Keying a Relay From the TX Line
A station's 12 V antenna changeover relay has a 100 mA coil. The transceiver's TX line is an open-collector output that sinks current to ground during transmit, rated 50 mA and 30 V maximum. The builder wires it to the base of an NPN transistor (minimum hFE 100 at 100 mA) through a resistor, with the emitter to ground and the coil between +12 V and the collector, powered from the rig's 13.8 V accessory socket rated at 150 mA. Base drive is taken from a 5 V rail. After assembly the relay pulls in, but the transceiver display flickers on every transmission and the TX line transistor inside the rig later fails. There is no diode across the coil.
1. Choose a base resistor and check the whole interface against the transceiver's ratings.
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Collector current
I_C = 100 mA (the coil).
Base current for saturation
I_B(min) = I_C / hFE(min) = 100 mA / 100 = 1 mA. That is only the edge of saturation, so overdrive by 3-5 times: aim for 3-4 mA.
Resistor
With 5 V drive and V_BE about 0.8 V: R = (5 - 0.8) / 0.0035 = 1200 ohm -> use 1.2 kohm (about 3.5 mA).
Check against the TX line ratings
- The TX line sinks only 3.5 mA, far inside its 50 mA rating.
- The base-emitter junction clamps the TX line near 0.8 V, far inside its 30 V rating.
- The coil draws 100 mA from a 150 mA accessory socket, leaving only 50 mA of headroom. That margin is why supply-rail droop is plausible and why the coil should have its own supply.
2. Explain the flickering display and the failed TX line transistor, and state the fix.
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The mechanism
With no flyback diode, the transistor switching off interrupts 100 mA of coil current. The collapsing field produces V = L di/dt, which can drive the collector tens of volts above the 13.8 V rail - often past the transistor's V_CEO. Two things follow:
- The spike couples back into the 13.8 V rail shared with the transceiver, so the display flickers.
- The spike also reaches the rig's TX line through the collector-base capacitance and drives current back into the open-collector output, which is why that transistor failed.
The fix
- Fit a 1N4007 across the relay coil, cathode to +12 V, so it is reverse biased normally and conducts only on switch-off.
- Power the coil from a separate supply, or at least add decoupling on the accessory socket and keep the coil's return current out of the rig's ground path.
- Keep the TX line doing only what it was rated for: sinking a few milliamps of base current.
Principle
Every inductive load switched by a semiconductor needs a path for its stored energy; the diode is not optional, it is part of the circuit.
3. The builder substitutes a transistor with hFE of only 30. Predict the outcome.
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Available base current
I_B = (5 - 0.8) / 1200 = 3.5 mA
Maximum collector current the device can support in saturation
I_C = 30 x 3.5 mA = 105 mA
Prediction
That is only just above the 100 mA the coil needs, so the transistor is barely at the edge of saturation. Expect:
- V_CE of a volt or two rather than near zero, dissipating 0.1-0.2 W in a small package - noticeably warm.
- The coil voltage falling short of 12 V, so the relay may pull in weakly, chatter, or fail to operate when cold.
- Worse behaviour as the transistor heats and hFE falls further.
Options
- Reduce the base resistor (470 ohm gives about 9 mA, still well inside the 50 mA TX line rating).
- Use a device with higher hFE, a Darlington, or a logic-level MOSFET.
- Or choose a relay with a lower coil current so the whole interface has margin.