# Why do AC SSRs have lower AC Voltage limits and what does this mean?

I want to use something like a Crydom D2450 SSR to switch AC current supplied by a variable transformer. This device is spec'd at 24-280VAC on the output. Why is there a lower output voltage limit (24VAC)? My device seems to work as low as a couple volts out of the transformer.

• How do you spec "seems to"? Jan 29 '20 at 21:38
• Because the output must be some amount of AC for the internal SCR's to disengage. It might also use a tiny part of that power internally, such that <24VAC cannot be guaranteed to operate correctly in all possible conditions. It might work for a few volts for you, but this is not guaranteed. Jan 29 '20 at 21:43
• Zero cross detection. Jan 29 '20 at 21:53
• @Cary, it's polite to include a link to the datasheet. It saves us looking it up and ensures that we're all using the same one. Jan 29 '20 at 22:21
• Thank you all for your answers and comments. I apologize for not including a link to the datasheet. Feb 3 '20 at 21:55

Why is there a lower output voltage limit (24 VAC)?

Because some voltage is required to operate the triac.

simulate this circuit – Schematic created using CircuitLab

Figure 1. An instant turn-on SSR features an opto-triac and a regular triac.

TR1 requires some headroom to turn on. The SSR may work at lower voltages but the turn-on point will be late in the cycle because the voltage has to rise high enough for TR1 and TR2 to turn on.

Figure 2. Resultant waveform if supply voltage is too low. Image mine.

The same effect is seen at higher voltages but because the voltage is rising at a much faster rate the turn-on happens much earlier in the cycle and is barely noticible in the overall waveform.

Figure 3. The zero-cross SSR features circuitry to suppress turn-on if the voltage has risen above a certain level. Source: How opto-triacs and zero cross works.