[AMRadio] Contactor


w1eof at hamnutz.com w1eof at hamnutz.com
Mon Sep 17 18:57:15 EDT 2007


If you put a relay in parallel with the plate switch on the Viking II it 
is not switching 600vdc. It switches the primary to the transformer. At 
least that's the way my PTT is wired in my Viking II.

A properly chosen relay can switch either load (110vac or 600vdc) all 
day long. The trick is to pick a proper relay. Every relay has contacts 
made for a particular purpose or load. Some can handle very very small 
voltages which believe it or not has it's own problems. Other switch 
many amps, or many thousands of volts. It's a matter of getting the 
correct part in the circuit.

73,

Mark W1EOF
Relay Designer



Geoff/W5OMR wrote:
> Edward B Richards wrote:
>> I thought that was why they put a diode across a relay coil. Can you put
>> a diode across the contacts?
>
> from 
> http://64.233.167.104/search?q=cache:5anM2XEYmcEJ:relays.tycoelectronics.com/appnotes/app_pdfs/13c3236.pdf+relay+contact+protection&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a 
>
> (or http://tinyurl.com/2zq7qf)
>
> -----------------------------
>
> _Contact Protection_
>
> Perhaps the most popular method of quenching an arc between separating 
> contacts is with an R-C network placed directly across the contacts. 
> As the contacts just begin to separate and an arc ignites, load 
> current feeding the arc will be shunted into the capacitor through the 
> series resistance, depriving the arc of some of its energy. As a result,
> arc duration will be shortened and [contact] material loss will be 
> minimized.
>
> (there's a section in there about what different contact materials 
> there are to be considered and what their properties are).
>
> Theoretically, the ideal arc suppression method would simply be a 
> capacitor placed directly across the contacts. However, with no 
> resistor in the circuit, when the contacts make, there is nothing to 
> limit capacitor discharge current. This nearly instantaneous discharge 
> current can generate a brief, but severe arc that may cause welded 
> contacts,
> depending on contact material and characteristics. Thus, the resistor 
> is necessary to limit capacitor discharge current. However, there is 
> one drawback. That is, the resistor tends to isolate the capacitor 
> from the very contacts the capacitor is supposed to protect. Because 
> of this, the amount of resistance should be kept as small as possible.
> Many relay users are unfamiliar with the selection of a capacitor for 
> arc quenching service. To begin with, AC differs from DC in that AC 
> crosses zero 120 times per second for 60 Hertz service while DC, of 
> course, is continuous current. In AC service, the capacitor need not 
> be as large as in DC service because the AC arc will extinguish at a 
> zero crossover point. In DC service, the capacitor must continue to 
> shunt load current sway from the contacts until the contacts separate 
> far enough apart for the arc to extinguish.
>
> --------------------------
>
> The site goes on and into detail about capacitor selection, with 
> formulas on how to figure such.
>
> Another section, worthy of quoting here, are practical applications:
>
>
> _Other Arc Suppression Methods_
>
> For quenching DC arcs in certain applications, relays are available 
> that have a permanent magnet located in close proximity to the 
> contacts. The magnet repels the DC arc, thereby stretching the arc and 
> causing it to extinguish quickly.  Some relay users connect a diode 
> across the inductive load to prevent countervoltage from reaching the 
> contacts. When the relay contacts open, the stored energy of the 
> inductance recirculates through the diode, not through the arc.While 
> this is an acceptable method of protecting the contacts, it does 
> result in lengthened hold-up time of the inductive load.  For those 
> applications that cannot tolerate lengthened hold-up time, a resistor 
> may be placed in series with the diode. The resistor does, however, 
> lessen the effectiveness of the diode and, usually, a compromise must 
> be reached by trial and error.
> By using a zener diode in place of the resistor, hold-up time is 
> greatly reduced. This is because the diodes cannot turn on until the 
> voltage across them equals the sum of their voltage drops.
>
> In some circuits, space is at a premium and there may not be 
> sufficient room for a zener and a regular diode. In such circuits, 
> some designers use a metal oxide varistor. The MOV performs in a 
> manner similar to back-to-back zener diodes. And, since the MOV is a 
> bidirectional device, it can be used in both AC and DC circuits.
> An added benefit of arc suppression is the minimization of EMI. An 
> unsuppressed arc between contacts is an excellent noise generator. 
> Such noise can be troublesome to sensitive components in a circuit, or 
> within the RFI field. In worst-case conditions, EMI can cause unwanted 
> turn-on of IC logic gates, SCRs, and triacs, and can cause damage to 
> other semiconductor devices.
>
> *Suggested capacitor types are metalized foil and film foil. Check 
> capacitor specifications for
> dv/dt and di/dt ratings
>
>



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