Power supply with changeable polarity. Stabilized current source with changing polarity. Temperature controlled fan

How to change the polarity of the power supply?

UP-08

Most high-voltage power supplies use so-called voltage multipliers to create the required output voltage. The basic multiplier circuit is shown below in a simplified power supply circuit diagram:

The multiplier circuit consists of capacitors and diodes arranged in a certain order. The polarity at the output of the block is determined by the orientation of the diodes. In the example above, the diodes should produce a positive polarity to ground at the output. If you change the orientation of all diodes, the multiplier will produce a negative voltage with respect to ground.

The above example shows a two-stage half-wave multiplier that uses four diodes. Full-wave multiplier stages are more efficient, use extra capacitors and twice as many diodes. To create high voltages, such as in Spellman power supplies, a large number of multiplication stages are connected in series. A 12-stage full-wave multiplier will contain 48 diodes.

As a rule, capacitors and diodes used to assemble multipliers are soldered directly into one, and sometimes into several printed circuit boards. Often, in order to isolate from high voltages, such boards are enclosed in a shell - they are filled with a compound.

To simplify the procedure for changing the polarity to the opposite (as in an instance of the SL series), at voltages above 8 kV, a second multiplier is provided - “opposite polarity”. The process of replacing the multiplier is not difficult, all you need is a screwdriver and a few minutes of your time. Due to the simplified design of the modular units, they generally do not allow polarity reversal on site.

When designing industrial devices, which are subject to increased requirements for reliability, I have repeatedly encountered the problem of protecting the device from reverse polarity of the power connection. Even experienced installers sometimes manage to confuse the plus with the minus. Probably even more acute such problems are in the course of experiments of beginner electronics engineers. In this article, we will consider the simplest solutions to the problem - both traditional and rarely used in practice methods of protection.

The simplest solution that suggests itself on the go is to turn on a conventional semiconductor diode in series with the device.


Simple, cheap and cheerful, it would seem that what else is needed for happiness? However, this method has a very serious drawback - a large drop voltage across an open diode.


Here is a typical I-V curve for a direct-on-line diode. With a current of 2 amperes, the voltage drop will be approximately 0.85 volts. In the case of low voltage circuits of 5 volts and below, this is a very significant loss. For higher voltages, such a drop plays a lesser role, but there is another unpleasant factor. In circuits with high current consumption, a very significant power will be dissipated on the diode. So for the case shown in the top picture, we get:
0.85V x 2A = 1.7W.
The power dissipated on the diode is already too much for such a case and it will noticeably warm up!
However, if you are ready to part with a little more money, then you can use a Schottky diode, which has a lower drop voltage.


Here is a typical IV for a Schottky diode. Let's calculate the dissipated power for this case.
0.55V x 2A = 1.1W
Already somewhat better. But what to do if your device consumes even more serious current?
Sometimes reversed diodes are placed in parallel with the device, which should burn out if the supply voltage is mixed up and lead to a short circuit. In this case, your device will most likely suffer a minimum of damage, but the power supply may fail, not to mention the fact that the protective diode itself will have to be replaced, and along with it, the tracks on the board may be damaged. In a word, this method is for extreme sportsmen.
However, there is another somewhat more expensive, but very simple and devoid of the above drawbacks, a method of protection - using a field effect transistor. Over the past 10 years, the parameters of these semiconductor devices have improved dramatically, while the price, on the contrary, has fallen dramatically. Perhaps the fact that they are extremely rarely used to protect critical circuits from the wrong polarity of the power supply can be largely explained by the inertia of thinking. Consider the following diagram:


When power is applied, the voltage to the load passes through the protective diode. The drop on it is quite large - in our case, about a volt. However, as a result, a voltage exceeding the cutoff voltage is formed between the gate and the source of the transistor and the transistor opens. The source-drain resistance sharply decreases and the current begins to flow not through the diode, but through the open transistor.


Let's get down to specifics. For example, for the FQP47Z06 transistor, the typical channel resistance will be 0.026 ohms! It is easy to calculate that the power dissipated in this case on the transistor for our case will be only 25 milliwatts, and the voltage drop is close to zero!
When the polarity of the power supply is reversed, no current will flow in the circuit. Among the shortcomings of the circuit, perhaps it can be noted that such transistors do not have a very large breakdown voltage between the gate and the source, but by slightly complicating the circuit, it can be used to protect higher-voltage circuits.


I think it will not be difficult for readers to figure out how this scheme works.

Already after the publication of the article, a respected user in the comments cited a field-effect transistor-based protection circuit, which is used in the iPhone 4. I hope he does not mind if I supplement my post with his find.

The peculiarity of this circuit is that by turning the control knob, you can change not only the output voltage, but also its polarity. Adjustment is made in the range from +12V to -12V.

Power supply circuit with polarity adjustment

In fact, these are two separate voltage regulators - on the "plus" and on the "minus" with a common regulating resistor R5.
The transformer for the source is also required with a double winding.
When the slider of the resistor R5 is in the middle position, then both stabilizers are closed and the output voltage will be zero. When moving the engine in one direction or another, one of the adjustable stabilizers will open - either "plus" or "minus" and, accordingly, the output voltage will change.

The capacitances of capacitors C1 and C2 should not be less than 1000 uF. Instead of transistors KT816 and KT817, you can use more powerful ones - for example, KT818 and KT819. The power of the power supply itself directly depends on the power of the transformer used.
The transformer must have two output windings of at least 12 volts each.
Instead of the diode assembly KTs405, you can use four simple diodes connected by a bridge.