DIY

# High voltage power supply for LED fixture repair

#### High-voltage power supply unit for repairing LED lamps.

This work was carried out at the request of one electrician friend who was obliged, among other things, to repair, as far as possible, LED floodlights, lamps and just light bulbs. The power supply of our unit (hereinafter referred to as the PSU) is a 230 V network, an adjustable constant output voltage is 0 … 300 V (for more details, in the description of the circuit), an adjustment of the output current limitation is 20 … 350 mA (see ibid.).
How it is known that in electrical engineering there are two types of malfunction – the absence of contact where it is needed, and the presence of the contact where it is not needed. So in LED lamps, either the driver is faulty, or the LEDs themselves. About the driver some other time. And LEDs, as a rule, are a line of up to several dozen pieces, with a “burning” voltage of 100 … 150, and sometimes up to 250 V.

Here, the input transformer is purely isolation, the fuse, thermistor, bridge and smoothing capacitor are classics. The regulator itself is a potentiometer R2 with an emitter follower on a KT838 transistor. Capacitor C2 provides a smooth increase in the output voltage when turned on and additional voltage filtering, a thermal interrupter from the screwdriver's battery when overheating turns off the control circuit. There is no voltage stabilization, it is simply not needed, there is a current limitation and stabilization. Output voltages up to 300 V, which is of course excessive, will be slightly different in the final design. The current limiting and stabilization circuit consists of a current sensor – a resistor R4, a regulating and limiting resistors R3 and R5, and a shunt transistor VT1. The calculation of the resistor R4 is shown in the figure below.

To open the silicon transistor, about 0.7 V must be applied to the B-E junction. We set the minimum current limiting level to 20 mA, then 700 mV/20 mA = 35 Ohm. There is a 33 Ohm 5 W resistor available. Maximum current 0.35 A, which is enough for most luminaires, it will withstand (see fig.).
The scheme is complete. Here added Zener diode VD2 for emergency current limiting, chain – transistor VT2, resistor R7 and LED HL2 to indicate current limiting mode, as well as resistor C5-16MV 2 W-1R-1% for continuous current measurement. Output – terminals J2 and J3. When measuring the voltage across terminals J1 and J2 with a millivoltmeter, the reading will correspond to the current in milliamperes. A small change in the circuit – the neon is actually connected in parallel with the capacitor C1.
In the working version, we use the TAN106-220-50 transformer (well, I turned up this one!). Its main function is galvanic isolation from the 230 V mains. Switching of windings – by jumpers.
Construction. I note right away that the power supply unit is designed to be built into something like a stand, so a separate building is not provided.

Selection of equipment and materials.

Approximate layout.
< br> Pre-assembly, front panel manufacturing.

Here's what happened .
Improvement. When setting up and running the power supply unit, it turned out that the output voltage regulator R3 heats up noticeably. It will not take long to replace it with a higher-resistance one, but taking into account the “stupidity” of the KT838, as, indeed, of most HV transistors, we put it on the buildup of the KT940, of average power. At the same time, Ku grows at times. Add. resistor R2 in the BB circuit needs no comments.
< b> Result.
Supplement. A little about the elimination of light pulsation with a frequency of 100 Hz.
As a rule, the smoothing of the rectified supply voltage (current) in the luminaires is not enough, which, given the complete inertialessness of the LEDs, sometimes leads to a noticeable pulsation of the luminous flux. If in spotlights and street lamps it is uncritical, then there are norms for household lighting, and even light such as sunlight is best for the eyes. For good smoothing of the rectified voltage, high-voltage capacitors of large capacity are required, which take up a lot of space and are not very fond of heat. Therefore, capacitors are also installed at the output of the driver, which are more efficient, because operate at a higher frequency (this does not apply to capacitor drivers!). And they work at a lower voltage, i.e. with the same dimensions, their capacity can be made larger, sometimes by several times, with a corresponding decrease in ripple. But there is a subtlety – when the LEDs break (this is the most common defect), the voltage at the output of the driver, especially the capacitor, jumps to more than 300 V. after all it can and bang. Therefore, manufacturers put capacitors at the output of the driver with the same maximum permissible voltage as at the input – 350 … 400 V, but insufficient for normal filtering capacity. You can understand them, but when you do it “for yourself”, you somehow want to stifle this pulsation, which is very clearly visible through the cell phone camera. The output is a capacitor of maximum capacity for a voltage slightly higher than the voltage across the LEDs, and a protective powerful zener diode for a voltage somewhere between these two voltages. And what zener diode and where to get it? They certainly exist, and there are many of them. From domestic – D815 … D817, KS600 … KS680, the mass of imports. But if you only need one right now? Then we sculpt just such a scheme.
Zener diode at voltage rating as close as possible (from below) to the required limiting rating. The transistor is such that it would withstand the required voltage and current, for example, KT857, KT858, their analogs. It is undesirable to put a trimmer resistor, it is better to choose the value and put a constant one. So, setting up such “zener diodes” is the second function of our power supply unit. But this is the second version of the “zener diode”, on a direct conduction transistor and with a body on the ground (if there is one, of course).
Surely instead of this huge power supply circuit, you can add something from Chinese” cubes “with even better parameters, and also cheaper. If anyone can describe this it will be interesting.

That's all, comment!

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