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Active and Passive Isolators

Functional principle:

Alternating voltages and alternating currents can easily be transmitted and electrically isolated using transformers. Transformers are reliable, easy to produce and suitable for high working or isolation voltages.

However, they cannot transmit direct voltage signals. Therefore the direct voltage signal is first converted into an alternating voltage using an electronic chopper. This AC voltage is transmitted to the secondary circuit by a transformer, where it is rectified in sync with the chopper frequency (see Fig. 3). The resulting DC voltage is then converted or amplified if necessary.

dAvoiding interference caused by differences in potential

Figure 3: Block diagram of an isolator with transformer-based isolation

The modern, switchable isolation amplifiers of the VariTrans series or the new 6-mm class transmitters are used for signal processing based on a different principle. The input signal is converted to a rectangular signal with a constant frequency. The duty cycle of the rectangular voltage is changed depending on the input voltages (pulse width modulation, PWM). The pulse-width-modulated rectangular signal is transmitted to the output side with electrical isolation by means of a transformer. There it is reconverted into a voltage or current using a low-pass filter (see Fig. 4).

PWM principle: Pulse width modulation

Figure 4: PWM principle: Pulse width modulation

The transmission ratio of the isolation amplifier or transmitter is controlled by a microcontroller. The settings are made using DIP and rotary encoder switches. Since these switches are not incorporated in the negative feedback of amplifier circuits, but instead only switch digital signals, they do not carry currents and cannot cause any contact resistance faults.

1.1 Externally Powered Isolation Amplifiers (Active Isolators)

Isolation amplifiers are the most frequently used type of device for galvanic isolation of measurement signals. Often they are not only used as potential isolators but also as transmitters for converting voltages or currents into standardized 20 mA or 10 V signals. When measurement signals are transmitted 1:1, they are also used to increase the signal load capacity. Loading of the input signal by the isolation amplifier is generally negligible. Isolation amplifiers generally require an external power supply. The switchable isolation amplifiers VariTrans P 27000 and P 15000 are typical examples.

Isolation Amplifiers for Unipolar Signal Processing

Isolation amplifiers which are only suitable for transmitting unipolar measurement signals can be used for many applications, such as for the processing of standard 0/4 ... 20 mA and 0 ... 10 V signals. The control range of all Knick unipolar isolation amplifiers extends a few percent into the negative range for exact transmission even in the vicinity of zero (see Fig. 5).

Block diagram of an active unipolar isolation amplifier

Figure 5: Block diagram of an active unipolar isolation amplifier

Isolation Amplifiers for Bipolar Signal Processing

It is often necessary to process bipolar measurement signals, for example when motor currents have to be measured in both directions of rotation. Bipolar signals are also processed when distances are measured or for better resolution of measurement signals. Knick also supplies different types of bipolar isolation amplifiers, for example, the VariTrans A 26000 for bipolar standard signals (see Fig. 6).

Block diagram of an active bipolar isolation amplifier

Figure 6: Block diagram of an active bipolar isolation amplifier

1.2 Loop-Powered Isolators (Passive Isolators)

Potential isolation of impressed current signals does not necessarily require active isolation amplifiers. Loop-powered isolators can often also be used without limitations. The passive isolators from Knick do not need a power supply, the power is provided from the measurement signal at the input terminals as a voltage drop. The load capability of the input signal is reduced by the voltage requirement of the passive isolator.

Passive isolators are suitable for 1:1 transmission of unipolar current signals. The suitability for the respective application should be checked under consideration of the the input signal load capability and output load.

Example 1:

A transmitter with a 0 ... 20-mA signal at the loop-powered isolator input can support a maximum load of 10 V
(Iin = 0 ...20 mA, Vin max = 10 V). The voltage drop Vdrop of the loop-powered isolator is specified as 2.5 V. With Vin = Vdrop + (Iout · RL), the maximum output load is
RL max = (Vin max – Vdrop) / 20 mA = 375 ohms (see Fig. 7).

Circuit diagram for example 1

Figure 7: Circuit diagram for example 1

Loop-powered isolators do not allow for signal amplification and are not free from feedback, i.e. the output load acts directly on the input signal. As a result, a current cannot flow in the input circuit when the output is open (infinite resistance) (see Fig. 8).

Loop-powered isolator with open output

Figure 8: Loop-powered isolator with open output

Until now, cases where interruptions of the output circuit could not be precluded were handled by connecting a suitable zener diode in parallel with the isolator output. In this case the input current flows through the zener diode at the output when the output circuit is open (see Fig. 9). This approach frequently turned out to be complicated and error-prone in practice.

Transmission of a measurement signal over large distances

Figure 9: Transmission of a measurement signal over large distances

Knick has solved this problem with the Bürdenstop® (load stop) function, thereby significantly expanding the possible range of applications for loop-powered isolators. Here, the current supplied at the primary side is maintained independently from the output load. This allows for compensating any excessive load increases at the output, such as those caused by line breaks or inconstant loads including complex impedances. The passive IsoTrans M12-A200 and IsoTrans A 20400 isolators are available with and without load stop function. When the output load (e.g. input resistance of the controller) is up to 60 Ω, a passive isolator with load stop) function is the best solution (see Fig. 10).

Transfer function with load stop

Figure 10: Transfer function with load stop

The operating current required for the passive Knick isolators is very low. It is approx. 2 μA to 150 μA depending on the model without appearing as an additional transmission error. Passive isolators are particularly advantageous due to their easy installation without additional supply lines. Loop-powered isolators are available in all housing versions.

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