How high can pull up/down resistances be? The value of a pull-up or pull-down resistor is limited by the leakage current of whatever it is pulling. For instance, say you have a 100K pull up resistor on a mosfet gate connection, and the mosfet has a gate leakage current specification of +-10uA. Your 100K resistor […]

A MOV is normally open circuit. If the voltage across it exceeds its nominal rated voltage it will very quickly clamp it by becoming very low impedance, shunting an overload through it for instance. When shunting a MOV is dissipating all of the energy across itself and therefore it’s often used in combination with a […]

PTC stands for Positive Temperature Coefficient and is simply another name for a thermistor. A PTC device will have a nominal resistance, for instance 1kohm, that it is normally at. If it gets hot then its resistance increases. PTC’s are often used to protect inputs for instance in multimeters but not they are no fast-acting. […]

Increasing voltage capability Place resistors in series. For example, 5x 200kohm 100V resistors is equivalent to 1 x 1Mohm 500V resistor. Using multiple lower voltage resistors is often cheaper than a single high voltage resistor. Also multiple resistors provides increased creepage distance against a single resistor of the same footprint (can be handy to avoid […]

Calculating any value for a potential divider Vin > R1 > Vout > R2 > 0V Vin = Vout * (R1 + R2) / R2          R1 = (Vin * R2 / Vout) – R2          R2 = Vout * R1 / (Vin – Vout)          […]

Potentiometer Input Impedance For AtoD Inputs This is based on the discussion found here. AtoD inputs will be specified with a maximum source resistance for them to function correctly to all of their specifications.  The source resistance of a pot is highest at its midpoint setting, but even at that position it is only 1/4 of […]