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A Guide to Switch Considerations - Voltage Switching

Published: 26th May 2021

Many different applications involve switching a voltmeter or voltage source to multiple devices, including testing batteries, electrochemical cells, circuit assemblies, thermocouples, etc.

The types of switch cards and the techniques used in these applications will depend on the magnitude and impedance of the voltages switched. The approximate level for low voltage switching is in the millivolt range or less, mid-range levels are from 1V to 200V, and voltages greater than 200V demand the use of high voltage switching methods.

Switching a Voltmeter to Multiple Sources in Series

Figure 1 illustrates switching a voltmeter to a series string of 30 batteries or voltage sources (VS). To avoid short-circuiting one or more of these sources, it is necessary to open a given channel before closing a second one (break-before-make operation). To guard against short-circuiting, add fuses in series with each voltage source to prevent damage to the switch card. Be sure not to exceed the common-mode rating of the switch card. In this example, each battery is 12V and the total voltage across the string is 360V. A channel-to-channel voltage rating and a common-mode voltage rating of at least 500V is desirable.

Figure 1. Switching a voltmeter to multiple sources in series

Switching a Voltage Source to Multiple Loads

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

Figure 2. Switching a voltage source to multiple loads

Switch Resistance

When switching a voltage source to multiple devices, it may become necessary to compensate for voltage drops due to switch resistance. In particular, if the devices have low resistance, the current flowing through the switches may cause a significant voltage drop. To prevent this problem, remote sensing can be used to correct for any voltage drops in switches and wiring. With remote sensing, external sense connections are made across the load. Therefore, the subsequent programmed output voltage will be the actual voltage across the load.

For example, Figure 3a shows a 5V source being switched to an integrated circuit (IC). The contact resistance for each switch is 1Ω. If the current drawn from the source is 500mA, the voltage drop across each switch will be 500mV, and the voltage at the integrated circuit will be reduced by a total of 1V. Operation of the IC will likely be unsatisfactory. Figure 3b shows a 5V source with remote sense. In this case, sense leads are also connected to the load. This will ensure that the actual voltage across the load will be 5V, and the IC will operate as intended. Note that the voltage at the source output terminals is 6V.

Do not use hot-switching with remote sensing since the voltage from the source may become excessive during switching.

Figure 3a. Voltage drops across the contact resistance cause improper results

Figure 3b. Using remote sense through a switch ensures the proper voltage delivered to the IC

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Content produced by Keithley, A Tektronix Company: Test and Measurement Equipment | Tektronix


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