Some proportional valves are designed to use constant voltage with variable current. This concept is difficult for a technician to understand when first encountered. At first it would seem that this scenario is impossible because of Ohm's law. Recall that current increases as voltage is increased if resistance is constant. The way that these types of circuits operate is that they use a constant-voltage amplifier circuit. The amount of current is adjusted from 0-800 mA when the input signal changes from 0-10 volts. This type of circuit is easy to test and troubleshoot if one is aware of it. For instance, if one used a voltmeter to test the constant-voltage valve and amplifier, it would show maximum voltage (6 V dc) even when the valve is at rest and the input signal is at 0 volts. This would tend to confuse you, since the maximum voltage signal is generally associated with the maximum input signal. An ammeter would be needed to test this type of valve because one would notice that even though the voltage is constant, the current would change proportionately with the input voltage signal. When the current is at minimum, this type of valve has full voltage (6 volts). Notice that even though the valve has 6 volts applied, it wouldn't be activated because the voltage is only a potential at this point and there is no current. When the current is changed, the valve will change.
It's important that one doesn't use visual indicators, such as LED indicators or small lamps, to indicate when voltage is present because they will be illuminated all the time since the constant-voltage amplifier has voltage applied at all times and merely varies the current when the valve is adjusted.
