11/24/2023 0 Comments Ac rf isolator![]() Q: That sounds safe, so what is there to worry about?Ī: The reality is that the AC ground is not always there as intended or assumed. Now, the dangerous current will flow through the case and then to ground, and the operator will be safe. The excess current will flow to the case, which is connected to Earth ground via the third wire of the AC connection. There is no galvanic path between the two sides, so the operating circuit is electrically isolated from the AC line.Ī: Assume there’s an internal power-line short circuit (such as from insulation rubbing and fraying) and the instrument case becomes “live,” thus putting the user at risk. ![]() Q: So why is line isolation needed, and how is it accomplished?Ī: Consider a standard line-operated product, which can be an appliance or a medical instrument, usually but not always using a transformer with a primary side and a secondary side, often at lower voltage via the step-down turns ratio. Fig 2: The term “ground” (left) for true Earth ground is often misused and conflated verbally and graphically with chassis ground (left) or common/signal ground (middle) there are distinctly different symbols for each. In fact, a battery-operated device such as a smartphone or even an airplane have no Earth ground. Note that “Earth ground” is not the same as “chassis ground” or “common” (signal ground), and there is a different symbol for each ( Figure 2), although the word “ground” is often causally used for all three. Fig 1: The code-specified wiring of standard AC plug and receptacle socket in the US has three wires: Hot (black), Neutral (white) and ground (green or even bare copper). (Why some items like decorative table lamps have two-wire plugs and do not need three-wire plugs – and that’s actually safe and OK – is explained later). Many do-it-yourself folks unknowingly swap the hot and neutral wires or have a broken ground connection – and things still work fine, that is, until a fault occurs. In standard house wiring, the ground wire (green) is often not insulated and left as bare exposed copper. Start with AC-line basics: A single-phase AC line has three wires: line (L), neutral (N), and ground, where ground is a true Earth connection and normally carries no current, Figure 1. First, let’s be clear what we mean by “ground,” the most misapplied and misused word in electronics. Q: I thought “grounding” and “ground” were supposed to remove AC-line shock risk?Ī: Ground is part of the solution and can be part of the problem when a fault occurs. However, if that currently has no return-flow path through the body, then there is no risk even if the person is touching a high-voltage line. The user is at risk if current, driven by the AC line’s potential, flows through the user’s body and back to its source. Q: What does isolation do in these cases?Ī: The answer starts with reviewing the first principles of electric-current flow and the paths this current flow takes. Q: Would this current flow occur during normal operation?Ī: No, it is strictly a fault-mode situation where isolation prevents a dangerous situation in the form of electrical shock in the event of certain types of faults and fault scenarios. Q: What is the objective of AC-line isolation?Ī: It is needed to prevent the undesired flow of current through a user, who can be someone operating an instrument or someone connected to it, such as in a medical situation. ![]() We’ll be looking only at the safety/failure perspective here. There are many solutions to signal isolation, but many of these do not apply to safety isolation. These two aspects sometimes overlap but often do not. In basic terms, if you connect an ohmmeter between points A and B, the reading will be near-infinite ohms.Ī: There are two primary reasons for needing isolation: 1) functional signal isolation, to ensure the overall circuit and system works properly during normal operation and 2) safety isolation, to ensure no danger to a user in case of occurrence of certain fault modes. Q: What is meant by isolation in circuits?Ī: In the electronics sense, isolation means there is no “ohmic” or “galvanic” path between two conductors, subcircuits, or subsystems. ![]() This FAQ discusses what AC-line isolation is, why it is needed, and how it is achieved. However, as with most topics, there’s more to isolation than its definition. “Isolation” is one of those terms which gets tossed around and is often cited as an almost universal, all-purpose cure for various ongoing or possible circuit and system problems – and it often is. Isolation from the AC line is essential for user safety in many situations and can be implemented using a special isolation transformer as well as other means.
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