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Most digital output magnetic sensors come with either an NPN "current sinking" or a PNP "current sourcing" output. If a sensor with an NPN output is shorted to the + supply voltage,or a sensor with a PNP output is shorted to Ground, it will destroy the output transistor. In a short circuit condition, there is nothing limiting the amount of current through the output transistor. Excessive current will surpass the output power capability of the transistor and it will be damaged.
Sensor Solutions has developed a current limiting feature that is now available for many of our Sensor Types. This feature turns off the output transistor when the current exceeds a pre-determined amount, thus saving the output transistor. The trade-off is that the saturation voltage (Vol for NPN and Voh for PNP) will be affected as the sensor is asked to sink or source more current.
When the NPN transistor is off, no current flows through the pull up resistor and the output (Vout) is high; approximately the + supply voltage (Vcc). When the transistor is turned on, it acts like a switch to ground and sinks current through the pull up resistor. This current causes the output to go low; approximately 0 volts (Ground). This pull up resistor limits the current through the NPN output transistor to approximately Vcc/Rpu.
When the output of a non-shorted sensor is low, the actual output voltage is not 0. This is referred to as the Vol, or Output Saturation Voltage Low. Most control systems will work fine with a Vol of less than 1.0 volts. The graphs below show a typical Vol vs. Output sink current:
These graphs show that Sensor Solutions short circuit protection circuit can sink up to 20mA and produce a Vol of less than 0.5 volts. As the circuit is required to sink greater amounts of current Vol will continue to increase up to a sink current of 60mAs, where the output transistor is turned off and Vol will approach Vcc. The sensor will protect itself, but should not be run continuously in this condition.
PNP Output Short Circuit Protection Output Characteristics
A sensor with an PNP output must be hooked up to a pull down resistor for the output to go low and high. The pull down resistor may be inside or outside the sensor as shown in the options below.
When the PNP transistor is off, no current flows through the pull down resistor and the output (Vout) is low; approximately 0 Volts (Ground). When the transistor is turned on, it acts like a switch to Vcc and sources current to the pull down resistor. This current causes the output to go high; approximately the + supply voltage (Vcc). This pull down resistor limits the current through the PNP output transistor to approximately Vcc/Rpd
In a short circuit condition, GND is connected directly to the output lead. When the sensor is trying to take the output high and the short is forcing it to GND the current limit kicks in. This current limit is set to between 40 to 60mA.
When the output of the non-shorted sensor is high, the actual output voltage is not the + supply voltage (Vcc). This is referred to as the Voh, or Output Saturation High Voltage. Most control system will work fine with a Voh that is more than 1 Volt below the + supply voltage (Vcc). The graphs below show a typical Voh vs. Output source current:
These graphs show that Sensor Solutions short circuit protection can source up to 20 mA and produce a Voh within 1 Volt of Vcc. As the circuit is required to source greater amounts of current Voh will continue to decrease until a source current of 60mA, where the output transistor is turned off and Voh will approach Ground. The sensor will protect itself but should not be run continuously in this condition.
Selecting A Sensor with Output Short Circuit Protection
Output short circuit protection is a feature in Sensor Solutions Load Dump protected electrical options. All sensors with the following electrical options have short circuit protection on the output(s).
If you need help finding a sensor with output short circuit protection, or you require a greater sourcing or sinking option, you can Contact us to discuss your options. Applications Engineers are available by phone Monday - Friday at 970-879-9900 and will respond to emails within 1 business day.