Solid State Relay Not Switching Off

I have a PCB Solid State Relay FSS1-102Z DC 5V 2A 250VAC with the following specs:

* INPUT *
Control Voltage 4-6 VDC
Max Current 15mA
Voltage On 4V
Current Off 1V
Zero Voltage Turn On

* OUTPUT *
Voltage 50 - 250VAC
Max Current 2A
Surge Current 30A

Basically I want to use an LED to trigger 12V load. I have chosen Solid State Relay due to reliability, sensitive to low current switching and quicker trigger response.

* RELAY PINS *
1 LOAD (Goes to 12V device to activate) and then to Ground from device
2 LOAD (Battery 12V)
3 INPUT Positive (Intercept LED positive)
4 INPUT Negative (Intercept LED negative)

Test results:
I am able to trigger using LED as input to relay which was as low as 2 volts which is what I intended, triggering works great and 12V device turns on, but....

Problem:
After relay triggered, relay stays permanently on, and does not turn off 12V device when trigger is released (when LED is off).

Solving:
Everytime LED is on would like to have 12V device on as well and when LED goes off then 12V device would switch off also. Can anyone solve my problem? Thanks.

I couldnt find the part you are using with a couple of searches but it sounds like it is a SCS(silicon controlled switch) this is basically adding another terminal to an SCR (silicon controlled rectifier) I am not trying to confuse you with this but basically it is working the way it is supposed to. it is a solid state switch triggd. by a low current. when this happens the output will remain on or high until "low current dropout" is achieved. This happens when the supply voltage is removed, not the trig. voltage.

what do you think will allow the supply to turn off when the LED is off? maybe add a capacitor to the supply line or is there a workaround?

Hi!
I've got the same problem with a solid state relay (Celduc SKB20440). HOw did you finally come around this or did you buy another relay?

If you are controlling a DC load you will need a solid state relay designed for DC loads.  They have output specs for a reason - yours is designed to control 50 - 250VAC, NOT 12VDC.  Using the right part for the job should get you the results you need.

It sounds like all you need is a transistor, or possibly a reed relay.

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Kevin Pierson

Yes. definitely a problem caused by the DC load.

The "contacts" of an AC switching solid state relay are either a TRIAC or a pair of SCRs in inverse parallel. When used as intended on an AC circuit, the contacts "drop out" each time the current passes through zero (2X each cycle of the sine wave).

When used on DC, the output stage will latch on, and stay latched as long as current flows through the contacts. The only way to drop the relay out is to remove power from the load.

You need a solid state relay specifically intended for DC loads, which uses a power MOSFET or IGBT for an output stage.

As KPierson said all you need is a transistor.

The output side of a solid state relay is powered, it needs the specified voltage to work properly.

If you do need isolation,
A DC solid state relay is very cheap and easy to build, I use an opto-isolator and an appropriate transistor as a DC solid state relay.

Another option is something like an HFS2 https://pdf1.alldatasheet.com/datasheet-pdf/view/194418/ETC/HFS2/A211DN.html This will also need an appropriate transistor to carry the load.