Nuts & Volts - April 2005

Q&A

capacitor across the contacts, the cap acts like a short circuit when the contacts open. At this point, the cap begins to charge using the formula t = RC, where R can be calculated from R = E/I, E being the voltage across the points and I being the current flowing at the time of the break.

If we can balance the charging time of the capacitor with the time it takes for the contacts to open wide enough that the voltage can’t jump the gap, the arc is suppressed. Typically, this is 0.1 mS for a frame relay of the type you describe. Plugging these values into our equations, we get R = 90 volts/1.5 amps = 60 ohms. Calculating for capacitance, we get C = t/R = 0.0001/60 = 1.67 µF.

If you’re trying to figure out the mathematics for an AC voltage suppressor, forget it. A sine wave is self-extinguishing because it crosses zero. That is, two times every cycle, the voltage across the contacts is zero as the voltage reverses itself from positive to negative. It’s very hard to maintain an arc under these conditions — as many a welder will attest to.

Relay Coil

MOV

Figure 2

EMF Suppression

same happens across the relay contact when connected to an inductive — like a motor described in the question “Relay Contact Life” — where this creates contact arcing. This is energy that must be dissipated somehow.

A diode placed across the inductor (or contacts) will conduct when the magnetic field collapses and will effectively short circuit the voltage. The decision whether to use a diode or not depends on the voltages and currents involved. A coil with higher inductance, like a frame relay, will store more energy — both voltage and current — than will a small coil, like a reed relay. As a rule of thumb, a diode should always be included — but other factors have to be considered, not the least of which is space. If the relay is small, space is limited, and Vcc is low, I often omit the diode.

Also, placing a diode across the inductor causes current to flow longer and lengthens the hold-in time of the relay. This is unac-

ceptable for some applications. By inserting a zener diode in series with the rectifier, hold-time is greatly reduced. This is because the diodes can’t conduct until the reverse voltage exceeds the zener voltage, plus 0.7 volts.

In some circuits, the diode(s) can

EMF Suppression

1uF

1k

Audio In

20W Power Amp

+12V

⁺LM1876

-12V

20k

1k

+

22uF

SPKR

Figure 3

QHow come you sometimes put a diode backward across the relay contacts and other times you don’t? I have read that the diode is a must. What’s the real story?

Mike Edwards Los Angeles, CA

AI think you’re talking about putting the diode across the relay coil, not the contacts, but they are both one and the same. When the current flow through an inductor — or coil — is interrupted, a high voltage in the reverse direction is generated by the collapsing magnetic field. This voltage can be as high as 10 times the Vcc source voltage.