[Boatanchors] Solid State Relays

[Brian Clarke]

Hello Dave,

A transformer in a power supply is not necessarily inductive. The load presented by the primary is that reflected from the secondary. If you use a:
*	choke-input filter after the diodes, during the 1st two or three mains cycles, the load presented by the primary 	will appear partly inductive
*	capacitor-input filter after the diodes, while the capacitors are charging, say, during the 1st two or three 	mains cycles, the load presented by the primary will appear partly capacitive. 
I say 'partly' because if the load is already connected when you switch on the power supply, the load presented by the primary will be resistive from switch-on.
Once all the filter capacitors are charged and there is a load on the secondary, the load presented by the primary is entirely resistive.

Actually, to be a purist, there will be a faint reactive component to account for magnetizing flux, iron and hysteresis 
losses. These will only be significant if there is no load, in which case only the inrush current will be a concern; these effects will be darn-near negligible at operating load.

However, because of the inrush current to charge the capacitors in a capacitor-input filter, you need to choose and specify a solid-state relay that can handle that instantaneous inrush load. Almost any book on power supply design will give you a graph from which you can calculate that inrush current. But, if you can't find one, get back to me with the details of your power supply and I can calculate the inrush for you. I've actually published a paper on power supply design that, in part, covers this issue.

Cheers es 73 de Brian, VK2GCE

On Tuesday, 13 August 2019 11:04 PM, you asked about Solid State Relays:

I'm planning a power supply which will switch transformer primaries.  
Have a bunch of "Solid State Relays" that are speced to handle the 
operating load, but I've never used these.  How are they switching an 
inductive load like a transformer primary?  Can they take the inductive 
"kick?"

73 Dave S.