Electronics > Projects, Designs, and Technical Stuff
Non-Zero Crossing AC SSR: Use with DC?
james_s:
The zero crossing switching offered by some SSRs refers to the turn-on point, a zero cross SSR will always turn on at the zero cross. All AC SSRs will turn off at the zero cross, this is part of the standard behavior of a thyristor. Once triggered they latch on until the current through them falls below a minimum threshold.
TimNJ:
--- Quote from: james_s on October 21, 2019, 07:59:58 pm ---The zero crossing switching offered by some SSRs refers to the turn-on point, a zero cross SSR will always turn on at the zero cross. All AC SSRs will turn off at the zero cross, this is part of the standard behavior of a thyristor. Once triggered they latch on until the current through them falls below a minimum threshold.
--- End quote ---
Thanks. Yes, I just learned (re-learned?) that. Never worked on a project with SCRs before so didn't have that engrained in my memory yet!
I think I'm getting the itch to feature-creep in a DIY MOSFET based solid state relay. Seems like a MOSFET based relay would be superior in this application. Plus it'll work with DC.
Why are the high power MOSFET based SSR options so limited? Is there something I'm missing? Why are SCR based relays still king? Inherent zero current switching?
james_s:
SCRs (and triacs) are inexpensive, robust, easy to trigger, and they have a low forward drop so the dissipation is reasonable.
SSR's are typically used to control AC line powered loads from low voltage DC control systems. When one needs to control a largish DC load it's usually easier to just use low side switching with a single transistor or use a mechanical relay. The advantages offered by a SSR aren't really needed.
TimNJ:
--- Quote from: james_s on October 22, 2019, 03:28:29 am ---SCRs (and triacs) are inexpensive, robust, easy to trigger, and they have a low forward drop so the dissipation is reasonable.
SSR's are typically used to control AC line powered loads from low voltage DC control systems. When one needs to control a largish DC load it's usually easier to just use low side switching with a single transistor or use a mechanical relay. The advantages offered by a SSR aren't really needed.
--- End quote ---
Thanks. I see that the price of 2 MOSFETs with comparable combined voltage drop (at rated current) is about $10-15USD, whereas the cost of the Crydom PF module is about $30. Crydom part is obviously very thoroughly tested and easy to use, without much fuss. Need to add in isolated DC-DC converter, optocoupler, etc. for MOSFET version, so in the end, it seems like the price might come out about the same.
I can see how the latching action of an SCR could be advantageous in some applications.
In terms of robustness, are you just saying they are "generally" robust? It seems like MOSFETs are also pretty robust these days.
Circlotron:
--- Quote from: TimNJ on October 20, 2019, 02:04:02 pm ---On that note, another thing I noticed is the relatively high minimum/holding current rating for these devices. For the Crydom PF, it says 60mA minimum. I'd like to make sure the relay stays on even if the SMPS is in a light load condition. To satisfy 60mA @ 90VAC, need about a 1.5K resistor. But, since I'd like this to also be compatible with high AC line voltage (264V), then the dissipation of the 1.5K resistor @ 264V will be about 50W. Yikes!
--- End quote ---
You could always put a normal mechanical relay across the SSR and trigger them both at the same instant. The SSR will turn on almost immediately and the mechanical relay with its contacts across the SSR will follow several milliseconds later.
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