Electronics > Projects, Designs, and Technical Stuff

Solid State Relay question

(1/3) > >>

ricko_uk:
Hi,
I haven't used a SSR before and am looking at this one: http://www.crydom.com/en/products/catalog/pf-series-ac-pcb-mount.pdf

Three questions:

1) on page 1 it says: "Zero-crossing (resistive loads) or random-fire (inductive loads) output". Should it not be the opposite? i.e. Zero-crossing (inductive loads) or random-fire (resistive loads) output?

2) is it suitable for capacitive loads?

3) how does it work? Does it switch on the output at zero crossing whenever the LED is on?

Many thanks :)

Doctorandus_P:
You would want to switch the SSR when the current is (near) zero.
For a resistive load, the current and voltage are in phase, so detecting the zero crossings of the voltage is just fine.

For an inductive load you also want to switch at the point where the current is (near) zero, but there is a phase shift between voltage and current.

For a 600VA toroidal transformer project I once did just for fun, I had extremely large current surges If I switched it at the wrong moment. After I figured out which phase delay to trigger the solid state relay, I still had very high surges about half the time I turned the transformer on.

It turned out that the remanent magnetism in the turned off transformer was important enough to cause an extra phase delay. When I figured this out, I always turned the transformer off during a negative going flank, and turned it on at a positive going flank, and then the current surges stopped.

During development I've measured the full mains peak voltage (>300V) over a 1 ohm shunt resistor, which I found quite scary. The transformer also hummed loudly for a few seconds after such an event.

ricko_uk:
Thank you Doctorandus_P :)

TimNJ:
General rules of thumb for any type of switch (mechanical switches, relays, transistors):
1.) Inductive loads: High stress at turn off
2.) Capacitive loads: High stress at turn on

For capacitive loads, the stress is caused by inrush current. SSRs usually have a 1 cycle max current rating. You can determine the peak inrush current through calculation, measurement, or simulation. For a zero-crossing turn-on triac/SSR,I’d say there shouldn’t be much issue, unless you are really switching a very large capacitance.

ricko_uk:
Thank you Tim,


--- Quote from: TimNJ on July 26, 2020, 06:29:46 am ---You can determine the peak inrush current through calculation, measurement, or simulation.

--- End quote ---

How can I determine it through calculation, at least a rough/safe value? Would it be just through ohms law using the capacitor ESR as the only resistance? Would that be the peak inrush current value?


--- Quote from: TimNJ on July 26, 2020, 06:29:46 am ---For a zero-crossing turn-on triac/SSR,I’d say there shouldn’t be much issue, unless you are really switching a very large capacitance.

--- End quote ---

What would be a "very large capacitance"? I have 1,500uF up to 3,000uF (i.e. 1.5 to 3 Farads) :)

Many thanks :)

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod