Leakage problem driving one SSR with another

[Bouquet]

Hi,

I'm controlling a high-current SSR with a lower-current SSR module
but have found that the 4mA leakage of the module is causing the
high current module to stay on permanently. The high-current SSR
has an impendance of 20KOhms and the control voltage is 240V AC.

I've calculated that a 5-10W resistor would be needed across the
SSRs to drain the leakage, but is there any other device I could
use to sink the 4mA @ <20VAC in the SSR off-state but not much more
current in the 240V on-state? Some AC constant current device, a
neon lamp maybe.

I'd appreciate any help you could offer.

Mark

 

[Walter Harley]

I'm controlling a high-current SSR with a lower-current SSR module
but have found that the 4mA leakage of the module is causing the
high current module to stay on permanently. The high-current SSR
has an impendance of 20KOhms and the control voltage is 240V AC.

I don't know the answer to your problem (hopefully someone else does). But
I wonder whether there is an alternate approach?

Seems to me, you only need to control 12mA of AC (to power the high-current
SSR). But your low-current SSR has *leakage* of 4mA, suggesting it's not a
very low-current SSR after all. Maybe the answer is to get a smaller
low-current SSR, one that has less leakage?

A brief web search shows, e.g., the Crydom AO241, with control voltage of
4-10v @ 15mA, leakage current of 0.1mA, max load current of 1A. I'm sure
there are a bunch of other ones, that's just the first one I found.

 

[Daniel Lang]

You may want to use a photomos optoisolator to drive the high current SSR.
Something like the NEC PS7141-1A or NEC PS7160-1A from Digikey should work
fine (use the 2 drain pins and leave the common source pin open).

Dan

 

[Boris Mohar]

Hi,

I'm controlling a high-current SSR with a lower-current SSR module
but have found that the 4mA leakage of the module is causing the
high current module to stay on permanently. The high-current SSR
has an impendance of 20KOhms and the control voltage is 240V AC.

I've calculated that a 5-10W resistor would be needed across the
SSRs to drain the leakage, but is there any other device I could
use to sink the 4mA @ <20VAC in the SSR off-state but not much more
current in the 240V on-state? Some AC constant current device, a
neon lamp maybe.

I'd appreciate any help you could offer.

Mark

Put an AC capacitor across the input to shunt the leakage.

--

Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
Aurora, Ontario