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The majority of speed drives for miniature trains consist of a transformer followed by a rectifier and a common wound rheostat. If such an assembly is satisfactory for a "toy" use, this is no longer the case for serious exploitation.
The realization of an electronic variator is necessary, but two schools confront each other: that of the DC voltage regulator, which we propose today, and that of the pulse width modulator, which will be the object of 'Another flash editing.
Whatever the advantages of one or the other solution, one fact is certain: they are both much better than the original solution, especially if, as in the case of our assembly, Acceleration and progressive slowdown very realistic.

The scheme of such a variator is not very original. After rectification and filtering, the available DC voltage is applied to the speed control potentiometer. The cursor of this latter attacks the input of an amplifier R-C consisting of P2 and C2. This cell prohibits any rapid variation of the input voltage of the amp and thus allows a progressive acceleration. When, after selecting a speed, the potentiometer is operated downwards, diode Di prevents the latter from acting on the input of the amp, which then sees its input voltage decrease only by the discharge of C2 through Of P3, which simulates a slow deceleration of the train.
The pusher P is an emergency brake which makes it possible to stop the train rapidly if necessary, but always with a certain inertia dependent on R1.
The operational amplifier is followed by a dlingling circuit consisting of T1 and T2, in order to be able to provide a sufficient output current. In addition, transistor T3 protects the circuit against short circuits by limiting the output current to 1 A with the values ​​of the elements visible in the diagram.

We have designed a printed circuit board that supports all components except the transformer, reversing gear, brake pusher and potentiometer P1.
The mounting does not present any particular difficulty. The transistor T2 is to be mounted on a radiator of sufficient size after interposition of the traditional isolation accessories. Its location on the printed circuit board facilitates this operation. If you want to make a high-end controller, you can exit on the front panel P2 and P3; You will then have the possibility of adjusting the inertia at acceleration by P2 and at deceleration by P3.
If the speed of the brake does not suit you, you can increase it by decreasing Ri or decreasing it by increasing
R1- If the output current limitation is not suitable for you, you can change it by changing the value of R2 and applying the relationship: R2 = 0.6 / I, where I is the maximum current expressed in amperes.
Be careful, however, if you exceed 1 A; The diodes D1 to D4 must then be changed for models that support the desired current (diodes of the 1N540X series for 3 A, for example).

IC1: LF351
T1, T3: BC107,108,109,547,548,549, 2N2222 2N2222A
T2: MJE 3055, TiP 3055
D1 to D4: 1N4002 to 1N4007
D5: 1N914 or 1N4148

Resistors 1/2 eu 1/4 WS 5%
R1: 22 R
R2: 0.68 R wound 3 W

C1: 1000 mF / 25 V
C2: 100mF 25V
C3: 2.2MF 25V

P1: 47 kR linear potentiometer
P2: adjustable potentiometer for C1 of 22 KR
P3: adjustable potentiometer for C1 of 470 KR
P: contact pushbutton by pressing
S1: switch 2 circuits 2 positions
TA: transformer 220 V-15 V, 1 A

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