Designers often test prototypes with a variable-voltage bench power supply. If they leave the bench for any length of time, someone may borrow the supply or otherwise change the voltage to a level that could damage components in the prototype.
This problem can be prevented by using a variable-voltage “crowbar” circuit connected across the power-supply terminals along with the prototype under test (see the figure). Such a circuit suits power supplies equipped with short-circuit-protection.
A multi-turn trimming potentiometer with screwdriver adjustment (P1) applies a fraction of the power-supply voltage to Zener diode Z1 through the 470-O, 1/8-W resistor. The Zener employed in this example circuit has a breakdown voltage of 3.9 V, which suits it for settings between 5 V and 12 V and use with a power supply with a 0- to 15-V range.
The user must first set the voltage at which the crowbar circuit should activate when needed. To do so, connect the crowbar to the supply without the prototype. Then set P1’s wiper to ground (point B). Switch on the power supply and adjust the supply voltage slightly higher than the level at which the prototype operates.
Next, trim the preset adjustment of the crowbar so the Zener diode conducts and the silicon-controlled rectifier (SCR1) fires. This action short-circuits the supply, creating the crowbar action. The current delivered by the supply will then be restricted to the preset level and a very small voltage will appear across the supply’s terminals.
The subsequent step is to lower the supply voltage and switch off the supply. Then connect the prototype and the crowbar circuit and switch on the supply and adjust it to the desired voltage. If this voltage is accidently increased, the SCR will fire and protect the prototype by removing the applied voltage. Capacitor C1, an electrolytic, avoids nuisance firing of the SCR caused by transients, although it also slows down the circuit’s response time a bit.
The SCR’s current rating should be higher than the short-circuit current delivered by the power supply. A heatsink may be used for the SCR. Because the trim pot is adjusted by a screwdriver, it isn’t likely to get disturbed accidentally.