Solid State Relays What do I need to know

[Jan Wagner]

I have a solid state relay but I can't get it to work.

It is a 25 amp 240v a.c. with 3 - 32v d.c. control. The problem is
when I connect the mains to post 1 it always delivers current to post
2 even with pin 4 and 3 shorted. The load I am driving is only in the
order of 0.04 amps (7k Ohm soenoid plus a neon indicator lamp), the
relay is not earthed but before I write it off as broken I am
wondering, is there a minimum load current? does the SSR need a
minimum (1-3v d.c.) off voltage, does it need to be earthed or is
there something else I should know. I will experiment further but in
the intrests of saving myself some time and possibly gain some
knowledge I though I would post here also.

These can leak a few mA, IIRC depending on which type of snubber network
they have inside. So it isn't necessarily broken. Other SSRs leak only
microamps or less, though. Maybe you should google for the datasheet, it
should state the off-state leakage current.

cheers,
- Jan

 

[Starbase]

I have a solid state relay but I can't get it to work.

It is a 25 amp 240v a.c. with 3 - 32v d.c. control. The problem is when I
connect the mains to post 1 it always delivers current to post 2 even with
pin 4 and 3 shorted. The load I am driving is only in the order of 0.04 amps
(7k Ohm soenoid plus a neon indicator lamp), the relay is not earthed but
before I write it off as broken I am wondering, is there a minimum load
current? does the SSR need a minimum (1-3v d.c.) off voltage, does it need
to be earthed or is there something else I should know. I will experiment
further but in the intrests of saving myself some time and possibly gain
some knowledge I though I would post here also.

Many thanks

Chris

 

[John Popelish]

I have a solid state relay but I can't get it to work.

It is a 25 amp 240v a.c. with 3 - 32v d.c. control. The problem is when I
connect the mains to post 1 it always delivers current to post 2 even with
pin 4 and 3 shorted. The load I am driving is only in the order of 0.04 amps
(7k Ohm soenoid plus a neon indicator lamp), the relay is not earthed but
before I write it off as broken I am wondering, is there a minimum load
current? does the SSR need a minimum (1-3v d.c.) off voltage, does it need
to be earthed or is there something else I should know. I will experiment
further but in the intrests of saving myself some time and possibly gain
some knowledge I though I would post here also.

A 25 amp relay includes some sort of snubber across the output that
carries a bit of current, even when the relay is off. You can prove
that this is the cause of your trouble by paralleling the relay with a
light bulb to suck up this leakage current. For this small a load,
you should probably be using a smaller relay.

 

[Starbase]

A 25 amp relay includes some sort of snubber across the output that
carries a bit of current, even when the relay is off. You can prove
that this is the cause of your trouble by paralleling the relay with a
light bulb to suck up this leakage current. For this small a load,
you should probably be using a smaller relay.

Many thanks John and Jan for your input on this problem, it would seem that
you are correct and I will therefore be using a small low power mechanical
relay in place of the SSR for the time being, (as the whole affair is Solar
power driven through an inverter so the aim is to save power, and I need to
learn a bit more as the system must be bullet proof as it controls the
frige/freezer and I will be in serious trouble if I accidently defrost it).
The sharing of your knowledge on the newsgroup is very much appreciated.

Best regards

Chris

 

[Starbase]

A 25 amp relay includes some sort of snubber across the output that
carries a bit of current, even when the relay is off. You can prove
that this is the cause of your trouble by paralleling the relay with a
light bulb to suck up this leakage current. For this small a load,
you should probably be using a smaller relay.

Thanks again, as a temporary measure I have sunk the excess current using a
15 k ohm (7 watt) resistor and the circuit now works as anticipated (less
the extra un-anticipated 4 watts power drain) but for now it will do the job
it is inteded to do until I can find a suitable replacement. Forever
upgrades!

Best Regards

Chris