# d Solid state relay minimum load current?

I am looking at using a solid state relay to drive a tiny pneumatic valve. The SSR spec has a load current range of 0.02 to 3.5A. Most SSRs specify a maximum load current, which is obvious as to why. Can someone explain why there would be a need to specify the minimum load current? Here is the SSR spec:

My valve is 0.45W at 24VDC = 0.01875A, so the SSR leakage current should not be a problem. Should I be concerned about the current draw of the valve being lower that the 0.02A spec? Could I just throw a resistor across (or in series?) with the valve coil to increase the load current?

## 1 Answer

SSR's are normally TRIACs with leakage current such that a minimum load current or maximum load resistance is needed to attenuated the leakage voltage seen by the load when switched off.

But in this case it is a leaky low voltage NPN transistor that can switch high current with minimal optical current. (See schematic below)

This is inherent with all low voltage bipolar transistors and reverse shunt clamp diodes on the output and are typically specified as 1 mA max and thus rated at 60Vdc , your looks like a 60kΩ pull-up resistor to your DC supply. However your SSR is rated at 1.5mA leakage so the Requiv.= 40kΩ

Since your pump is like a coil with resistance, your load specs indicate 0.45W @24V so R=V^2/P = 1280 Ω. Thus with the device switched off, you may see 1.280k/(40k+1.280)*24V=0.74V across the part, which is not enough to activate it. The leakage current for release relays and valves is normally 10~20% of rated current so the device should release. if it releases too slow and is noisy from DC ripple, the you can always put a 3.3K 1/4W R in parallel with the valve or find a better SSR.

Normally you don't need an opto isolated SSR to drive an isolated valve, just a reed relay will do or just an open collector transistor for low side switch and reverse diode clamp across output.

• I apologize for use the term SSR. This is a solid state "output module" used for signal isolation. It has is rated for DC load only not and AC load. I didn't think they used TRIACs for DC output modules. Here is the full spec sheet. It shows a transistor on the output, not a TRIAC. Does all that still apply? mccdaq.com/GetPDF.aspx?t=/PDFs/specs/SSR-ODC-05-SPEC.pdf – JoeChiphead Oct 18 '16 at 20:08
• I started thinking it was AC and said triacs then corrected myself mid-stream.. with the explanation about bipolar transistors. actually triacs are made from 2NPN and 2PNP transistors as well so same applied except leakage is always lower on high V rated Triacs. sorry for confusion – Tony Stewart Sunnyskyguy EE75 Oct 18 '16 at 22:10
• I could suggest a Reed relay for you rated for 500mA or less with low coil current – Tony Stewart Sunnyskyguy EE75 Oct 18 '16 at 22:11
• I am limited on the choice of output modules since they are part of a larger IO "rack" - so a reed relay is . You mentioned "can always put a 3.3K 1/4W R in parallel with the valve". How did you arrive at this value? – JoeChiphead Oct 19 '16 at 13:53
• 24V/3k3=7.3mA additional load 24V*7.3mA= 175mW just enough to meet SSR minimum with some margin – Tony Stewart Sunnyskyguy EE75 Oct 19 '16 at 14:05