Introduction: Using Chopper Transformers From Old PC Power Supplies.
At the address:
https://www.instructables.com/Recovering-Old-PC-Power-Supplies/
I showed how various components can be recovered from old PC power supplies.
I was saying there that perhaps the most useful component that can be recovered is the chopper transformer, basic component (which can be used as such or rewound) in the construction of another power supply power switching.
This is what we will be talking about here and I hope it will be useful for enthusiasts of power switching source constructions.
Supplies
We will need the following:
-Double spot oscilloscope min. 1MHz (photo1). For ease of graphic editing I used Hantek DSO5102P, but any 1MHz double spot oscilloscope is OK.
-Any sinusoidal generator that can provide 50KHz /3Vpp (peak to peak). I used Grundig TG 40 (photo2).
-Fludor and tin soldering tools.
-Digital multimeter. Any kind.
If we want to disassemble a chopper transformer in order to rewind it, we will need those from photo3:
-Hot air gun, scissors, tubular key and large pliers. Small pieces of cardboard or rubber will be provisionally glued to the tubular key and the large pliers, as in the photo, in order not to break the ferrite core of the transformer during disassembly.
-For the actual winding, we need: a screwdriver with variable speed and the tool that is mounted in it, made of copper wire with a diameter of 2.5 mm, as in photo 4.
-CuEm wire of 0.2...0.4 mm. diameter, photo 5. This can be recovered from degaussing coils, old TVs. color if it is in good condition.
Step 1: Identification of Chopper Transformer Windings With the Oscilloscope and the Generator
The simplest is to use the chopper transformer as such, without rewinding them (it is a more difficult operation and with the risk of breaking the magnetic core).
We can thus use the transformer in sources that supply the primary transformer with high voltage, as described in:
https://www.instructables.com/Using-Components-Recovered-From-Old-PC-Power-Suppl/
The trafos we will use here are recovered. It is good that when we can get data from the power supply where it comes from, see photo14 in the link from Introduction. Thus we can have an idea about the maximum power that the transformer can deliver, the configuration of the secondaries and even the diodes used in the secondaries.
Often it is not possible and then it is necessary to identify the exact configuration of the windings on the workbench, before connecting the transformer to the source where it will work, because a wrong connection will lead to the destruction of the source.
Of course, not all chopper transformers from the old PC sources have the same style of windings, but most of them have the constructive form from sketch 1.1 and photo 1.2 (view from the pins).
On the left, the high voltage primary is connected to pins 1,2,3, pin 2 not being connected in the diagram (it has only a technological role). Thus, the primary is actually connected to pins 1 and 3.
On the right are 6 pins to which the secondaries are connected, all having a common ground, secondary GND.
Photo 1.2 shows the physical construction of a chopper transformer, the pins having the same notation as in sketch 1.1.
What we propose next is to find out what is the pin arrangement of the secondary windings, which is essential in using the transformer.
First, with the help of the digital multimeter we will test the continuity between the secondary and secondary GND pins, to establish how many secondary there are and how they are arranged (the resistances of the windings are of very low values).
Then we will make the circuit seen in photo 1.3, as follows:
The TG 40 generator is adjusted to f=45...50KHz (working frequency for many transformers). 3Vpp sine level. We will connect the output of the generator through a BNC cable to 2x crocodile clips at pins 1 and 3 of the transformer. The polarity of the connection is indifferent.
The GNDs of the two probes of the oscilloscope are connected to secondary GND.The probes are connected in turn to the secondaries and the waveforms are visualized.
As examples, we will test 3 types of tafo chopper:
- Trafo type EI-33-38T
With this type of transformer, a manufacturing short circuit can be observed between pins 6,7. On the PCB where it was mounted, I found a short circuit between pins 8 and 9. Thus, the secondaries are connected as shown in sketch 1.4. This connection will be used for tests and in the use of the transformer.
With a digital multimeter you can read the very low resistance between the secondary GND and all the pins in the secondary.
With the montage of photo 1.3, we will measure the secondary voltages, Uprimary=3Vpp. These are:
Photo 1.5: voltages between pins 4, 5 and secondary GND: 2 anti-phase voltages of approx. 0.6 Vpp
Photo 1.6: voltages between pins 4, 8(9) and secondary GND: 2 anti-phase voltages of approx. 0.6 Vpp and respectively 0.22Vpp
Photo 1.7: voltages between pins 4, 6(7) and secondary GND: 2 in-phase voltages of approx. 0.6 Vpp and respectively 0.22Vpp.
2.Trafo type LPJ050003330
With this type of transformer, a manufacturing short circuit can be observed between pins 6,7.
With a digital multimeter you can read the very low resistance between the secondary GND and all the pins in the secondary.
With the montage of photo 1.3, we will measure the secondary voltages, Uprimary=3Vpp. These are:
Photo 1.8: voltages between pins 4, 5 and secondary GND: 2 in-phase voltages of approx. 0.22 Vpp
Photo 1.9: voltages between pins 4, 8 and secondary GND: 2 in-phase voltages of approx. 0.22 Vpp and respectively 0.6Vpp
Photo 1.10: voltages between pins 4, 6(7) and secondary GND: 2 anti-phase voltages of approx. 0.22 Vpp.
Photo 1.11: voltages between pins 8, 9 and secondary GND: 2 anti-phase voltages of approx. 0.6 Vpp.
3.Trafo type AETOJEI33DN0011
With this type of transformer, a manufacturing short circuit can be observed between pins 6,7.
With a digital multimeter you can read the very low resistance between the secondary GND and all the pins in the secondary.
With the montage of photo 1.3, we will measure the secondary voltages, Uprimary=3Vpp. These are:
Photo 1.12: voltages between pins 4,5 and secondary GND: 2 anti-phase voltages of approx. 0.6 Vpp.
Photo 1.13: voltages between pins 4,8 and secondary GND: 2 anti-phase voltages of approx. 0.6 Vpp and respectively 0.22Vpp.
Photo 1.14: voltages between pins 4,6(7) and secondary GND: 2 in-phase voltages of approx. 0.6 Vpp and respectively 0.22Vpp.
Photo 1.15: voltages between pins 8,9 and secondary GND: 2 in-phase voltages of approx. 0.22Vpp.
Common to all these transformers is the connection between pins 6 and 7, which can be seen externally and which is made by the factory.
In the transformer LPJ050003330, pins 4 and 5 are connected to each other, a fact that emerges from the measurement of the voltages on these pins that are equal and in phase. For the same reason, pins 8 and 9 of the AETOJEI33DN0011 transformer are connected to each other .
Step 2: The Use of Transformer Chopper in Specific Cases
Once we have seen what the secondary voltages are (for a certain voltage applied to the primary), we can determine the way the secondaries are connected, so the way the chopper transformer is made up. This allows us to connect the transformer in a certain concrete circuit configuration, in a certain power source. In the following we will refer to the power source given as an example in Step1. But these transformers can be used as such in other sources such as those with TL494 powered at 110...230Vac.
- Trafo type EI-33-38
Taking into account the amplitudes and phases of the voltages in the secondaries of this transformer, it is constructed as in Sketch 2.1.
It can be connected in the circuit as in A, a double diode MBR20100 being recommended. An output voltage of approx. 28...29V/10A will be obtained.
It can be connected in the circuit as in B, a double diode type SB3045 being recommended. A voltage of approx. 9...10V/20A will be obtained.
Both variants can be used simultaneously, if we have a PCB provided in this sense, the total power of the source being max. 200W.
Photo 2.2 shows the chopper transformer mounted on the power supply. Wires W1 and W2 make the connections between pins 4, 5 and the 2 anodes of diode D3 (the PCB is designed flexibly, to allow us to mount a wide range of chopper transformers, whatever their configuration.
Photo 2.3 shows the bottom view power source. w3 connects pins 8 and 9 of the chopper transformer.
This is how the configuration from sketch 2.1, A is realized.
2.Trafo type LPJ050003330
Taking into account the amplitudes and phases of the voltages in the secondaries of this transformer, it is constructed as in Sketch 2.4.
It can be connected in the circuit as in A, a double diode MBR20100 being recommended. An output voltage of approx. 28...29V/10A will be obtained.
It can be connected in the circuit as in B, a double diode type SB3045 being recommended. A voltage of approx. 9...10V/20A will be obtained.
As in the previous case both variants can be used simultaneously if we have a PCB provided in this sense, the total power of the source being max. 200W.
3.Trafo type AETOJEI33DN0011
This transformer, it is constructed as in Sketch 2.5.
It can be connected in the circuit as in A, a double diode MBR20100 being recommended. An output voltage of approx. 28...29V/10A will be obtained.
It can be connected in the circuit as in B, a double diode type SB3045 being recommended. A voltage of approx. 9...10V/20A will be obtained.
As in the previous case both variants can be used simultaneously if we have a PCB provided in this sense, the total power of the source being max. 200W.
Although the three transformers have different codes and manufacturers, they are practically identical in operation, differing only in the way of connecting the secondaries in the circuit and possibly the maximum power delivered by them.
The power supply described is very flexible from the point of view of connecting the secondaries in the circuit, thus allowing the use of a variety of chopper transformers recovered from old PC power supplies.
Step 3: Disassembling a Chopper Transformer
Obviously, not in all cases the voltages provided by the chopper transformer recovered as such from the old PC are suitable for our needs.
In this case, it is necessary to disassemble the transformer and rewind the secondaries or even the entire transformer.
Disassembling a chopper transformer is not a very simple thing because the ferrite core is very brittle and is well impregnated from the factory with electrotechnical varnish.
There is a danger of breaking the magnetic core during disassembly, which would make it unusable.
But with patience and using a proper disassembly method, the operation will succeed.
I have seen several methods on the internet, including boiling the entire transformer in water.
I personally imagined another method that gave results and which I will show next.
We will use the tools in Photo 3, supplies.
After removing the strip of electrotechnical material that surrounds the ferrite core and on which the transformer type is written, we will heat the ferrite core for a few minutes with the hot air gun, as in Photo 3.1
When the magnetic core reaches a temperature of 100...130 degrees Celsius, the electrotechnical varnish with which it is impregnated will soften and the transformer can be disassembled.
To begin with, we will use the large pliers with the cardboard pieces temporarily glued to avoid breaking the magnetic core and we will disassemble the "I" part of the E+I core, as in Photo 3.2.
In Photo 3.3 you can see this part and here we also see how we will press the "E" part of the core with the tubular key that has a piece of cardboard temporarily glued to avoid breaking the ferrite core.
Photo 3.4 shows the partially disassembled "E" core.
Photo 3.5 shows the transformer with the magnetic core completely disassembled.
It must be taken into account during the entire time of these operations that everything is very hot, so care must be taken not to suffer burns on the hands, which can be very unpleasant. It is recommended to use cotton thermal protection gloves.
At the end, the components will be allowed to cool down to room temperature.
Step 4: Rewinding a Chopper Transformer
There are many theoretical methods for determining the number of turns and the diameter of the winding conductor, in the form of formulas or tables, on the Internet or in specialized works.
I propose here to use simple, practical and very effective methods, each suitable for the place where it will be applied.
I disassembled the transformers discussed in Step 1, 2 to see how they are factory wound.
In all cases, half of the primary is wound directly on the winding case and there are 21 turns CuEm 0.85 mm. diameter. This winding is connected to pins 1 and 2 , see sketch 1.1.
A good quality insulation follows, which ensures primary-secondary galvanic isolation.
The secondaryes follow, made with 1mm. diameter CuEm wire.
These are connected to pins 4...9, specific to each transformer.
A good quality insulation follows, which ensures primary-secondary galvanic isolation.
The second half of the primary follows (the same number of turns and the same diameter of the wire).
This winding is connected to pins 2 and 3, see sketch 1.1.
The end of the first part of the primary is connected to the beginning of the second part.
For power sources whose link can be found in Introduction or Step1 (with high voltage transformer primary) we can use chopper transformer with unmodified primary. We will only rewind the secondary ones.
To find out the number of turns in the secondary we will apply an empirical method: if with 2X8 turns in the secondary (as they were counted during disassembly) a voltage of 28Vdc is obtained, then for a voltage of approx. 14Vdc we will wind 2X4 turns of the same type of wire or with multifilar wire, as will be shown below.
We will unwind the outer part of the primary and the secondaries, we will rewind 2X4 turns with the wire used in the secondaries, we will restore the isolation of the primary and secondary, we will rewind the outer part of the primary and we will reassemble the transformer.
For power sources that are powered at low voltage, the primary will also have to be rewound. I will write an article about this in the future.
In any case, rewinding the windings is better done using multifilar conductors. And this for two reasons:
-The multifilar conductor fits better on the winding housing, taking up space more efficiently.
-The losses in such conductors are lower, which leads to a better overall efficiency of the power supply.
For the transformers described here, multi-wire conductors of 4 wires CuEm 0.4 mm or 6 wires CuEm 0.3 mm can be used.
These can be easily made in the personal workshop, according to the following method, in which I made a multifilar conductor with 4 CuEm wires of 0.4 mm diameter, approx. 50 cm long.
On a wooden support I hammered a nail of approx. 5 cm. which I bent to 90 degrees see Photo 4.1.
I tied the end of the 0.4 mm wire to it, measured at approx. 50 cm. (Photo 4.2).
I inserted the tool from Photo 4 (Intro) into a screwdriver with variable speed.
Here we will pass through the wire ring at a distance of 50 cm. See Photo 4.3.
We will repeat the operation three more times, so that we get 4 wires in parallel, as in Photo 4.4.
We will gently stretch the wires and start the screwdriver at low speed, until we get the twisted cable as in Photo 4.5.
We will cut the ends of the cable (Photo 4.6).
We will obtain the multifilar cable as in Photo 4.7.
We will clean the ends of the wires with a sandpaper and then they can be soldered with tin to the pin of the chopper transformer as in Photo 4.8.
The successful realization of a chopper transformer means solving a difficult and important problem for a switching power supply.
Good luck!