A Guide to Switch Considerations - Low Voltage & High Voltage Switching
Published: 26th May 2021
Low Voltage Switching
When switching low voltage signals (millivolts or less), special techniques must be used to avoid unwanted voltage errors. However, the required accuracy will determine if these techniques are necessary. These unwanted voltage errors may be due to thermoelectric offset voltage in the switch card and connecting cabling, switch film contamination, magnetic interference, and ground loops. For more information on low voltage measurements, refer to Keithley’s Low Level Measurements Handbook.
Thermoelectric Offset Voltage
The contact potential or thermoelectric offset voltage is the key specification of a switch card designed for low voltage switching. Thermoelectric voltage is the voltage generated by thermal differences at the junction of dissimilar metals, such as between the nickel-iron reed relays and the copper conductor to which they are connected. The temperature gradient is typically caused by the power dissipated by the energized coil. The offset voltage adds directly to the signal voltage and can be modeled as an unwanted voltage source in series with the intended signal. The offset voltage will cause an error in the applied stimulus to a device under test or the value measured by the voltmeter.
As shown in Figure 4a, the offset voltage (E) of a single pole relay is added into the circuit. As a result, the measured voltage (VM) will be the sum of the source voltage (Vs) and the offset voltage (E). To minimize this offset voltage, a low voltage switch card uses a two-pole relay, as shown in Figure 4b. Here, the offset voltage (ET1) in the circuit HI is largely canceled by the offset voltage (ET2) in circuit LO. The contact potential of some low voltage cards is specified with the card used as a two-pole switch to take advantage of this cancellation. It may be specified as “per contact pair.” However, total cancellation cannot be achieved because temperature differences will cause ET1 and ET2 to be slightly different.
The drift due to the thermoelectric voltage of a switch card depends on several factors, including the type of relays used (reed, solid-state, or electromechanical). The drift also depends on the coil drive technique (latching or non-latching). Finally, the material used for the contact plating (for example, nickel alloy or gold) also affects the thermoelectric voltage.
The power dissipated in the coil of the reed relay may cause the temperature to increase for several minutes after it is energized, so it is important to make low voltage measurements within a few seconds after contact closure. If many measurements are taken over several minutes after closure, a steadily increasing thermoelectric voltage will be added to the reading. Thermal time constants may range from seconds to hours. Even though solid-state relays have no coil dissipation, heat generated by internal IR drops can still produce thermoelectric drift. Latching relays use a pulse of current to actuate them, so they have very low thermoelectric drift.
Figure 4a. Potential in loop is added to measurement
Figure 4b. Canceling of potentials in loop
The connections to the switch card itself represent another source of thermally generated voltages. Wherever possible, make connections to the card with untinned copper wire, and keep all leads at the same temperature.
The thermoelectric offset voltage due to the switch card and the interconnecting cable may be compensated for by using a short-circuited channel to establish a zero reference. Connect a clean copper wire connected between the HI and LO terminals of an unused channel. Close the channel and measure the offset voltage. Then open this channel. This value can be subtracted from subsequent readings made on other channels. This approach is not ideal because the offset will change over time due to self-heating and changes in the ambient temperature.
When switching low voltages while making low resistance measurements, the thermoelectric offset voltages may be canceled by using offset compensation. This technique requires making two voltage measurements with two different values of current. To determine the resistance, the difference between the two resulting voltages is divided by the difference of the two test currents:
Switch Film Contamination
Over time, a contaminating film can form on the surface of a relay contact. This film can increase the relay contact resistance, which can make the switched voltages erratic when measuring or sourcing low voltage. Voltages greater than 100mV are usually sufficient to clear this contamination. Using scanner cards with solid state switches is one way to avoid this problem
Magnetic interference can be a problem in low voltage circuits. A high rate of change in magnetic flux, such as that produced by a switching power supply or by switching a high current signal on and off, can induce a pulse of many microvolts in an adjacent circuit. This can easily cause significant error in a low voltage circuit. This type of interference can be minimized by separating the noise source and the sensitive circuit as much as possible, by magnetic shielding, and by reducing the enclosed area of the noise source and signal conductors. Twist the HI and LO wires of each channel together to minimize the enclosed area.
Ground loops can easily occur in a complex test system. If a small potential difference exists between two ground points, some ground currents may flow through a sensitive part of the system. This may occur only when certain switches are closed, so it can be very difficult to diagnose. When possible, try to maintain a single system ground point. When this is not possible, isolation techniques using optical coupling or balanced transformers may help by increasing the effective resistance between the two points, thereby reducing the common ground current to a negligible level
High Voltage Switching
Some applications, such as testing insulation resistance of cables and printed circuit boards and high-pot testing, may require switching high voltages. To avoid switch card damage, be particularly careful when switching voltages of ~200V or higher.
Choose a card rated for the desired voltage and power levels. Cold switching, if feasible, will extend the relay life and make it possible to increase the allowable current. Be sure to use appropriately rated cables when switching high voltages.
Reactive loads can cause excessive current and voltage transients, so current surge limiting for capacitive loads and voltage clamping for inductive loads are required to prevent damage to the relays and external circuitry.
The surge current from a capacitive load is i = C dV/dt and must be limited to less than the rated current to protect the relays. Figure 5 shows a series resistance (R) used to limit the charging current. The resistor must be able to withstand the applied voltage; otherwise, the high voltage may arc across the resistor, damaging the device under test and the switch card. All components must be rated for peak voltage and current.
When determining the current limit for a reactive load, consider the maximum load in VA. For example, if the maximum load is 10VA and 500V is switched, then the current must be limited to 20mA. The series resistance is then calculated as:
Inductive reaction voltage (L(di/dt)) must be less than the scanner card’s maximum voltage rating. Figure 6 shows two typical clamping circuits, one using a diode for clamping DC voltages and the other using back-to-back zener diodes for clamping AC voltages.
Figure 5. Limiting capacitive reaction current
Figure 6. Limiting inductance reaction voltage
High Impedance Voltage Switching
High impedance voltage switching may be necessary in applications
such as monitoring electrochemical cells and measuring semiconductor
resistivity. Switching and measuring voltage sources with high internal impedance are subject to a number of errors, including offset currents, stray leakage paths, and electrostatic interference. Shunt capacitance may increase the settling time. This section discusses these error sources and provides an example application.
When choosing a card to switch high impedance voltage, make sure the card has a low offset current. Any offset current flowing through a high impedance device will cause an unwanted voltage to appear across the device. This offset voltage will be added to the voltage measurement.
High impedance circuitry is susceptible to electrostatic interference, so use shielding to avoid noise pickup. The device under test, as well as the connecting cables, should be well shielded.
Leakage paths can cause error by reducing the measured voltage. Such leakage paths may be present in the test instrument, switching cards, cables, and fixtures. To minimize errors due to such leakage paths, choose a switch card with high isolation resistance and use guarding wherever possible, including in the test fixturing and cabling. Also, select insulating materials with the highest possible insulation resistance.
Response time is another concern when switching high impedance voltage signals. Excessive response time may be caused by shunt capacitance, both in the switch itself and in the associated cables. In some cases, the shunt capacitance can be largely neutralized by the use of a driven guard, which will keep the shield of the cable at nearly the same potential as the center conductor (or high impedance lead) of the cable. Figure 7a shows a high impedance voltage connected through a switch to an electrometer voltmeter. Notice the slow response to a step function. To guard the signal, make a connection between the guard output (unity gain or preamp output) of the electrometer and the shield of the switch card, as shown in Figure 7b. Some electrometers, such as Keithley’s Model 6517B and Model 6514, can make this connection internally by enabling the internal guard connection. Enabling the guard effectively reduces the cable and switch capacitance, thereby improving the electrometer’s response time.
Switch cards appropriate for high impedance voltage switching include the Models 7158 and 6522. A card with triax connections is necessary if the guard voltage could exceed 30VDC. This precaution is necessary to ensure safety.
Figure 7a. Switching a high impedance voltage source to an electrometer
Figure 7b. Using a driven guard to neutralize shunt capacitance
A Guide to Switch Considerations - Signals Involving Reactive Loads
Switching circuits that include reactive elements need special care to avoid problems due to transient effects. The methods used to limit transient effects depend on whether the load is capacitive or inductive.