I Device for Overload Protection of Galvanometric Chart Recorders David Lunney Department of Chemistry, East Carolina Uniuersity, Greenuille, N.C. 27834
GALVANOMETRIC CHART RECORDERS are still fairly widely used because of their rapid pen response, reliability, and simplicity. It is frequently desirable to protect their movements from damage by severe overloads, but if this is done with the usual diodes or zener diodes used to protect meter movements, the damping and pen response may be adversely affected. What is needed is a true current-limiting device which is insensitive to the voltage across the galvanometer coil. The circuit shown in Figure 1A has the desired characteristics. As long as the voltage drop ZmRsacross the sensing resistor R, is less than about 0.5 volt, the transistor is off and essentially all the current flows through the coil. When the drop exceeds about 0.5 volt, the transistor turns on and shunts the current around the coil. The diode prevents current from flowing back through the collector-base junction and damping the motion of the coil when it is being moved downscale by its spring. Since this was intended to be a student-proofing device and students frequently fail to observe the polarity of connections, a second transistor and diode were added to protect the coil in both directions (Figure 1B). We used R, = 470 ohms to protect a 1-mA movement; the transistors turn on at about 1.25 mA. There is no detectable scale compression at full scale deflection. There are several precautions that must be observed in applying this circuit: If overloads from low-impedance sources are anticipated, RL must be included to limit the transistor collector current to a safe value. R. = --*
maximum anticipated voltage ~maximum collector current
For protection against really severe overloads, the transistors should have high forward current gains at high collector currents, and low collector-emitter voltages in the saturation region. (The complementary pair 2N4401 and 2N4403 meet these requirements.) The effectiveness with which the device protects the movement can be determined by calculating bZm/3VTwhere I, is the current through the movement and VTis the total applied voltage, or by calculating bZm/bZTwhere IT is the total current through the protection circuit. These quantities are given by
where hfe is the forward current gain of the transistor and hie is its input impedance. Usually
OOUT
A
+
I
B Figure 1. Schematics for circuits A . Basic protection circuit E . Circuit for protection against overload currents in both directions
so
+
1 hte Rs -(T-) IT = hfe Since we want this quantity to be small, hfe should be large. bZm -
The voltage across the coil at full scale deflection should be at least about a volt. If the coil has a very low resistance, a series resistor should be introduced. The reason for this is that at overload, the voltage across the coil is equal to the collector-base voltage of the transistor plus the forward drop across the diode (about 0.6 V for silicon). For most galvanometric recorders, this is not a problem because a typical 1-mA movement has a resistance of about 1500 ohms, giving a voltage of about 1.5 volts at full scale. RECEIVED for review May 24, 1971. Accepted July 26, 1971.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
643
