Solid State Relay Load Minimum Voltage Range - Page 1
Forum section: electronics / projects

author: FoxesRule
author_rank: Newbie
timestamp: November 06, 2023, 10:55:39 am
content: Hi everyoneI am confused with the concept of aminimumload voltage range for a solid state relay.For example, the SSR: KSI240D80 in the datasheet states a voltage range of 48V to 280V AC.Datasheet link:https://docs.rs-online.com/276f/A700000006781971.pdfThe SSR has a random-on and a zero crossing version. Why would the SSR have a minimum voltage range (48V) if it has a zero crossing version? Would the zero crossing version not turn on at 0V? (Especially with a resistive load where there is no lag between the voltage and current waveforms?)Does that not mean that the SSR has a voltage range of 0V to 280V AC?What would happen if you use a random-on SSR and try to turn it on if the voltage is at, for example, 24V AC?Thank you!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 06, 2023, 11:24:40 am
content: The higher current triac type SSRs usually are kind of 2 stage: first a low power optocoupler and than a large triac for the main current. To triger the parte triac it needs some power from the switch side and thus a minimum voltage and possibly also a suitable waveform / frequency. So it may not work with 16 Hz (some railway systems), or need a higher min voltage. With only 24 V the random on version may not trigger in all circumstances, or at least not all units.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2023, 12:07:01 pm
content: Despite its name, a zero-crossing SSR will not turn on at 0V, because there will be no current flowing at that voltage. It'll turn on, when the voltage is between around 0.7V and a certain volage, decided by the zero-crossing circuit: normally a few 10s of V. This is of course assuming the minimum load current rating is satisfied.A zero-crossing SSR will switch on/off at lower voltages than indicated on the datasheet, but the zero-crossing functionality will not work. This is because the zero crossing voltage is high, compared to the peak voltage of the supply. Suppose the zero-crossing detector inhibits firing when the volage is above 20V. If it's run with a power supply volage of 12VAC RMS, the peak voltage will never exceed 17V, so it will behave the same as a random on SSR.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 06, 2023, 03:50:14 pm
content: I once wanted to switch a 12 VAC load with an optotriac and actually took some waveforms.As others have mentioned, for the sinewave’s first few volts after the zero-cross, the device would not have yet fired. For 120/240 loads, this is negligible. At 24 VAC, this may be visible, but causes no ill effects.However at lower voltages, the waveform starts to look similar to a phase-control waveform.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 06, 2023, 08:57:32 pm
content: Quote from: FoxesRule on November 06, 2023, 10:55:39 amNot all 'Solid State Relays' are created the same.Some use MOSFETs (usually smaller and lower voltage ones) and they can work to very low voltages, and currents.Ones like you link, use triacs or SCRs and they need some voltage present to trigger that SCR.You also need to worry about the holding current of the Triacs, and minimum currents, due to any snubbers.It also varies with how they are driven. The AC driven ones, have a longer delay time of 40ms.It gets complicated quickly and the data is often sparse.I recently used a 10A puck style triac SSR (just like your link) to control some low power slave LED lights. Way overkill in rating, but it's what you can buyThe master unit had a low power limit, but was AC-dimmed, and the LED lights (not dimmable) were above that limit, but also low power.Experiments found a 5k6 power resistor in series, isolated any inrush issues so the SSR 'looked invisible', and the snubber RC was low enough to not keep the slave unit ON.The low MIN IN voltage was still low enough that the slave LEDs stayed on as the master side dimmed, but went off as they turned fully off.The phase triangle measurements & a quick Spice run suggested SSR AC-in mode, was roughly 150nF and 3k3 (usually into a bridge)

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: November 06, 2023, 09:28:50 pm
content: On my speaker protect circuit with solid state relay I used mosfets as switches.The gate power was from a transformer secondary winding and on/off was controlled by an opto coupler.This way there is no cross over distortion and switching is immediate.