How to deal with common challenges in voltage swit

2022-08-17
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How to deal with the common challenges in voltage switches

to design an automatic test system, the switch needs to understand the characteristics of the signal to be switched and the test to be performed. For example, the most suitable switch cards and techniques to withstand switching voltage signals in test applications depend on the magnitude and impedance of the voltage involved

medium voltage switch

medium voltage applications (1V to 200V) usually need to switch a voltmeter or voltage source to multiple devices, such as test batteries, electrochemical batteries, circuit accessories, thermocouples, etc. Switching multiple power supplies and switching multiple loads have corresponding problems respectively

switch from one voltmeter to multiple series power supplies

Figure 1 shows the situation of switching voltmeters to multiple series 30 voltage sources (VS). In order to avoid a short circuit in one or more of them, one channel must be opened (disconnected before operation) before closing the other channel. In addition, fuse shall be connected in series to each voltage source to avoid exceeding the common mode rated voltage of the card. In this example, each power supply is 12V, and the total voltage of the whole series power supply is 360V. It is best to use a channel of at least 500V - the channel to see whether the software is connected to the rated voltage and common mode rated voltage of the machine

Figure 1: switch from a voltmeter to multiple series power supplies

switch from a voltage source to multiple loads

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

Figure 2: switch from one voltage source to multiple loads

switching resistance

when switching a voltage source to multiple devices, it may be necessary to compensate for the voltage drop caused by the switching resistance. In particular, if the device has a low resistance, the current flowing through the switch may produce a large voltage drop. In remote detection, the external detection circuit is connected across the load, which is helpful to correct all voltage drops on the switch and wiring

low voltage switch

when the signal level controlled by the switch is MV or even lower, the use of special technology helps to prevent voltage error. These errors may come from the thermoelectric offset voltage on the card or in the connecting line, switching film pollution, magnetic field interference or grounding loop

thermoelectric offset voltage

a key indicator of a low voltage card is its contact. The report points out that the potential, that is, thermoelectric offset voltage. Thermoelectric voltage is the voltage generated by the temperature difference on the nodes composed of different metals, such as nickel iron reed relay and the copper conductor they are connected to. This temperature gradient is mainly caused by the power consumption of the excitation coil. This offset voltage is directly superimposed on the signal voltage, which can be modeled as an unwanted voltage source in series with the target signal. The offset voltage will cause errors to the excitation applied by the device to be tested (DUT) or the results measured by the voltmeter

a variety of factors can affect the drift level of the card caused by thermoelectric voltage, including the type of relay used (reed, solid-state or electromechanical), coil drive technology (latch or non latch), and materials used for contact electroplating (such as nickel alloy or gold)

after the reed relay is energized, the power consumption on its coil will increase its temperature for several minutes, so it is very important to complete the low-voltage measurement within a few seconds after the contact is closed. If many measurements are made within a few minutes of closing, the increasing thermoelectric voltage will be added to the reading. The size of the thermal time constant can range from a few seconds to a few hours. Even if the solid-state relay has no coil loss, the heat generated by the internal IR voltage drop will still produce thermoelectric drift. The latching relay is excited by current pulses, so it has very low thermoelectric drift

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

when switching low voltage and measuring low resistance, offset compensation can be used to offset thermoelectric offset voltage, which requires two different current values for two voltage measurements. Divide the difference between the two voltage measurement results by the difference between the two test currents, and then the computer can calculate the resistance value:

switch film pollution

with the extension of time, a layer of pollution film will be formed on the surface of the contact point of the relay, thereby increasing its resistance, which will make the switch voltage unstable under the condition of low voltage measurement or power supply.> The voltage of 100mV is usually not affected by this pollution. Solid state switch scan card can prevent this problem

magnetic interference

the high-speed change of magnetic flux, such as the switching power supply or high current signal, will induce several microvolts in the adjacent low-voltage circuit, causing obvious errors. Magnetic interference can be minimized by separating noise sources from sensitive circuits as far as possible, shielding magnetic fields, using shielded twisted pairs, and reducing the limited area of noise sources and signal conductors

grounding loop

if there is a small potential difference between two grounding points, some sensitive parts of the system may generate a certain amount of ground current. This situation only occurs when some switches are closed for complex diagnosis. Whenever possible, try to maintain a system grounding point. If this cannot be achieved, the isolation technology based on optical coupling or balance transformer can be adopted to increase the effective resistance between the two points and reduce the common ground current to a negligible level

high voltage switch

insulation resistance test or voltage withstand test of cables and printed circuit boards usually involves high-voltage switch switching. In order to avoid damaging the switch card, you must be very careful when switching the voltage above 200V. You should choose the switch card whose rated index meets the required voltage and power, such as the giseley 3720 dual 1x30 multiplexer card for 3706 system switch/multimeter (as shown in Figure 3) and the cable with appropriate rated index. If feasible, the use of cold switching can prolong the service life of the relay and increase the allowable current

Figure 3: yoshiri 3706 system switch/multimeter

reactance load will cause excessive current and voltage jump. Therefore, in order to prevent damage to relays and external circuits, current surge limiting measures need to be taken for capacitive loads, and voltage clamping measures need to be taken for inductive loads

high impedance voltage switch

high impedance voltage switch needs to be used in applications such as monitoring electrochemical batteries and measuring semiconductor resistivity. Switching and measuring voltage sources with high internal impedance will encounter errors such as offset current, stray leakage current and electrostatic interference. The stability time can be extended by using parallel capacitor technology

but this item has little impact on the price (except for gate type). When selecting a switch card with high impedance voltage, ensure that the card has a low offset current. Any offset current flowing through a high impedance device will generate unwanted voltage on the device and add it to the voltage measurement

high impedance circuit is very sensitive to electrostatic interference, so both DUT and connecting line should be well shielded to prevent noise induction

the leakage current in the test instrument, switch card, cable and fixture will bring errors due to reducing the measurement voltage. Therefore, the switch card with high isolation resistance should be selected, the protective circuit should be used in all possible places, and the insulator with the highest insulation resistance should be selected as far as possible

response time is another key factor of concern when switching high impedance voltage signals. The parallel resistance in the switch and associated cables will cause additional response time. In some cases, using the excitation protection circuit can greatly eliminate the shunt capacitance, so that the shielding layer of the cable and its central conductor (or high impedance lead) maintain almost the same potential. Figure 4A shows a high impedance voltage case with an electrostatic voltmeter connected by a switch. Note its slow response to the step function. To protect this signal, a connection can be set between the protection output end of the electrometer and the shielding end of the card, as shown in Figure 4B. Some electrometers, such as Keithley's 6517b, can realize this connection internally by turning on the internal protection connection function. Turning on this protection function effectively reduces the cable and switching capacitance, thereby improving the response time of the electrometer

figure 4A: switch from high impedance voltage source to electrometer

figure 4B: using excitation protection circuit to offset parallel capacitance

if the protection voltage exceeds 30VDC, it must be based on the current development status of the domestic plastic machinery industry that there are cards with low independent innovation ability and less advanced and personalized special varieties than three-axis connection to ensure safety. Cards suitable for high impedance voltage switches include jishili 7158 card for 7000 series switch host (as shown in Figure 5) and 6522 card for 6517b electrometer (as shown in Figure 6)

figure 5

figure 6

for more details about the voltage switch in the automatic test system, please enter the free download of the switch Manual of jishili. (end)

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