This discussion deals with a simple technique for protecting PIC or Stamp based circuitry from permanent damage due to accidental connection of the +5V power lead to +12 VDC or to -12 VDC. This technique is useful for designs requiring 100 mA or less. However, it may adapted to designs requiring as high as 500 mA.
There is nothing more frustrating than being on the verge of a break through, only to accidentally connect your serial LCD panel to -12V. There is no thrill associated with the break through and perhaps a bit of heart ache as you realize your LCD will never work again.
This is all the more frustrating for me as a professor. Students might use all the care in world, but with 40 students sharing four LCD panels, you just know there is going to be more than one accident over the course of a semester.
Thus, our assembled serial PIC-n-LCD 20X4 LCD panel uses this simple technique to protect agaist permanent damage due to accidentally connecting the +5V power lead to +12V or -12V or reversing the +5V and ground. However, it is applicable to virtually any +5V design where the operating current is less than nominally 100 mA.
The design consists of a series positive temperature coefficient PTC thermistor and a shunt 5.6V zener diode (1W).
At 25 degrees C, the nominal resistance of the thermistor is 10 Ohms. Thus, for normal operation, +5V is connected to the unit and there is a minimal voltage drop across the PTC thermistor. For the PIC-n-LCD, this is about 20 mA and thus, the voltage actually seen by the PIC is the +5V less the 0.1V drop across the thermistor.
If, however, the power lead is accidentally connected to +12V, the shunt zener turns on leaving nominally 7.0 Volts across the series thermistor. Thus, the initial power dissipated by the thermistor is (V^2) / R_therm or about 5.0 W. The initial power dissipated by the zener 0.7 Amps * 5V = 3.5 Watts which is far in excess of its 1.0 W rating. However, this is brief.
The thermistor rapidly heats, the resistance rises and the current decreases and stabilizes at nominally 150 mA. Thus, the power dissipated by the zener diode falls to 0.75 Watts.
The important point is that the maximum voltage seen by the PIC and the associated circuitry is the 5.6 zener voltage which is well within the 7.0 Volt specification.
If the power lead is accidentally connected to -12V, the zener diode is forward biased (0.7V) and initially 1.1 Amps flows. Thus, initially, the power dissipated by the thermistor is 11 Watts. It rapidly heats, the resistance increases and the current stabilizes at nominally 150 mA.
Once the accidental connection of the power leads is recognized by the user and the power leads disconnected, the thermistor cools in about 10 seconds.
The PTC thermistor is a Keystone RL3006-10-110-12-PTF. It is available from Digikey as KC003R for $2.66 (single unit).
The zener diode is a 1N4734 5.6V 1W zener. (About 25 cents).
In the above, I have discussed this approach in the context of designs which normally require less than 100 mA. The limiting factor is the voltage drop across the thermistor.
For designs requiring higher operating currents, Digikey has similar thermistors;
Digikey Number Resistance Limiting at 25 degrees C Trouble Current KC001R 1.0 Ohm 650 mA KC014R 2.5 400 KC002R 5.0 250Note that when using these, either a 2W or 5W zener should be used.