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Non-Zero Crossing AC SSR: Use with DC?
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TimNJ:
Hi all,

I'm looking for a relay to use in an SMPS inrush tester I am designing. After pushing a button (or driving the input with a square wave signal), the circuit fires the relay at the peak of the next AC cycle. The circuit uses a zero-crossing detector and then applies a delay to turn the relay on exactly at the peak of the AC wave.

I was planning on using an solid state relay (SSR) because I think it should have cleaner turn on/off characteristics. It's important that there isn't a crazy amount of ringing on the measurement.

Many SSRs are zero-crossing-switching, meaning they will only turn on or off at a zero-crossing. But, many manufacturers will also offer an instantaneous-switching version. Here is one I am considering: http://www.crydom.com/en/products/pcb-mount/ac-output/pf-series/

Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?

Thanks!
Tim

P.S. Current schematic attached, for reference.


MagicSmoker:

--- Quote from: TimNJ on October 20, 2019, 02:28:43 am ---...
Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?
...

--- End quote ---

If the SSR uses a triac or back-to-back SCRs you can't use it on DC unless you are fine with cycling the incoming power to turn it off (or load current drops below the holding current of the thyristor(s)). If the SSR uses MOSFETs for the output switch then you can use it on AC or DC (note that for AC there needs to be two MOSFETs wired in inverse series - that is, source to source).
TimNJ:

--- Quote from: MagicSmoker on October 20, 2019, 09:56:40 am ---
--- Quote from: TimNJ on October 20, 2019, 02:28:43 am ---...
Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?
...

--- End quote ---

If the SSR uses a triac or back-to-back SCRs you can't use it on DC unless you are fine with cycling the incoming power to turn it off (or load current drops below the holding current of the thyristor(s)). If the SSR uses MOSFETs for the output switch then you can use it on AC or DC (note that for AC there needs to be two MOSFETs wired in inverse series - that is, source to source).

--- End quote ---

Ah. Thanks. I knew they had back-to-back SCRs but I don't really understand SCRs very much, so that limitation didn't really make sense to me.

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!

It's only a test fixture, so maybe it's okay, but seems a bit crazy.

Thanks again.
MagicSmoker:

--- 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 ---

There's a couple of solutions to the minimum holding current issue that don't require wasting any heat in an external minimum load resistance. The first is to wire a decent quality film capacitor rated for use across the mains in parallel with the SMPS-under-test; 2.2uF should do the trick. The second is to simply provide a continuous trigger signal to the SSR for the entire time you need the load to be on.  ;)


EDIT - added the wink at the end.
TimNJ:
Oh...well that's interesting. Do I not understand how an SSRs is typically triggered? My plan was to provide continual 5V or 12V to the "coil" terminals just like a normal non-latching relay.

Regarding the capacitor, I thought of that, but thought I wouldn't want it because it would throw off the inrush measurement. However, I realize now that you can avoid measurement of the X-cap charging by simply measuring the current on the output of the tester, instead of on the input side. |O

Thanks once again.
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