Node protection PA and acoustics from overcurrent. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers
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Based on Audio Analogue Aida amplifier protection circuit - from the site of his designer Federico Paoletti, pi.infn.it/~federico/unofficial.htm. I developed it a little and used it in my new mobile amplifier - also borrowed from Paoletti. The circuit is applicable in any PA, the output of which is a composite emitter follower (from 2 stages or more, or one Darlington, naturally - silicon, or one CMOS stage).
In the original circuit, two optocouplers were added to the traditional protection circuit (the voltage drop across the emitter resistor opens the key that shunts the BE junction of the output composite transistor) (in the base shunt circuit for the upper and lower arms). The current drawn by the shunt from the base flows through the LED of the optocoupler and opens the galvanically isolated key. Fine, but the same node can not only autocratically turn off the amplifier, but also do it on command from the outside. You will need a couple more optocouplers that shunt the BE junctions of the output transistors:
(click to enlarge)
The operating threshold is set by the R1-R2 divider based on the current limit for each transistor and the resistance in the emitter-output circuit. The total resistance R1 + R2 should be in the range of 10..100 Ohm. Thanks to the R3-C1-R4-D1, R8-C2-R9-D2 (LPF) circuit, the DC response threshold is much higher than on the AF. In addition, the threshold drops with increasing temperature T1, T2.
The first and fourth (vertically) optocouplers are protection sensors (closed = failure), the second and third are keys controlled by an external signal (inflowing current = turning off the PA). A current-limiting resistor must be provided in the forced shutdown circuit. Resistors R9-10 - I put instead of jumpers on the board, are they needed as such - probably not.
Despite the additional voltage drop on the LED, the circuit is guaranteed to turn off the PA with a follower on single and dual Darlingtons (0.33 Ohm under the emitter). The main thing is that the current taken by T1, T2 from the "triangle" - the voltage amplifier - is sufficient to guarantee the inclusion of optocouplers (at least 1 mA for TLP521). Optocouplers - any except for Darlingtons (well, to hell with them, and you don’t need a lot of gain here). The required current transfer ratio is determined by the currents and levels of automatic protection, as well as the current that the shunt must take from the "delta". T1, T2 - any low-power with low saturation voltage.
By the way, one and a half volts on the LED, memorized by heart, is not entirely true. On the TLP521 LED in this circuit, the drop at room temperature is no more than 0.9V.
When designing the board, first we place the PA itself "in the mind", and the protection circuit - in the second place. In this case, the "ground" of the protection should be carried out separately and connected to other "earths" at a common point of the "star". If in a car amplifier the analog ground is current-decoupled from the on-board ground, and the automation unit usually sits on the on-board, then connect the R4D1 R9D2 circuits to the power analog, and the rebound ground to the on-board.
Publication: klausmobile.narod.ru
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