[Amps] Solid State Relays Again

[Larry Benko]

Unfortunately you don't get the choice of when to switch off the SSR 
usually.  Triacs and SCRs only switch off when the current falls below 
some minimum hold current.  There are ways to force a turnoff by 
injecting reverse current etc. but this is not done in the SSRs we are 
talking about.

As far as the "transformer" is concerned, the current/voltage seen is a 
function of a lot more than just the transformer.  The load, filter 
capacitors, the small leakage inductance, etc. all affect the result.  
Many switching power supplies make use of this to minimize the switch 
size.  I have never seen a transformer circuit where the current that 
flowed after the input was opened exceeded the maximum current that 
flowed while powered.  However the voltage can rise to huge values.

Here would be my take on the demise of the SSR:
1.) A 40/50A SSR in a 20A circuit is not going to be damaged by current 
if a step start circuit is in place and working.  I also assume the SSR 
was adequately connected to a heat sink that is appropriate for the SSR..
2.) If the SSR worked for many on/off cycles you would have seen all the 
phase angles for turn on (assuming not a zero crossing device) and the 
turn off occurs at the same current every time by the nature of the 
triac/SSR.
3.) I am assuming that nothing in the amplifier was just recently 
changed and the amplifier did not suffer a fault on the last operational 
power cycle.

A reasonable guess would be a AC line transient caused the failure and 
MOVs would be a good choice for future protection.

On a different note, the quescent power of a relay is about 1 to 2W for 
the coil for a 20A relay independent of load current while an SSR is a 
device that drops a voltage that is in the order of a volt or a little 
more.  At 20A this means the SSR dissipates over 20W which is why they 
need to be heat sunk.

73,
Larry, W0QE


On 2/10/2014 8:50 PM, Roger (K8RI) wrote:
> On 2/10/2014 2:22 PM, TexasRF at aol.com wrote:
>> Additionally, the step start R will alter the phase relationship as the
>> load is primarily resistive when it is in the circuit.
>>
>> So, perhaps the more critical scenario is at switch off?
>
> Having worked with SCRs and Triacs I'd much rather switch on at peak 
> power than switch off while drawing current.  The Collapse of the 
> magnetic field can produce a tremendous voltage kick, or back EMF.
> It's my understanding that even transformers that are low voltage, 
> high current, if turned off while drawing much current can produce 
> many times their regular input voltage.
>
>
> 73
>
> Roger (K8RI)
>