Crowbar Speaker Protection

Posted on Feb 5, 2014

Crowbar circuits are so-called because their operation is the equivalent of dropping a crowbar (large steel digging implement) across the terminals. It is only ever used as a last resort, and can only be used where the attached circuit is properly fused or incorporates other protective measures. A crowbar circuit is potentially destructive - if th

Crowbar Speaker Protection
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e circuitry only has a minor fault, it will be a major fault by the time a crowbar has done its job. It is not uncommon for the crowbar circuit to be destroyed as well - the purpose is to protect the device(s) attached to the circuit - in this case, a loudspeaker. There`s really nothing to it. A resistor / capacitor circuit isolates the trigger circuit from normal AC signals. Should there be enough DC to activate the DIAC trigger, the cap is discharged into the gate of the TRIAC, which instantly turns on. hard. A TRIAC has two basic states, on and off. The in-between state exists, but is so fast that it can be ignored for all intents and purposes. The BR100 DIAC (or the equivalent DB3 from ST Microelectronics) is rated for a breakdown voltage of between 28 and 36V - these are not precision devices. Needless to say, using the circuit with supply voltages less than around 40V is not recommended, as you will have a false sense of security. The supply voltage must be higher than the breakdown voltage of the DIAC, or it cannot conduct. Zeners cannot be used as a substitute for lower voltages - a DIAC has a negative impedance characteristic, so when it conducts, it will dump almost the full charge in C1 into the gate of the TRIAC. This is essential to make sure the TRIAC is switched into conduction. The TRIAC is a common type, and may be substituted if you know the specifications. It`s rated at 12A, but the peak current...

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