[AMRadio] Vacuum or electronic relay

Jim Candela jcandela at prodigy.net
Sat Dec 2 15:52:56 EST 2006


 You are correct that the SSR has numerous applications and advantages over that of a mechanical relay (EMR). That said, the marketing data sheets usually ignore some of the disadvantages:

The on resistance of a dual SCR or single triac SSR is usually 1.5 volts. This means that if your load is 20 amperes RMS, the SSR dissipates 20 X 1.5 or 30 watts of heat, and the load sees line voltage minus 1.5 vac. The requirement to heatsink a SSR and losing voltage are not a big plus for the SSR.

If the forward current through the SSR drops to ~ .15 ampere, the SSR turns off for the duration of that 1/2 AC line cycle

If the SSR load has significant inductance, and the rectifiers are dumping stored leakage inductance every half line cycle resulting in back-emf spikes sent back to the SSR, the SSR may turn off again for the duration of that 1/2 line cycle. A R-C Snubber circuit can remedy this problem.

If the AC zero cross circuitry gets fooled because the load is highly reactive (voltage not in phase with current), then the SSR may fire at wrong point, and in effect act as a light dimmer (load does not see full 360 degree of AC line cycle). Solution here is to get SSR without zero cross circuitry.

I recall maintaining a fleet of Crown M600 audio amplifiers (600 watts RMS @ 8 ohms @ 20 Khz or 1340 watts RMS @ 4 ohms @ 1 Khz) back in the 1980's. These had a large heat sinked and fan cooled Crydom SSR. I always had SSR spares in my tool kit. The older Crowns had a EMR, and those suffered from contact pitting. A soft start turn on and EMR was in my opinion the solution that Crown chose to not do.

I like the idea of using a suitable SSR to key those 115 vac Dow Key relays. Do you have any models that you recommend?


----- Original Message ----
From: "DOXEMF at aol.com" <DOXEMF at aol.com>
To: Discussion of AM Radio in the Amateur Service <amradio at mailman.qth.net>
Sent: Saturday, December 2, 2006 1:39:49 PM
Subject: Re: [AMRadio] Vacuum or electronic relay

  I have used SSR blocks, 25-40 A @ 120-240V with 5 V turn on control with several multi Audio Amplifier rack installations 
with any difficulty. Easily a 40-50 A turn on surge or higher peak. RF filtering on the control lines is a must for transmitter use. Failure mode is ON so there should be a backup manual switch just in case, altho have never actually seen one short out in rated service. 
  I have used some of the smaller SSR relays to interface between a lower voltage or DC PTT or Antenna Relay switch line when using 120 volt Dow Keys. Works well. 

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