help driving relay

[Paul Taylor]

Hello to you all a quick question has anyone in the past driven a high
powered relay 25A at 240v from a SSR which is being switched using TTL from
some logic? im having problem getting the relay to reset itself when the ttl
is removed from the SSR the relay basically stays latched I can hang a load
on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
any ideas is much appreciated thanks in advance.

-----------------------------------------------------
Paul Taylor BSC (Hons)
Electronics Technician
School of Environmental Science
University of East Anglia
Norwich
NR4 7TJ

Phone: +44 (0)1603 592502
Fax: +44 (0)1603 591327

Email: Paul.Taylor@uea.ac.uk
Web: http://www.uea.ac.uk/~e087

------------------------------------------------------

 

[Ol' Duffer]

In article <dcvfdc$2k2$1@cpca14.uea.ac.uk>, paul.taylor@uea.ac.uk
says...

Hello to you all a quick question has anyone in the past driven a high
powered relay 25A at 240v from a SSR which is being switched using TTL from
some logic? im having problem getting the relay to reset itself when the ttl
is removed from the SSR the relay basically stays latched I can hang a load
on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
any ideas is much appreciated thanks in advance.

Many SSR's don't like highly inductive loads (like your relay coil).
High dv/dt (back emf) can make the SSR self-trigger. RC snubber
networks are relatively common in this application. Your SSR's
maker should have some application notes to help you sort it out.

 

[Spehro Pefhany]

On Fri, 5 Aug 2005 11:35:26 +0100, the renowned "Paul Taylor"
<paul.taylor@uea.ac.uk> wrote:

Hello to you all a quick question has anyone in the past driven a high
powered relay 25A at 240v from a SSR which is being switched using TTL from
some logic? im having problem getting the relay to reset itself when the ttl
is removed from the SSR the relay basically stays latched I can hang a load
on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
any ideas is much appreciated thanks in advance.

You'd do better with an SCR-based SSR (or possibly an alternistor
type). These types are more tolerant of switching inductive loads,
particularly the "off" part of switching.

Kudos for mentioning that it's a high power relay (or contactor) and
thus we can pretty much eliminate the possibility of the few mA of
leakage through the snubber holding the relay on.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Fred Bloggs]

Paul Taylor wrote:

Hello to you all a quick question has anyone in the past driven a high
powered relay 25A at 240v from a SSR which is being switched using TTL from
some logic? im having problem getting the relay to reset itself when the ttl
is removed from the SSR the relay basically stays latched I can hang a load
on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
any ideas is much appreciated thanks in advance.

How are you measuring the off state, with a high impedance meter and no
load? The SSR leakage will show the full voltage across the meter.
Almost any kind of SSR will handle a relay coil drive, why don't you
measure it with the relay in place, the meter should show much less than
10% line voltage in the off state.

 

[John Popelish]

Paul Taylor wrote:

Hello to you all a quick question has anyone in the past driven a high
powered relay 25A at 240v from a SSR which is being switched using TTL from
some logic? im having problem getting the relay to reset itself when the ttl
is removed from the SSR the relay basically stays latched I can hang a load
on the SSR o/p and this sorts the problem I feel this isn't a very good fix?
any ideas is much appreciated thanks in advance.

-----------------------------------------------------
Paul Taylor BSC (Hons)
Electronics Technician
School of Environmental Science
University of East Anglia
Norwich
NR4 7TJ

Phone: +44 (0)1603 592502
Fax: +44 (0)1603 591327

Email: Paul.Taylor@uea.ac.uk
Web: http://www.uea.ac.uk/~e087

------------------------------------------------------


I think the cause of the problem is the highly inductive nature of the

load. Because of that, the load current is not in phase with the
voltage, so at the moment of zero current (when the SSR tries to
switch off) there is considerable line voltage that is being bucked by
the inductive rate of change of current produced voltage. But at
switch off, the rate of change of current snaps to zero, allowing the
line voltage to appear, essentially instantaneously across the SSR.
This exceeds its dv/dt limit, and the voltage swing capacitively
couples into the SSR device gates, switching it back on.

Adding a series RC across the SSR output will provide a path for a
little current at that moment, to greatly reduce the dv/dt. You may
get by with something like 100 to 470 ohms 1/2 watt and .1uF, but the
capacitor needs to be rated for across the 240 volt line, since it
sees this all the time the SSR is off.
E.g.
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=59424&Row=406818&Site=US

Prefab (resistor potted with capacitor in single case) snubber
networks like this are also available.

 

[Arie de Muynck]

"Spehro Pefhany" wrote...

Kudos for mentioning that it's a high power relay (or contactor) and
thus we can pretty much eliminate the possibility of the few mA of
leakage through the snubber holding the relay on.

Question of the week:

Why don't people put the snubber acroos the load to solve the snubber
leakage problem? The turnoff dV/dT would be the same.

I did it - solved most problems with small solenoids, and I never had any
problem with powerspikes triggering the SSR's. When zero-voltage switching
at turnon the inrush current is no problem either.

Regards,
Arie de Muynck

 

[Spehro Pefhany]

On Fri, 5 Aug 2005 19:39:24 +0200, the renowned "Arie de Muynck"
<send.spam.to@spammer.org> wrote:

"Spehro Pefhany" wrote...
Kudos for mentioning that it's a high power relay (or contactor) and
thus we can pretty much eliminate the possibility of the few mA of
leakage through the snubber holding the relay on.


Question of the week:

Why don't people put the snubber acroos the load to solve the snubber
leakage problem? The turnoff dV/dT would be the same.

1) You're assuming there is no line inductance. This is the sort
of thing that can work in one place, but possibly fail in some
godforsaken windswept customer location without an airport,
sushi, cappuccinos, fresh-baked nan or any of the other
necessities of a civilized existence. A grim thought.

2) The SSR maker typically has only two terminals to work with.

I did it - solved most problems with small solenoids, and I never had any
problem with powerspikes triggering the SSR's.

This method does potentially aggravate the issue of transients on the
line. You can always put supression in both places (across the load
and across the line), but you have to deal with the unknown (and
possibly low) impedance of whatever is creating those spikes, rather
than the usually known (and relatively high) impedance of the load.

When zero-voltage switching
at turnon the inrush current is no problem either.

No, but if it's triggered by the dv/dt or voltage of a spike it won't
likely be at the zero crossing.

Regards,
Arie de Muynck

On the plus side, for this method is that a failure of the snubber
tends to put the load in a safer condition. Depending.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com