Crowbar Speaker Protection

Crowbar circuits are named for their function, which resembles dropping a crowbar across electrical terminals. They are employed only as a last resort and should only be used where the connected circuit is adequately fused or includes other protective measures. A crowbar circuit can be destructive; if the circuitry has a minor fault, it may result in a major fault once the crowbar has activated. It is common for the crowbar circuit itself to be destroyed, as its purpose is to protect attached devices, such as loudspeakers. The circuit consists of a resistor and capacitor that isolate the trigger circuit from normal AC signals. If there is sufficient DC voltage to activate the DIAC trigger, the capacitor discharges into the TRIAC's gate, turning it on instantly. A TRIAC has two primary states: on and off, with the transition state being so rapid that it can generally be disregarded. The BR100 DIAC (or the equivalent DB3 from ST Microelectronics) has a breakdown voltage ranging from 28 to 36V, making it unsuitable for supply voltages below approximately 40V, as this could create a false sense of security. The supply voltage must exceed the DIAC's breakdown voltage for it to conduct. Zener diodes cannot replace DIACs at lower voltages because a DIAC has a negative impedance characteristic, allowing it to dump nearly the entire charge in C1 into the TRIAC's gate, which is critical for triggering the TRIAC into conduction. The selected TRIAC is rated for 12A, with a peak non-repetitive current of 95A, sufficient to protect the circuit until a fuse or output transistor fails. A heatsink is advisable, although the TRIAC may fail if it is required to protect speakers. The 0.47-ohm resistor is included to prevent an absolute short circuit, slightly limiting the current and marginally increasing the likelihood of the TRIAC surviving. An alternative TRIAC, such as the BT139 rated at 16A continuous and 140A non-repetitive peak current, may be used. The peak short circuit current for a ±60V supply is typically around 90A, accounting for wiring resistance and the TRIAC's internal resistance. Such high currents can cause significant damage to any components in the discharge path and are not recommended for filter capacitors, although they may endure due to their non-repetitive nature. Construction is not critically demanding, but wiring must be of adequate gauge and secured with cable ties. The capacitor C1 should be polyester; while a non-polarized electrolytic capacitor might seem acceptable, it risks drying out over time, leading to loss of capacitance and potential false triggering of the crowbar during normal operation, which could damage the amplifier. All connections must be secure and well-soldered, as this circuit acts as a final safeguard for speakers and must remain inactive for extended periods. The crowbar circuit does not need to be housed within the amplifier chassis; it can be installed within the speaker cabinet. Excessive heat is only a concern during operation, and the primary goal is to protect the speakers from damage. It is critical to ensure that the crowbar circuit does not activate due to normal signals; for example, a high-level bass signal could trigger the circuit and potentially damage the amplifier. To confirm that no signal can inadvertently activate the crowbar, a small lamp can be temporarily substituted for R2, and the amplifier can be driven to maximum power with bass-heavy material, without connecting a speaker. If the lamp flashes, it indicates that the amplifier would have been damaged, suggesting an increase in the value of C1. Bipolar electrolytics should never be used for C1 due to their tendency to dry out and lose capacitance over time, potentially leading to false triggering of the circuit.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 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 (non-repetitive) is 95A, and it only needs to sustain that until the fuse (or an output transistor) blows.

A heatsink is preferred, but there is a good chance that the TRIAC will blow up if it has to protect your speakers, so it may not matter too much. The 0. 47 ohm resistor is simply to ensure that the short circuit isn`t absolute. This will limit the current a little, and increases the chance that the TRIAC will survive (albeit marginally).

Feel free to use a BT139 if it makes you feel better - these are rated at 16A continuous, and 140A non-repetitive peak current. The peak short circuit current will typically be about 90A for a ±60V supply, allowing ~0. 2 ohms for wiring resistance and the intrinsic internal resistance of the TRIAC, plus the equivalent series resistance of the filter capacitors.

That`s a seriously high current, and it will do an injury to anything that`s part of the discharge path. Such high currents are not advised for filter caps either, but being non-repetitive they will almost certainly survive.

Apart from the obvious requirement that you don`t make any mistakes, construction is not critical. Wiring needs to be of a reasonable gauge, and should be tied down with cable ties or similar. C1 must be polyester. While a non-polarised electrolytic would seem to be acceptable, the circuit will operate if the capacitor should dry out over the years. This means it will lose capacitance, and at some point, the crowbar may operate on normal programme material.

This would not be good, as it will blow up your amplifier! Make sure that all connections are secure and well soldered. Remember that this is the last chance for your speakers, so it needs to be able to remain inactive for years and years - hopefully it will never happen. The circuit doesn`t have to be mounted in the amplifier chassis - it can be installed in your speaker cabinet.

Nothing gets hot unless it operates, at which point no-one really cares - it just has to save the speakers from destruction once to have been worthwhile. Remember that the crowbar circuit absolutely must never be allowed to operate with any normal signal.

A perfectly good amplifier that triggers the circuit because of a high-level bass signal (for example) will very likely be seriously damaged if the crowbar activates. To verify that no signal can trigger it, you may want to (temporarily) use a small lamp in place of R2, and drive the amp to maximum power with bass-heavy material.

A speaker does not need to be connected. If the lamp flashes, your amp would have been damaged. If this occurs, you may want to increase the value of C1. Note that bipolar electrolytics should never be used for C1, because they can dry out and lose capacitance as they age. This could cause the circuit to false-trigger. 🔗 External reference