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Hydraulic-Electric Analogies: Conditioners

Last month we introduced the conditioner, which is integrated into virtually every hydraulic and electrical system. This installment concludes the discussion on these important components.

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Conditioners are widely used in both electrical and hydraulic circuits. In electronics, they’re referred to as signal conditioners; in hydraulics, they’re called fluid conditioners. Fluid conditioners consist of filters, heat exchangers, and, sometimes, fluid heaters. These were described last month and compared to their electrical counterparts: electronic filters and heat sinks. No other fluid conditioners exist, but it’s a much different story for electronic signal conditioners.

The term signal conditioner, ingrained in electrical jargon, has no standardized definition. Countless devices can be, and are, called signal conditioners. Some more common devices and synonyms include amplifier, buffer, impedance matcher, clipper, clamper, rectifier, isolator, demodulator, modulator, pulse-width modulation, and on and on. Needless to say, not all will be covered in this introductory discussion on analogies. Though no analogous hydraulic device exists for the many electronic “signal conditioners,” we’ll discuss those that are in play today.

The Two Most Important Electronic Functions

In this context, electronic refers specifically to the myriad solid-state devices, such as radio and TV transmitters and computers. They use silicon and germanium elements that perform the functions. Rectification (unidirectional current) and amplification are the two basic functions from which all others emanate. Complete circuits are constructed by adding all the necessary external resistors, capacitors, inductors, switches, etc.

Diodes and rectifiers: Diodes are silicon-germanium devices (solid-state junctions) that pass current in one direction, but block it in the other. Thomas Edison discovered the diode effects in his quest for an electric lamp. Later, the phenomenon was used to create vacuum-tube diodes that provided rectification. They prevailed in electronic circuits until the invention of the transistor and the subsequent explosion in solid-state electronic technology.

3. Diodes conduct in the direction of the schematic arrow when using conventional current. The anode must be at a higher voltage than the cathode in order to conduct.

Edison noted that with a filament glowing in an evacuated bulb, a third conductor placed inside could be made to conduct current with applied voltage of one polarity, but blocked current in the other direction. It was a solution looking for a problem, which did not come until the invention of radio in the early 20th Century. The Edison diode required the high temperature (hundreds of degrees) of the bulb filament in order to conduct current.

The process is called themionic emission, wherein the electrons in the heated element are said to “boil off.” Therefore, they can be attracted to any additional conducting element that is positive with respect to the heated element. The vacuum-tube diode conducts when the heated element—the cathode—is negative, and the other unheated element—called the anode—is not.

The anode must be positive relative to the cathode for conduction to occur. In contrast, solid-state diodes conduct current in both directions at room temperature, making them infinitely more useful than hot vacuum tubes. A diode junction is nothing more than the intimate contact of two pieces of silicon, each with a different “doping agent” (deliberately added impurity) in each piece. The result is a diode. Diodes can be so small that it is possible to integrate millions of them onto one solid-state chip no larger than a fingernail.

The schematic symbol for a diode (with descriptions provided to explain operation) is shown in Fig. 3. If you recall, conventional current assumes that a fictitious positively charged particle moves around a circuit. Electron flow is the opposite, and electrons move in the opposite direction to conventional current.

4. Shown are volt-ampere characteristics of a typical diode for positive voltage difference. Diode bias voltage is inherent in the semiconductor junction and must be overcome before free flow can exist.


The apex of the diode arrowhead is the direction of conventional current free flow. It requires that the anode voltage be higher than the cathode voltage. Another way of saying it is that the voltage difference across the diode must be positive relative to the anode. The diode is said to be “forward biased” and will be in free flow if it exceeds the forward-bias voltage inherent in the silicon junction.

Figure 4 illustrates the typical volt-ampere characteristics of a diode with a conventional-current designation. It is normal in both silicon and germanium junctions that a small forward voltage must be applied to the anode in order to overcome the junction threshold. The amount of voltage depends on the semiconductor material. It is 0.5 to 0.7 V for silicon diodes, but between 0.1 and 0.3 V in a germanium diode.

Check Valves

5. An inline check valve uses a hollow poppet to conduct free flow. With no pressure on the inlet side, a very weak bias spring forces the poppet into a sealing seat with a positive metal-to-metal contact. It will open only when the pressure on the inlet side exceeds that on the outlet side, in addition to overcoming the bias spring.


The hydraulic check valve is analogous to the diode. The check valve lets fluid flow in one direction, but prevents flow in the opposite direction. The cutaway drawing in Fig. 5 shows the principles at work in a high-pressure hydraulic check valve.

6. The schematic symbol for a check valve is an implied ball that fits into a seat.

At very low pressures, a very light spring biases (pushes) the poppet closed to create a positive, metal-to-metal seal. When the pressure on the left side of the sketch acting on the end area of the poppet exceeds the spring force (bias), the poppet moves to the right (off the seat) to let fluid flow from left to right. The poppet shown has been hollowed out to allow for drilled passages from the poppet bevel into the interior chamber to carry flow in the check valve’s “forward direction.”

If pressure on the outlet side of the sketch is elevated, the pressure acts on the rear area of the poppet, forcing it more firmly into the metal-to-metal seat and thus assuring that there will be no flow from right to left in the drawing. Figure 6 shows the schematic symbol for a check valve, and mimics the actual function of the valve. It consists of a circle (shown as a ball in this case) that lifts off its seat and allows flow in the free-flow direction when the inlet is at a higher pressure than the outlet. Orientation is the same in both Figs. 5 and 6. Figure 5 shows the bias spring, which may or may not be shown in other schematics.

Figure 7 contains the pressure-flow characteristics of a typical check valve and shows the effects of the bias spring. The differential pressure must exceed the effects of the bias spring before free flow can occur in the forward direction. Once free flow starts, the weak bias spring allows the poppet to move to its full open position. Once this occurs, the check valve performs as a fixed-opening orifice.

7. The check valve will be in its free-flow condition any time pressure exceeds that required to overcome the spring bias.


Not all check valves require the bias spring. In cases where gravity helps seat the poppet in its closed position, the spring can be eliminated and free flow will occur whenever pressure exceeds zero.

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