Introduction
This is a simple crowbar circuit to protect your radio from damage if your power supply regulation fails and the voltage rises too far above the normal 13.8 volts DC set point.
Most ham radios (and many 12 volt appliances) are designed to operate on 13.8 volts +/- 15%. 13.8 volts is the nominal charging voltage from automotive electrical systems. the +/-15% works out to a minimum of 11.73 volts and a maximum of 15.87 volts.
If the voltage falls below 11.73 then your radio may shut off or begin to behave erratically, but low voltage does not typically cause damage. If the voltage goes above 15.87 there is a chance that your radio will be damaged. The higher the voltage, the greater the chance of damage.
A crowbar circuit monitors the power supply voltage. If the voltage goes above a preset value, the circuit creates a dead-short across the power supply. The circuit name comes from the fact that it acts like dropping a crowbar across the + and – power supply terminals. The short circuit is intended to blow a fuse or trip a circuit breaker to shutdown the power supply before it can damage whatever equipment it is powering. Therefore, in order for a crowbar circuit to be effective, the power supply must have an appropriate fuse or circuit breaker either on its output or input.
Some higher-quality power supplies have internal crowbar circuits to protect connected devices in the event of a regulation failure. Astron is one brand that have internal crowbars. There are others. Be cautious of very inexpensive supplies. There are many stories of radios being damaged by faulty power supplies.
MiniBar Description
The MiniBar is a classic crowbar circuit with a couple minor additions. The MiniBar includes a green output LED to show that power is available at the output. It also has a red, fault LED that lights when the built-in fuse has blown.
The default configuration of the circuit, shown above includes Anderson PowerPoles for the input and output connections. This allows the circuit to easily be inserted between a radio and power supply in a typical setup. If you choose to build your own version, you can use wires or any other terminals for the connections.
Theory
The MiniBar is a very simple circuit. Power comes into the circuit through J1. The crowbar circuit is designed for a nominal trip voltage of about 15.3 volts. The main components that control the crowbar are SCR, D1 and Zener diode, D2. To understand how the circuit works, it is important to understand how these two key components work.
D1 is a Silicon Controlled Rectifier (SCR). You may also see this device referred to as a thyristor. SCRs were among the earliest solid state devices. The schematic symbol looks like a diode with one extra lead. The extra lead is called a Gate. An SCR only conducts in one direction, just like a diode. The difference is that an SCR doesn’t conduct at all until a voltage and current are applied to the Gate lead that are above the threshold level for the device. Once an SCR is switched on, it keeps conducting regardless of what happens at the Gate lead until the current through the SCR drops below the minimum level needed to keep it on. Another characteristic of SCRs is that they are very robust devices. The SCR that I chose for this circuit is a small surface mount part. Even so, it is rated to withstand a peak surge current of almost 400 amps! This gives it plenty of capability to blow a 20 or 30 amp fuse sustaining any damage.
D2 is a Zener diode. Zener diodes conduct in the forward direction just like any diode. In the reverse direction, a Zener diode blocks current flow until the voltage across the diode reaches a specific value called the Zener Voltage. Once the voltage is above that, a Zener diode will try to hold the voltage across it to the Zener value. This makes them useful as a voltage regulator.
In our crowbar circuit, D2 is a 14 volt Zener diode. The diode is connect from Vout to V- through R1. R1 is there to limit the current through the diode. It is also there to pull the gate of D1 to ground or V-. More on that in a minute.
As long as Vout is less than 14 volts, D2 doesn’t conduct. No voltage or current is flowing through R1 and the gate of D1 is held at 0 volts. If Vout goes above 14 volts, then D2 begins to conduct. Remember that D2 wants to keep the voltage across it at exactly 14 volts. So the voltage at the junction of R1, D2 and D1’s gate will be: Vout – 14 volts.
D1 has a gate threshold voltage (VGT) of 1.3 volts and a gate threshold current (IGT) of 15ma. So, the gate of D1 won’t conduct until it sees at least 1.3 volts. Once it sees 1.3 volts, then it needs about 15ma of current to turn on the SCR. D2 connected directly to Vout can easily supply 15ma.
We calculated the voltage at D1’s gate above. So the trip voltage for this crowbar circuit is:
VGT(D1) + VZENER(D2) = 1.3 + 14 = 15.3 Volts
Note that there is some variability in the trip voltage. The selected Zener diode has a +/-2% accuracy and the gate threshold voltage of the SCR changes a little with temperature.
C1 provides just a little bit of filtering to prevent a noise spike from tripping the crowbar.
One of the nice features of a crowbar circuit is that it draws absolutely no power until it trips. Because the MiniBar includes a green, power on LED (D3), it does draw a small amount of power with no other load. R2 limits the current to about 4ma with a 13.8 volt supply. You could easily eliminate the LED if you wanted no current draw from the circuit.
The MiniBar also includes FAULT LED D4. If the crowbar trips and blows F1, and a load is connected at J2, the red FAULT LED will light. R3 limits the LED current (and current through the load) to less than 6.5ma with supply voltages as high as 20 volts.
Current handling
Normal current handling for this circuit is limited by the Powerpoles, the fuse holder and the conductors between them. The Powerpoles are rated for 45 amps. The fuse holder datasheet doesn’t give a current rating. So presumably it can handle the highest rated ATO or ATC fuse that is available. I believe this is 40 amps.
I have tested the PC board version of the circuit. The top and bottom planes on the board are made with 2 oz. copper instead of the usual 1 oz. With 15 amps of continuous current, the PCB got slightly warm, but not hot. At 20 amps continuous, the board got pretty hot after 1 minute. I believe the PC board version should be fine with up to 30 amps at a 50% duty cycle of 30 seconds transmit and 30 seconds receive. This should be more than adequate for a 100 watt radio.
Documentation
Schematic:
If you use KiCad you can download the complete KiCad V7.0 archive and make your own PC boards or modify the circuit as you wish: