In the original scheme in the traditional circuit protection (voltage drop across the emitter resistor opens key bypass transition BE composite output transistor) added two optocoupler (circuit shunt base for the upper and lower arm). Current drawn from the shunt base flows through the LED of optocoupler and opens galvanically isolated key. Fine, but the same node can not just arbitrarily turn off power, but doing it on the pitch outside. Need another pair of optocouplers, shunt BE transitions output transistors:
Threshold set divider R1-R2 based on the current limit for each transistor and the emitter resistance in the circuit output. The total resistance R1 + R2 must be in the range 10 .. 100 ohms. Thanks circuit R3-C1-R4-D1, R8-C2-R9-D2 (LPF) threshold DC is much higher than the AF.Furthermore, the threshold decreases with increasing temperature T1, T2.
The first and fourth (vertical) optocouplers - security sensors (closed = alarm), second and third - the keys controlled by an external signal (sink current = off UM). In the chain of forced shutdown necessarily provide current-limiting resistor. Resistors R9-10 - I put instead of jumpers on the board, whether they are as such - probably not.
Despite the additional voltage drop across the LED, the scheme is guaranteed off the PA with a repeater on single and dual Darlington (0.33 ohms under the emitter). The main thing that current drawn T1, T2 in the "triangle" - the voltage amplifier - was sufficient to guarantee the inclusion of optocouplers (not less than 1 mA for TLP521). Optocouplers - any except Darlington (well, they go to hell, and do not need much gain here). Requires current transfer ratio is determined by currents and levels of automatic protection, as well as current shunt which will select from the "triangle". T1, T2 - any thin with low saturation voltage.
Incidentally, memorized by heart half volt LED on - not quite right. The LED TLP521 in this scheme falls at room temperature not more than 0.9V.
When designing the board first, "the mind" is placed own mind and protection circuit - second. In this "ground" defense to hold separate and connect with other "lands" at a common point "star." If a car amplifier analog ground unleashed current board of land and automation unit usually sits on the board, the circuit R4D1 R9D2 connect with analog power and ground otpronov - with on-board.
Original article sourse cxem.net
