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 voltages (isolation voltages) when configured accordingly.
The transformers are not suitable for the transmission of direct current (DC) signals. The DC measuring signal is therefore 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) and then amplified or converted if necessary.
Another principle of signal processing is used in the modern, switchable isolation amplifiers from the VariTrans® series. The input signal is converted into 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 using a transformer (isolated) and then reconverted into a voltage or current using a low-pass filter (see Fig. 4).
The transmission ratio of the isolation amplifier is controlled by a microcontroller. The settings are made using DIL switches. Since these DIL 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 Active isolators (isolation amplifiers)
Isolation amplifiers are the best known type of devices for galvanic isolation of measuring signals. In addition to potential isolation, these devices can also be used as transmitters for signal conversion of voltages or currents into standardized 20 mA or 10 V signals. When measuring signals are transmitted 1:1, they are also used to increase the signal load capacity. The loading of the input signal by the isolation amplifier is generally negligible. Isolation amplifiers generally require an external power supply. Typical examples are the switchable VariTrans® P 27000 and P 15000 isolation amplifiers.
1.1.1 Isolation amplifiers for unipolar signal processing
Isolation amplifiers that are only suitable for transmitting unipolar measuring signals can be used for many applications, for example, for processing standard 0/4 ... 20 mA and 0 ... 10 V signals. For the exact transmission even in the vicinity of zero, however, the control range of the Knick unipolar isolation amplifiers extends a few percent into the negative range (see Fig. 5).
1.1.2 Isolation amplifiers for bipolar signal processing
Bipolar measuring signals frequently need to be processed when, for example, motor currents are to be measured in both directions of rotation. Bipolar signals are also processed when distances are measured or for better resolution of measuring signals. Knick also supplies different types of bipolar isolation amplifiers, for example, the VariTrans® A 26000 for bipolar standard signals (see Fig. 6).
1.2 Passive isolators
Active isolation amplifiers are absolutely necessary for electrical isolation of injected current signals. Passive 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 measuring signal at the input terminals as a voltage drop. The load capability of the input signal is reduced by the natural voltage requirement of the passive isolator. Passive isolators do not allow signal amplification and do not work isolated, i. e. the output load is directly on the input signal.
As a result, a current can no longer flow in the input current circuit when the output is open (endless resistance) (see Fig. 7). If temporary interruption of the output current circuit cannot be ruled out, the DC transformer output can be connected with a Zener diode that has a voltage value well below the maximum voltage transmitted by the DC transformer and above the actual load voltage required. When the output current circuit is open, the input current then flows via the Zener diode in the output of the passive isolator (see Fig. 8).
The operating current required to operate the devices is very low. It is approx. 2 µA to 100µA depending on the model without appearing as an additional transmission fault.
Passive isolators are suitable for 1:1 transmission of unipolar current signals. The suitability for the respective application should be checked taking the load capability of the input signal and output load into consideration (see Fig. 9).
Passive isolators are particularly advantageous due to the simple installation without additional supply lines.
Passive isolators are available as modular cases with up to 4 channels (e. g. IsoTrans® A 47 H1/4), solder-in modules and on Eurocards with up to 8-channels.