This is a circuit found abroad. Two Darlington tubes are used to control a coil. Let's talk about two diodes D1 (surge protection tr3). What is the function of D2?
Kasia ポストする August 27, 2020
D1 and D2 protect TR1 and TR3. When TR1 and TR3 are cut off, the relay discharges through D1 and D2.
Lainie ポストする August 27, 2020
Personal opinion: D1, it can be seen, but generally it is not directly lead back to the positive end of the power supply, and D2 can not see how to protect it.
Lashay ポストする August 27, 2020
Disagree with the view upstairs
The effect of connecting a diode between 4A and 4B is different from the effect of two diodes in the figure. When the current is cut off, the drop rate of the relay coil current is different. A diode is connected between 4A and 4B, the current drops slowly, and the current "tails" longer; the freewheeling method of the two diodes in the picture is equivalent to applying 24V voltage to the coil in reverse, and the current drops faster.
