A Guide to Switch Considerations - Current Switching

Current Switching

Applications for current switching include testing of power supplies, insulation resistance, capacitor leakage, resistivity of materials, batteries, and semiconductors. This section discusses current switching in general. Special techniques are required when switching high current or low current. Current switching applications may require connecting multiple current signals to a single meter, or may require connecting a single source to multiple loads. Each case has different switch configurations and concerns.

Connecting Multiple Current Signals to a Single Ammeter

Most current measuring applications require that all current paths be continuous, even when a particular current signal is not connected to the ammeter. To accomplish this, switch cards designed for current switching often use SPDT or Form C relays. As shown in Figure 8, when a channel is in an open state (i.e., not connected to the output), signal HI is connected to signal LO through the normally closed (NC) contact. In this case, only signal HI is switched. Signal LO is hardwired to the ammeter and to all other signal LOs. This is normally the case when testing components with relatively low currents (<1μA). When a channel is selected, the current source is connected to the ammeter through the normally open (NO) contact. Note that the current will be interrupted briefly when the Form C relay is actuated. This could cause problems when used with high speed logic or other circuits sensitive to a momentary break in the current flow. This problem can be overcome by using a switch card with a pair of Form A isolated switches to provide a make-before-break (make/break) connection.

Figure 8. Using Form C relays to switch multiple current sources

Figure 9 illustrates using a switch card with isolated Form A relays to maintain the current path. In this example, one relay (Ch. 1) is used to connect Current Source #1 to the ammeter. A second relay (Ch. 2) is kept in the closed position to provide a path when the current is not being measured. If using a card such as the Model 7013 Relay Switch Card in a Series 7000 mainframe or the Model 3740 Isolated Switch Card in the Model 3706A mainframe, Ch. 2 is normally energized. This relay is opened when the measurement is made, but not until after Ch. 1 is closed. This circuit is used when switching moderate-level current (milliamps to amps range) when the current cannot be interrupted even for a moment. Note that with this circuit, the LO terminals of the current sources are isolated from each other, unlike the circuit in Figure 8.

The Model 7053 High Current Card is another card that is often used to switch current. As shown in Figure 10, this card uses a dual Form C relay. The jumper ensures a continuous path, but it can be removed when using the card to switch voltage. Because this is a Form C relay, the current will be momentarily interrupted when the relay is energized or de-energized. An alternative to switching the current loop is to place a shunt resistor in the loop. The voltage drop across the resistor (RS) is then measured as shown in Figure 11. The voltage readings are then converted to current using Ohm’s Law. This technique can be used when the range of current is relatively narrow, such as approximately two decades. The resistor (Rs) should be no greater than the ratio of the maximum allowable voltage drop to the maximum current. Choose a value with a convenient scale factor, such as 1, 10, or 1000Ω, so that it will be easy to calculate the current. Once the value of the resistor is chosen, verify that the minimum voltage drop will not be so small that it will be difficult to measure with the voltmeter.

Figure 9. Using the Model 7013 Relay Card to switch current

Connecting a Current Source to Multiple Loads

Unlike switching current signals to an ammeter, switching a current source to multiple loads usually does not require maintaining the current path at all times.

For this case, a card with Form A switches is suitable, as shown in Figure 12. To connect the current source to Load #1, close relay Ch. 1. Ensure that Ch. 1 is opened before Ch. 2 is closed if the loads are capable of
storing energy, such as a battery or a large capacitor. Two high energy loads connected in parallel may damage the relays.

Often, cards designed for voltage switching can be used for current switching for currents from 1A to 1μA. For current less than 1μA, choose a card that has high isolation resistance (across open contacts, from high to low, and between channels) and low offset current.

Cold switching is always preferable to ensure maximum relay life. Cold switching involves turning the source current off (placing it on standby) before opening or closing a relay.

Figure 10. Using the Model 7053 High Current Card to switch current

Figure 11. Using shunt resistors to measure current

Figure 12. Connecting a current source to multiple loads

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