Common Challenges in Designing Voltage Switches for Automated Test Systems

Designing switches for automated test systems requires understanding the characteristics of the signals to be switched and the tests to be performed. For example, the most appropriate switch card and technology to handle switching voltage signals in a test application depends on the magnitude and impedance of the voltages involved.

Medium voltage switch

Medium voltage applications (1V to 200V) often involve switching a single voltmeter or voltage source to multiple devices such as test cells, electrochemical cells, circuit accessories, thermocouples, etc. Switching multiple sources and switching multiple loads each present their own set of problems.

A voltmeter to switch multiple power supplies in series

Figure 1 shows the case of switching the voltmeter to multiple 30 volt voltage sources (VS) in series. To avoid shorting one or more of them, one channel must be opened before another (disconnected before operation). In addition, fuse each voltage source in series to avoid exceeding the common-mode voltage rating of the card. In this example, each supply is 12V, and the total voltage of the series supply is 360V. It is best to use a channel-to-channel voltage rating and common-mode voltage rating of at least 500V.

Figure 1. A voltmeter to multiple series power supplies switching [page]

Switching a voltage source to multiple loads

Figure 2 shows a single voltage source connected to multiple loads. If two or more loads are connected to the power source, the voltage across each load may be less than expected due to the current flowing through a common impedance (R), such as test leads and line resistance. As additional loads are connected, the total current will increase, increasing the voltage drop across the common impedance (R).

Figure 2. Switching of a voltage source to multiple loads

Switch resistance

When switching a voltage source to multiple devices, it may be necessary to compensate for the voltage drop caused by the switch resistance. In particular, if the devices have low resistance, the current flowing through the switch may cause a large voltage drop. In remote sensing, an external sense circuit is connected across the load, which helps correct for any voltage drops on the switches and wiring.

Low voltage switch

When the signal levels controlled by switches are millivolts or even lower, special techniques can help prevent voltage errors that may arise from thermal offset voltages on the card or in the connecting wires, switch membrane contamination, magnetic field interference, or ground loops.

Thermoelectric offset voltage

A key specification for a low voltage card is its contact potential, also known as the thermoelectric offset voltage. Thermoelectric voltage is the voltage generated by the temperature difference at a junction made of dissimilar metals, such as between a nickel-iron reed relay and the copper conductor to which they are connected. This temperature gradient is mainly caused by the power dissipated in the excitation coil. This offset voltage is directly superimposed on the signal voltage and can be modeled as an unwanted voltage source in series with the desired signal. The offset voltage will cause errors in the excitation applied to the device under test (DUT) or the results measured by the voltmeter.

Several factors affect the level of card drift caused by thermoelectric voltages, including the type of relay used (reed, solid-state, or electromechanical), the coil drive technology (latching or non-latching), and the material used for contact plating (for example, nickel alloy or gold).

After a reed relay is energized, the power dissipated in its coil will cause it to heat up for several minutes, so it is important to complete low voltage measurements within a few seconds of contact closure. If many measurements are taken within a few minutes of closure, a growing thermoelectric voltage will be added to the readings. The thermal time constant can vary from a few seconds to several hours. Even though a solid-state relay has no coil losses, the heat generated by the internal IR drop will still cause thermoelectric drift. Latching relays are excited by current pulses and therefore have very low thermoelectric drift.

The connection to the switch card is also a source of heating voltage. We should try to use untinned copper wire to connect the switch card and keep all leads at the same temperature. The offset voltage can be compensated by using a short-circuit channel to create a zero base value. However, this compensation method is not ideal because the offset voltage will change over time due to self-heating and changes in ambient temperature.

When switching low voltage and measuring low resistance at the same time, offset compensation can be used to offset the thermoelectric offset voltage. This requires two voltage measurements with two different current values. The difference between the two voltage measurements is divided by the difference between the two test currents to calculate the resistance value.

Switch membrane contamination

Over time, a layer of contamination will form on the surface of the relay contact, increasing its resistance, which will make the switching voltage unstable in low voltage measurement or power supply. Voltages >100mV are usually not affected by this contamination. Using a solid-state switch scanner card can prevent this problem.

Magnetic interference

High-speed changes in magnetic flux, such as those caused by switching power supplies or high-current signals, can induce voltages of several microvolts in adjacent low-voltage circuits, causing significant errors. Magnetic interference problems can be minimized by separating the noise source from the sensitive circuit as much as possible, shielding the magnetic field, using shielded twisted-pair wire, and reducing the limited area of ​​noise sources and signal conductors.

Ground loop

If there is a small potential difference between the two grounding points, some sensitive parts of the system may generate some ground current. This situation only occurs when certain switches are closed and complex diagnostics are performed. Whenever possible, try to maintain a single system grounding point. If this is not possible, isolation technology based on optical coupling or balanced transformers can be used to increase the effective resistance between the two points and reduce the common ground current to a negligible level.

High voltage switch

Insulation resistance testing or withstand voltage testing of cables and printed circuit boards often involves switching high voltages. To avoid damaging the switch card, caution must be exercised when switching voltages above 200V, and select a switch card rated for the required voltage and power, such as the Keithley 3720 Dual 1x30 Multiplexer Card for the 3706 System Switch/Multimeter (Figure 3), and appropriately rated cables. If feasible, cold switching can extend the life of the relay and increase the allowable current.

Figure 3. Keithley Model 3706 System Switch/Multimeter

Reactive loads can cause excessive current and voltage jumps, so in order to prevent damage to the relay and external circuits, current surge limiting measures are required for capacitive loads and voltage clamping measures are required for inductive loads. [page]

High Impedance Voltage Switch

High impedance voltage switches are needed in applications such as monitoring electrochemical cells, measuring semiconductor resistivity, etc. Switching and measuring voltage sources with high internal impedance can encounter errors such as offset current, stray leakage current, and electrostatic interference. Using shunt capacitor techniques can extend settling time.

When selecting a switch card for switching high impedance voltages, make sure the card has low offset current. Any offset current flowing through a high impedance device will produce an unwanted voltage across the device, adding to the voltage measurement.

High impedance circuits are very sensitive to electrostatic interference, so the DUT and connecting wires should be well shielded to prevent noise induction.

Leakage currents in test instruments, switch cards, cables, and fixtures can all introduce errors by reducing the measured voltage. Therefore, choose switch cards with higher isolation resistance, use protection circuits wherever possible, and select insulators with the highest insulation resistance possible.

Response time is another key concern when switching high impedance voltage signals. The parallel resistance in the switch and associated cabling can cause additional response time. In some cases, the use of an excitation guard circuit can largely eliminate the parallel capacitance, so that the cable shield is at nearly the same potential as its center conductor (or high impedance lead). Figure 4a shows a high impedance voltage connected to an electrostatic voltmeter through a switch. Note the slow response to the step function. To guard this signal, a connection can be made between the guard output of the electrometer and the shield of the card, as shown in Figure 4b. Some electrometers, such as Keithley's 6517B, can make this connection internally by turning on the internal guard connection feature. Turning on this guard feature effectively reduces the cable and switch capacitance, thereby improving the electrometer's response time.

Figure 4a. Switching of high impedance voltage source to electrometer

Figure 4b. Using an excitation protection circuit to offset the parallel capacitance

If the protection voltage exceeds 30VDC, a card based on triaxial connections must be used to ensure safety. Cards suitable for high impedance voltage switches include Keithley's Model 7158 card for the 7000 Series Switch Mainframe (see Figure 5) and the Model 6522 card for the 6517B Electrometer (see Figure 6).

Figure 5. Keithley Model 7001 Switch Mainframe

Figure 6. Switch Card for Keithley Model 6517B Electrometer/High Resistance Meter