In electronics, a clamp is a circuit to limit a voltage in a system by clipping it. A clamp circuit consists of a series resistor and one or more zener diodes or other devices that exhibit high resistance until their breakdown voltage is reached. Clamps are useful for protecting circuitry from an overvoltage, or limiting a higher voltage in a circuit to make it compatible with a system that uses a much lower voltage. A possible use of this would be converting the +/- 12vdc used by RS-232 serial ports to a 5 volt TTL-compatible level.
For a DC circuit,
+ o------\/\/\-+------------+
R | Output
input __|__|
| / \ zener
/___\ diode
|
- o------------+-------------
For an AC circuit, where current must be allowed to pass in either direction, two zener diodes are used 'back to back'.
In either case, the clamp will limit the voltage on the circuit to the reverse breakdown voltage of the diode. If two are used back to back, the voltage will be the sum of the forward and reverse breakdown voltages. MOVs or varistors can also be used in place of zener diodes.
The resistor is sized to allow only the needed amount of current to flow through the circuit, and the zener diodes must be rated for this amount of current.
Although the clamp is very useful for low-power logic signals, it wastes too much power for practical use as a voltage regulator above a few milliamps.
A precision voltage reference is usually based around a zener diode clamp.