- Electrical features of working with LEDs
- Point sources and matrices: selection, procurement
- Body and reflector parts
- Installation of radioelements
- Power supply question
Until LED products have finally entered our life and production has not fully become on the rails, the price of LED lighting equipment will continue to bite. But why wait or, even worse, overpay if you can assemble a searchlight of the required power with your own hands according to our instructions.
Electrical features of working with LEDs
If you intend to use LED technology, it will not hurt you to learn about some of the intricacies of working with it, which can partly be called disadvantages. On the one hand, LEDs are compact, economical and durable light sources, and on the other hand?
Solid state semiconductor elements are critically sensitive to high core temperatures. The phenomenon, called degradation, is the loss of dopants by the semiconductor, which is expressed in a decrease in luminous flux or final failure.
a) construction of a conventional LED: 1 – anode; 2 – cathode; 3 – conductor; 4 – crystal; 5 – plastic lens
b) construction of a powerful LED: 1 – body; 2 – conductor; 3 – heat sink; 4 – crystal; 5 – lens; 6 – cathode
At a temperature of 60 ° C, the LED degrades very quickly and the 50 thousand hours declared by the manufacturer eventually turn into 3-5 thousand. And the more powerful a single LED, the higher the likelihood of its rapid aging due to overheating. Therefore, in the development of lighting devices, a high-quality heat removal system, as well as dividing the emitter into several points and their correct layout.
Another feature of LEDs is that they can only transmit a limited number of electrons per unit of time. The network supplying the LED must be current stabilized, otherwise severe overheating and associated negative consequences occur. The supply current is regulated by the applied voltage and limited by a resistor on each of the LEDs. When developing a connection diagram, a careful calculation is needed: overestimate the voltage and the LEDs will quickly fail, and if you make it too low, they will shine half-heartedly.
Most simple floodlights have only one light-emitting element, while in high-power devices it is recommended to distribute the load for more efficient heat dissipation. In such cases, the connection can be serial, parallel, or mixed. The first is not entirely safe: if one of the LEDs burns out, it can either break the circuit or bypass it. With a parallel (and especially mixed) connection, there is a great risk that after one consumer is disconnected from the circuit, the current in the supply network will increase to unacceptable values.
Point sources and matrices: selection, procurement
There are three types of LEDs that are wise to use in floodlights. Please note that when assembling a lighting device from several LEDs, they must be identical in both type and volt-ampere characteristics. It is also recommended to purchase up to a dozen spare diodes as a repair kit and in case of damage during installation..
LEDs in the form of a plastic capsule with pins are suitable for the manufacture of small spotlights and flashlights. This is the cheapest type of product, and the final product will ultimately be relatively easy to repair..
The second type is ultra-bright white LEDs on a metal substrate. They should be used in high-power lighting devices, it is quite easy to remove heat from them..
Another type of LED is high power LED matrices. It is not recommended to independently make floodlights with a matrix power of 20 W and above: it will not be possible to effectively remove heat by simple measures.
Body and reflector parts
There are a number of solutions for the body of a homemade floodlight. If a high degree of dust and moisture protection is required for a street lamp, then a car headlight will do. The rim of the lamp base will need to be cut and secured over the LED array panel. The disadvantage of this method is the limited power of the searchlight, despite the fact that only one matrix will fit in it.
If you are placing multiple LEDs or arrays on a single PCB or mounting plate, the enclosure can be made from sheet metal or sheet steel. On the workpiece, mark the unfolding of the truncated pyramid: a square in the center and identical isosceles trapezoids on the sides. Do not forget to leave a “tongue” on one of the sides of each trapezoid for joining the petals together. Also, in the smaller base of the trapezoid, leave a rectangular strip of about 15–20 mm, and in the center of the square, cut another one with a side 20–25 mm smaller.
When the pattern is ready, sand the edges, bend the body and fasten the seams with rivets. Prime the inner surface, open it with white spray paint without gloss and leave to dry for 2-3 days. From the front side of the case, wind a square piece of glass of suitable dimensions diagonally and lean it against the folded shelves from the inside. Walk abundantly along the contour of the glass with white silicone, apply it to the seams of the case.
Fasten the mounting plate or board with eight 4 mm bolts, pre-drilling holes along the edges of each shelf on the narrow side of the case. Use a PVC foam door seal to keep the plate tight. It will be difficult to tighten the bolts, their heads are not accessible, so use a couple of lock nuts at the end.
Installation of radioelements
If you chose LEDs with pins, you will need a PCB plate to mount them. Think over the layout and draw the conductive tracks with a permanent marker. It is permissible to assemble the anodes of all LEDs (long tails) on one “ground” bus. The cathodes are also collected at one point, but a current-limiting resistor must be sequentially connected to the power supply circuit of each LED..
Its calculation is simple: subtract the LED voltage from the supply voltage and divide by the maximum permissible current. To play it safe in the event of fluctuations in the source voltage, the permissible LED current can be deliberately underestimated to 90–95% of the passport value.
An example of a circuit of an LED matrix of diodes with an operating voltage of 3 volts and an operating current of 20 mA
The approximate supply voltage for one LED is 4 V. If the source produces more, it is advisable to include diodes in a mixed circuit, where strings are connected in parallel, each of which has one LED for every 4–5 V voltage. The permissible current for such a series assembly is determined as the sum of the permissible currents of each, and the forward voltage remains the same, provided that this parameter is the same for each LED..
After placing the elements and drawing the tracks, etch the PCB plate in a solution of citric acid (30-50 g), 3% hydrogen peroxide (100 ml) and sodium chloride (2 teaspoons), periodically checking the degree of dissolution of the unprotected areas. Drill holes for the pins with a 1.5-2 mm drill, drill eight holes for fixing the board to the case, and then carefully plummet the live parts with rosin solder.
A circuit board can also be used for the LED matrix
If you assemble diodes or matrices on a cooling substrate, they are mounted in a hinged manner. Choose an aluminum comb-type radiator as the mounting panel. Each LED is attached using two or three holes, mark them all at once and drill from the back of the radiator with a 2.5 mm drill.
For fastening, use short self-tapping screws 3.5×11 mm for metal profiles, but without a drill at the end. Before fixing the diode, apply a small amount of KPT-8 thermal paste to the substrate.
The cathode (-) and anode (+) of LEDs with a substrate are marked, the connection diagram and calculation of protective resistors are the same for all types. The elements should be connected to each other by means of a piece of telephone wire. In order not to do unnecessary work, the anodes can be immediately soldered with short jumpers to the case of an aluminum radiator.
Power supply question
After assembling the LEDs, you will have two leads to which it would be nice to apply voltage, but where do you get it? Household power supplies are of little use here; to power the LEDs, an LED driver is needed that produces a pulsating constant current of a stable value.
For most products, a driver for interior lighting systems or for LED strips is suitable. It is better to purchase a power supply in advance in order to calculate the number and connection diagram of diodes according to the output voltage and the total stabilization current.
For small crafts, you can use general-purpose power supplies with an output ripple current of 0.5–1.5 A and a voltage of 3–5 V higher than the forward voltage of the diodes. The power supply can be stabilized with the LM317 microcircuit; for more powerful floodlights, use the LM350 and LM338, respectively, increasing the power of the source.
LED current stabilizer
The current limiting of the microcircuit can be adjusted by changing the resistance of the resistor. Its rating is defined as 1.25 / I, where I is the current of the LED or assembly.