Bill,
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?
Regards,
Jim
JKO
----- Original Message ----
From: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
To: Discussion of AM Radio in the Amateur Service <[email protected]>
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.
Bill,
KB3DKS/1
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