Switch selection: AC and DC behave very differently

Rudy Ramos, Mouser Electronics

Contrary to popular belief, many people believe that the current rating of a switch is applicable to both alternating current (AC) and direct current (DC) and use this misconception when picking a switch for use in an application. However, as the ratings on the side of the switch shown in Figure 1 show, AC or DC circuits are capable of carrying very different currents.

Understanding the types of currents and switch ratings is essential to selecting the right switch for any given application.

Switching speed

The choice of available switches is huge although at the simplest level they all have an identical function; turning power to an electrical circuit on or off by opening or closing the circuit. The speed at which this connection is made / unmade is important and the speed requirement varies depending on whether the circuit uses AC or DC.

The difference between AC and DC is all in the name; with DC there is a constant current flowing in a single direction while with AC the direction of flow changes, as does the magnitude. To illustrate the effect of the differing current types on switching behavior, we should consider two separate circuits, one AC and the other DC, carrying the same current.

Switching off an AC circuit causes a voltage spark (sometimes known as an ‘arc’) inside the switch that, fortunately, dissipates quickly. Most AC is based upon a sine wave so this is only at its peak twice per cycle, and also is exactly zero twice per cycle – therefore there is a very high likelihood that the current will not be at peak levels when the switch is operated. In 50% of all cases, the power when the switch is operated will be less than half.

The situation is quite different with DC – as the voltage and current are constant (for a constant load) then the power is always at a peak level. As a result, when DC power is switched the arc can take significantly more time to dissipate. This time is affected by the speed of the switch – the faster the contacts move apart when turning off, the shorter the duration of the arc. The slower a switch operates, the longer the contacts are open but in close proximity, promoting arcing which is not desirable. Long term arcing can result in pitted contacts and in some cases can cause overheating, premature failure or, in extreme cases, fire.

Load types make things more complex

Alongside knowing the switching speed, understanding the nature of the load; particularly whether it is purely resistive or has some inductive aspects will affect the voltages and currents seen at switching and will have an important impact on whether any particular switch type will be suitable for a given application.

Devices such as ‘normal’ incandescent light bulbs or heaters are purely resistive and present very simple loads. Here, when the circuit is energized, the current rises rapidly to a steady-state value with no overshoot and the voltage remains constant, assuming that the load does not change.

Loads such as electric motors or transformers are highly inductive and, at the point of being switched on, will draw a significant instantaneous current, known as an ‘inrush current’ before returning to a steady-state current based on the load. Switching off an inductive load leads to a huge voltage being developed across the contacts. Known as a ‘back emf’, this voltage can lead to significant arcing which will cause pitting.

When selecting a switch, the peak values must be taken into consideration. Failing to do so will, at best, shorten the switch life considerably and may, in extreme cases, lead to immediate and catastrophic switch failure.

In general the DC voltage rating for a switch will be far lower than the AC voltage rating for a given current – a switch rated at 250 VAC at 15 A will only carry a 12 VDC rating at the same current of 15 A. Clearly, the situation is very different, based only on the type of current being carried by the circuit in question.

Certifications give confidence

When searching through catalogues or websites, there will inevitably be multiple switches that appear to be suitable for any given application. One differentiator is to choose a switch that carries an independent third-party certification that demonstrate that it has been tested under laboratory conditions to meet certain safety and performance standards. Two of the leading standards bodies with good reputations are US-based U (Underwriters Laboratories) and European / German VDE (Verband der Elektrotechnik, Elektronik und Informationstechnik).

The roles of the two organizations are slightly different. UL offers safety certification, testing, validation, inspection and audit as well as other training and advisory services. UL’s client list includes manufacturers, retailers, consumers and service companies, as well as to policymakers and regulators.

VDE is one of the largest technical and scientific organizations in Europe and is involved in a very wide range of safety-related and other issues.

When selecting a switch, the manufacturer will normally quote the relevant standards on the datasheet and the agency’s logotype will be printed somewhere on the switch. In the case of UL, designers can visit their website to verify that the approval is genuine and not applied to a counterfeit products. It is generally best to buy all products, especially safety certified products, from a manufacturer-approved source.

Switch selection – getting it right

With a little knowledge, switches can be chosen reliably and safely for any application. Understanding whether the current is DC or AC and whether the load is purely resistive or has inductive components will allow a correct (and safe) switch to be selected. Further assurances are available by selecting a safety agency approved switch.

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