Electronic ignition unit for chainsaw

Encyclopedia of radio electronics and electrical engineering / Home, household, hobby

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Despite the widespread use of imported chainsaws, the population, especially in rural areas, still uses many domestic Druzhba and Ural devices. Both saws have a common drawback, which I also had to face - the fragility of the electronic ignition unit. This problem is not new - see the article by P. Ivanov "Repair of the ignition block of a chainsaw" in Radio, 2003, No. 2, p. 45. Currently, it is not difficult to buy a block, but it is expensive and does not last long. I decided to start developing my own design, which I bring to your attention.

Unlike the one mentioned above, the ignition block does not contain external elements and fits entirely into the original dimensions of the factory block. The board of the old unit must be removed.

The block diagram is shown in fig. 1. Generator coil L1, ignition coil (high voltage transformer) T1, capacitor C1, inductive ignition pulse sensor L2 and duralumin base are used from an old ignition unit. The remaining elements are newly introduced.

Rice. 1 Schematic diagram of the device

When the flywheel rotates, the generator coil L1 generates an alternating current, which, after being rectified by the diode bridge VD1-VD4, charges the capacitor C1. At a certain position of the flywheel, a short pulse of positive polarity appears at the terminals of the sensor coil L2, which, having passed through the diode VD5 and the current-limiting resistor R1, opens the trinistor VS1. Capacitor C1 is discharged through an open trinistor and the primary winding of the ignition coil T1. Its secondary winding generates a high voltage pulse, which is then applied to the spark plug.

On transistor VT1, resistor R2 and zener diode VD6, an amplitude limiter for the opening pulse is assembled. As long as the voltage at the control electrode of the trinistor VS1 does not exceed the stabilization voltage of the zener diode VD6, the transistor VT1 is closed and does not affect the control electrode circuit. When the zener diode VD6 is opened, current begins to flow through it and the resistor R2. A voltage appears on the resistor R2, which opens the transistor VT1, shunting the control electrode circuit of the trinistor VS1. As a result, the pulse amplitude is limited to about 4 V with the zener diode indicated in the diagram. This voltage is enough to confidently open the trinistor.

In order for the described design to fit into the dimensions of the factory ignition unit, it is necessary to modify the trinistor. To reduce its length, the threaded shank was shortened (1-2 threads were left), the cathode and control leads were also shortened to a length of 4...5 mm. Before shortening the lead, it must be compressed in two places near the body with side cutters with blunt cutting edges. Then, above these places, the output is bitten off and the cut is soldered with solder.

You can compress the output no closer than 2 mm from the body of the trinistor, otherwise the insulator will crack. This compression is needed to increase the contact area between the inner conductor coming from the crystal and the outer terminal of the trinistor.

The unit is mounted with a rigid copper wire with a diameter of 0,4-0,45 mm in vinyl insulation. Diodes VD1 - VD4 are assembled closely into a block and their leads are soldered in such a way that AC leads are obtained on one side of the block, and DC leads on the other. The transistor is fixed with a screw, which was used to secure the factory board. A petal is placed under the transistor, to which the leads connected to the case are soldered. Resistors, diode VD5 and zener diode VD6 are soldered on the terminals of the transistor VT1 by surface mounting.

Capacitor C1 is placed in the same place, a bridge VD1-VD4 is placed in the same compartment. The wires from the L1 coil to the bridge are flexible, of the same section. The wire going to the anode of the trinistor is soldered to its body. Before the compound is poured, the trinistor is held "on weight" on rigid wires in such a way that it does not protrude beyond the dimensions of the block, and a gap of about 2 mm remains between the trinistor case and the duralumin base of the generator. The assembled block, after checking for operability, is poured with an epoxy compound, making sure that the parts of all elements close to the edges and the body of the trinistor are covered with a layer of compound. After the compound hardens, the trinistor is rigidly fixed to the base of the generator. The view of the finished block is shown in fig. 2.

Rice. 2 View of the finished block

When installing the unit in a chainsaw, it may be necessary to adjust the ignition timing. In practice, more often it was necessary to set an earlier one relative to the factory mark. If you plan to install the block in the Ural chainsaw, before assembly, you need to remove the part of the landing ledge from the back of the base, painted in fig. 3 in blue, flush with the base plane. Under the remaining parts of the protrusion, when installing the block in place, it is advisable to place gaskets made of heat-insulating material, such as asbestos cardboard, with a thickness of not more than 0,5 mm. With a greater thickness, the flywheel may touch the block parts. Gaskets are needed because the design of the Ural chainsaw provides for the installation of an electronic ignition unit directly on the wall of the engine crankcase, which becomes very hot during prolonged operation.

Rice. 3 Reverse side of the part

In the above block, instead of those indicated in the diagram, you can use diodes KD105G, KD209 with any letter index, as well as others suitable in size with a reverse voltage of at least 400 V and an average forward current of at least 0,3 A. Replace the KS133A zener diode with KS 139, KS 147, KS 156 with letter indices A, B, G or their imported analogues, provided that the sum of the stabilization voltage of the zener diode VD6 and the voltage of the IBE transistor VT1 does not exceed the allowable voltage at the control output of the trinistor. Trinistor KU202N can be replaced with KU202M, KU205V, KU205G. SCRs in a plastic case should not be used due to their insufficient resistance to overheating.

In conclusion, I will add that according to the description presented, more than 20 blocks were assembled and they work for a long time and reliably. The block assembled by me and installed 6 years ago on my chainsaw has never failed.

Author: A. Karpov, p. Imissskoe Krasnoyarsk Territory; Publication: radioradar.net

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