Thread Starter

jmh474

Joined Jun 26, 2014
38
That Chinese power supply kit is a copy of a 15 years old Greek kit that does not work because most of its parts are over-loaded. I helped fix it years ago but this Chinese kit does not use my fixes. Many of my fixes will not fit on the little Chinese circuit board.
Did you notice that the text says the kit has an LED display? They lied, it doesn't have one.
Any chance of a link to your fix ask I'm struggling to find it thanks
 

Tonyr1084

Joined Sep 24, 2015
5,572
I do have another 12v transformer BUT there not matching amp rating my other one is only 1.5 amps would it be OK to run them in series to get 24v??
Generally yes you can as long as you get the phasing right. But it will only be as strong as the weakest link. In other words, 1.5 amps. Generally not a recommended solution, but in a pinch, it can be done.

I bought a transformer for my doorbell. It has three terminals. Between A & B is 8 VAC. Between B & C is 16 VAC. Between A & C is 24 VAC. It's not a high powered transformer though. Only rated for a doorbell. But like others have said, there ARE transformers out there. I especially like using 24 volt transformers from old heaters. I have a few. But again, they are not rated at the power you're looking for.

Another transformer (or two) I have came out of an old stereo unit. Its high power output voltage is 96 VAC with center tap, making 48 VAC from one leg to the center tap. Using just two diodes it can be used as a 48 volt full wave rectified power source. I'm sure smaller stereo's (less powerful) will have transformers closer in voltage to what you seek. This particular transformer is a multi-tapped multi secondary transformer. I can pick 5 VAC 12 VAC (center tap) or 24 VAC (same coil, not utilizing the center tap). In fact, there are a number of voltages I can attain from this one transformer. The only drawback is the power output of each secondary. That information I do not have.
 

Tonyr1084

Joined Sep 24, 2015
5,572
Worse comes to worst you can get a microwave transformer and cut out the high voltage windings and wind your own transformer. I've never done that so I can't tell you exactly how to go about it, but whatever starting voltage you have, I would think (I said "I would THINK") if you have 240 VAC, building a 10:1 transformer would give you 24 VAC. Just make sure you use the proper wire size and be careful not to nick the insulation. Also - extreme caution is required when working with mains voltage. In fact, topics discussing mains voltage is a prohibited topic on this forum. I'm not advocating working with mains, but if you're using a transformer to go from mains to project voltages, then you're already dealing with that aspect. Just make sure you don't come in contact with the leads of the high voltage capacitor or any bare metal leads connected to that cap. Best is to take a pencil lead and short it across the capacitor leads. The pencil lead will act like a low resistance bleeder resistor and remove any dangerous charge that may be hiding in the cap. A shock from that cap can be painful, can even be deadly. So caution is advised. And don't attempt to operate the high voltage transformer outside of the oven. More precisely - don't mess with the high voltage.

Once you've safely removed the Microwave Oven Transformer (or MOT) you can use a hack saw to cut through the high voltage coil - it's the one with the very fine wire. Using pliers, pull out the guts of the HV winding and recycle the copper. If you're really good at mechanical work you can cut through the weld on the transformer to separate the core and slide the primary (mains voltage) winding out of the way and begin building the 24 VAC winding you want/need. I can't give you a ratio of how many windings OR what gauge wire to use. But trial and error will mean you will have to wire it up and test it. If the voltage is too small you will have to wind additional windings. If it is too high then all you need to do is to remove some of the windings. And as always, when working with enameled copper wire be sure not to nick the enamel coating.
 

Audioguru

Joined Dec 20, 2007
11,249
Any chance of a link to your fix ask I'm struggling to find it thanks
There are many links to fixing that power supply circuit and a few say that its Chinese pcb is too small for most of the fixes.
The 24VAC transformer voltage causes the circuit to produce only 25VDC at 3A and even with the voltage being too low, the opamps it has will be operating at a supply voltage higher than is allowed for them.

I recommend a 28VAC/4.2A transformer (120VA) because at 3ADC the maximum output from the transformer has a peak voltage of 28V x 1.414= 40V. 40V x 3A= 120VA.
I recommend TLE2141 opamps because they do not need a negative supply (but a low voltage negative supply is needed for the current regulation circuit) and they have a maximum allowed supply of 44VDC.
I recommend two output transistors to share the massive amount of heat.
I did not look at the Chinese parts list and note how many resistors must be large enough for the heat.
Here are my fixed schematic and parts list:
 

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MisterBill2

Joined Jan 23, 2018
7,776
One question I have is did the TS need 24 volts AC? And how much current did they need? Series connection is the way to go, rewinding a microwave oven power transformer is a major task and requires some understanding of exactly how it needs to be done. And removing the magnetic shunts can be a real challenge as well. That was discussed in a thread this past January, I believe.
 

Audioguru

Joined Dec 20, 2007
11,249
One question I have is did the TS need 24 volts AC? And how much current did they need?
What is a "TS"?
The original Greek kit and recent Chinese copies of this power supply wrongly used a 24VAC 3A transformer. It destroyed the opamps since its voltage was too high for them, it caused the 3A max voltage output to be only 25V instead of 30V because the transformer voltage was too low, and it caused the transformer to be overloaded and maybe fail because it is has (24V x 3A=) 72VA rating overloaded with (24V x 1.414 x 3A=) 102VA.
 

Thread Starter

jmh474

Joined Jun 26, 2014
38
iv got hold of a 24v switching supply can I use this instead of a transformer and just bypass the bridge rectifier??

Also I intend to change all the caps on the board for better quality ones and better values

I'll look into changing the opamps later in the day
 

MisterBill2

Joined Jan 23, 2018
7,776
Thread Starter. We used to call them the “OP”, Original Poster.
It took me a while to figure that out, also. The first thing was to realize that the string of comments is called a "thread", and then that the originator was referenced as the "starter". Spelling it out would be less confusing to newcomers, but take a lot more keystrokes.
And as I look at the circuit I see that it looks a whole lot like the one that I had for a screen saver many years ago. the circuit appears to be valid but the component selection is probably poor, and some of the values may need to be adjusted when using better transistors and op amps.
 

Audioguru

Joined Dec 20, 2007
11,249
If you use another 24V power supply then the maximum output at 3A will be about 25VDC, not 30VDC. The Chinese circuit has no calibration trimpots so if you turn up the voltage and current to maximum then the circuit will not regulate voltage and current anymore. The max output current will try to be about 4.2A until something breaks. The driver and output transistors will be overheating when the output current is high and the output voltage is low.
 
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