Note: If any of the tests fail, stop further testing, set the power supply to 5 volts, and go to the Troubleshooting section below.

Open up F1 by removing the fuse. This disconnects the Protected +5 line from the +5 power. Connect an adjustable power supply to the +5 and GND attachment points on the left-hand side of the board. Set the voltage to 5v and the current (if adjustable) to 1 amp, and power up the device.

If everything was connected correctly, nothing should happen, meaning no smoke and your power supply should not go into shutdown or anything like that. Place a voltmeter across the two fuse clips with the positive probe on the upper fuse clip. You should not see a voltage present.

Now, adjust the voltage up past the trigger point until LED1 illuminates – this indicates the crowbar circuit has been activated. This should be around the calculated trigger voltage. At this point, you should see a voltage present between the fuse points, indicating that the SCR is trying to bridge the Protected +5 line and GND. Remove power from the device.

If you want to perform complete testing of the device, you need to be willing to sacrifice a 2A fuse or two. Your power supply should also be able to supply at least 5 amps. Anything lower, and the fuse may take longer to blow.

Install the fuse, set the power supply back to 5 volts and set the current to 5 amps, and reapply power to the device. Using a meter, you should be able to see the supply voltage present between the Protected +5 connection on the right-hand side of the board and GND, indicating that power is being delivered through the fuse.

Now, adjust the voltage up toward the trigger point again. When the crowbar circuit kicks in it will once again try to bridge the Protected +5 line and GND, maximizing the current draw across the fuse, causing it to break. If the supply amperage is high enough, the fuse will break very quickly and the LED will again illuminate indicating that the circuit has been activated. If the supply is not large enough, the fuse may start to glow and may glow brightly for a little while before it eventually breaks. Note that if this happens, power will still be removed from the protected side of the circuit because the SCR will be bridging it.

An example of the device being tested with a load can be found here.

The remote trigger, available via the Trigger connection on the right-hand side of the board, can be used to remotely activate the crowbar circuit. This is an active high input and must be brought to 2.0 volts or higher in order to activate it. It is important to note that the remote trigger is not latching – as soon as you remove the remote trigger signal, the crowbar circuit returns to normal operation.

The remote trigger can be tested by setting the power supply back to 5 volts and reapplying power to the device. Using a jumper, connect the Trigger input to +5 and the crowbar circuit should activate, illuminating the Activated LED. Remove the jumper and the circuit should return to normal operation.

The remote trigger input should not exceed 5v. It does not track VCC.

First thing to do is to double-check your soldering work again. Look for any bridged connections, cold solder joints, incomplete joints, and look at the parts side of the board to make sure that no excessive solder has flowed through and is causing a problem.

During the smoke test, if the Activated LED does not illuminate when the trigger voltage has been crossed, check for a voltage between the fuse connections. If a voltage is present, that means that the crowbar circuit is working, and the problem may be with the LED or its circuit (i.e. its current-limiting resistor).

If the Activated LED illuminates when the trigger voltage has been crossed but no voltage can be detected between the fuse connections, this means that the IC circuit is working, and the problem may be with the SCR or the fuse connections.

If the Activated LED does not illuminate when the trigger voltage has been crossed, and voltage is not present between the fuse connections, check that you have power present at the IC by metering between pins 1 (VCC) and 7 (VEE/GND). Be sure to measure at the pins of the IC and make sure its pins are correctly in the socket. If no power is present, double-check that all its pins are in the socket and none of them are are bent. If power is present, meter between pins 8 and 7 and check for a voltage present when you cross the trigger voltage, and when placing a jumper between +5 and the Trigger connection. If no voltage is present for either scenario, the IC may be bad. If a voltage is only present when Trigger and +5 are connected, there may be a problem with R1 and/or R2.