Forums

arc suppressor for relays

Started by Jon Elson October 3, 2012
We have one of those $1K high efficiency washing machines. It does work, and
clearly reduced our water and gas bill, so I'm not complaining.

Lately, it has been getting "slow fill" errors, and I tried a bunch
of typical stuff before finding the REAL problem.  They have a bank
of electromechanical relays to operate a bank of solenoid water valves.
There is no arc suppression on these relays, and the cold water valve
relay is totally shot.  I have replacement relays on order, but want to
add arc suppression to prolong the life of the relays.

The coils are 120 V AC, the relays are TINY things.  I ordered some
Zener-type transient suppressors which indicate a 193 V trip voltage,
bipolar.
I'm not clear about whether this is the level at which all units will
start to conduct or the level at which none will conduct.

Also, would it be better to put the suppressor across the relay contacts
or across the valve solenoid?  If the device fails to short when across the
contacts, it will just turn the valve on all the time, causing a flood.
If across the valve coil, it will blow something, hopefully not circuit
traces off the board.

So, I'm looking for practical suggestions on how to choose the supressor
type (Zener or MOV) style, trigger voltage and where to put it in the
circuit.

Thanks,

Jon
On Wed, 03 Oct 2012 14:32:02 -0500, Jon Elson <jmelson@wustl.edu>
wrote:

>We have one of those $1K high efficiency washing machines. It does work, and >clearly reduced our water and gas bill, so I'm not complaining. > >Lately, it has been getting "slow fill" errors, and I tried a bunch >of typical stuff before finding the REAL problem. They have a bank >of electromechanical relays to operate a bank of solenoid water valves. >There is no arc suppression on these relays, and the cold water valve >relay is totally shot. I have replacement relays on order, but want to >add arc suppression to prolong the life of the relays. > >The coils are 120 V AC, the relays are TINY things. I ordered some >Zener-type transient suppressors which indicate a 193 V trip voltage, >bipolar. >I'm not clear about whether this is the level at which all units will >start to conduct or the level at which none will conduct.
You ordered parts without looking at the data sheets? Or do you need help in interpreting the data? Here's a typical TVS data sheet: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000720.pdf Take, for example, the P6KE220CA. No more than 500nA at 188V At 1mA nominally 220V, but no less than 209V or more than 231V At 1.85A (surely beyond your required clamping current) no more than 328V. At 10.3A, no more than 388V. It's a bipolar device, so the ratings apply for either polarity of applied voltage.
> >Also, would it be better to put the suppressor across the relay contacts >or across the valve solenoid?
I'd put it across the solenoid coil. You could also try one of these, which you may have parts for in your junk box: http://www.digikey.ca/product-detail/en/104M06QC100/338-2581-ND/2503688 Those built-up ones are stupid expensive, you can just try a 630V 0.1uF film cap series with a 1/2W 100R resistor for < 1/10 the cost.
>If the device fails to short when across the >contacts, it will just turn the valve on all the time, causing a flood. >If across the valve coil, it will blow something, hopefully not circuit >traces off the board. > >So, I'm looking for practical suggestions on how to choose the supressor >type (Zener or MOV) style, trigger voltage and where to put it in the >circuit. > >Thanks, > >Jon
Jon Elson <jmelson@wustl.edu> wrote:

> We have one of those $1K high efficiency washing machines. It does work, and > clearly reduced our water and gas bill, so I'm not complaining. > > Lately, it has been getting "slow fill" errors, and I tried a bunch > of typical stuff before finding the REAL problem. They have a bank > of electromechanical relays to operate a bank of solenoid water valves. > There is no arc suppression on these relays, and the cold water valve > relay is totally shot.
As an aside (which doesn't actually answer your original question), were you aware that relays need to be mounted so that the gap between the contacts is vertical if you want the longest life? This is one of these useful facts that was known by designers in the 1920s (or even earlier) and appeared in various textbooks on telephony, but now seems to have been forgotten. The reason is that it reduces the chance of dirt falling on the contact surfaces and helps to dislodge any loose debris resulting from contact wear. I have come across an industrial diswasher that was forever burning out small power relays simply because they were wrongly mounted. -- ~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk
Spehro Pefhany wrote:


> > You ordered parts without looking at the data sheets? Or do you need > help in interpreting the data? >
I did look at the data sheets, they cover a wide range of devices, and have very little descriptive info on what the columns of numbers actually mean. I think I could have ordered a slightly higher voltage part and been sure it would be fine. I ordered the P4KE200CA. But, looking a bit closer, they show a range of min and max clamping voltage that look like they will be OK, but right on the edge. For peace of mind, I may just install one and then order some 220 or 240 V ones later to retrofit the rest of the unit.
>>Also, would it be better to put the suppressor across the relay contacts >>or across the valve solenoid? > > I'd put it across the solenoid coil. > > You could also try one of these, which you may have parts for in your > junk box: > http://www.digikey.ca/product-detail/en/104M06QC100/338-2581-ND/2503688 > > Those built-up ones are stupid expensive, you can just try a 630V > 0.1uF film cap series with a 1/2W 100R resistor for < 1/10 the cost.
Not a lot of space to put this on the board, I've got maybe 3mm on the back of the board and can put it between the pins of the relay. I don't think the caps would fit, but the 400W TVS should. Thanks much, Jon
Adrian Tuddenham wrote:


> > As an aside (which doesn't actually answer your original question), were > you aware that relays need to be mounted so that the gap between the > contacts is vertical if you want the longest life? This is one of these > useful facts that was known by designers in the 1920s (or even earlier) > and appeared in various textbooks on telephony, but now seems to have > been forgotten. > > The reason is that it reduces the chance of dirt falling on the contact > surfaces and helps to dislodge any loose debris resulting from contact > wear. > > I have come across an industrial diswasher that was forever burning out > small power relays simply because they were wrongly mounted.
These relays are semi-sealed (totally enclosed by plastic, but probably not hermetic (although the boards are probably washed after assembly, so they may be darn close to hermetic) so there is no external dirt to get in them. It is now burning the contacts severely, and you can hear the solenoid valves rattling and buzzing as the contacts arc. So, this is not a little speck of dust, but massively burned-away contacts. I expect on post-mortem, the contacts will be totally vaporized. Jon
"Jon Elson" <jmelson@wustl.edu> wrote in message 
news:LrOdnT0aBNYvDfHNnZ2dnUVZ_sqdnZ2d@giganews.com...
> We have one of those $1K high efficiency washing machines. It does work, > and > clearly reduced our water and gas bill, so I'm not complaining. > > Lately, it has been getting "slow fill" errors, and I tried a bunch > of typical stuff before finding the REAL problem. They have a bank > of electromechanical relays to operate a bank of solenoid water valves. > There is no arc suppression on these relays, and the cold water valve > relay is totally shot. I have replacement relays on order, but want to > add arc suppression to prolong the life of the relays. > > The coils are 120 V AC, the relays are TINY things. I ordered some > Zener-type transient suppressors which indicate a 193 V trip voltage, > bipolar. > I'm not clear about whether this is the level at which all units will > start to conduct or the level at which none will conduct. > > Also, would it be better to put the suppressor across the relay contacts > or across the valve solenoid? If the device fails to short when across > the > contacts, it will just turn the valve on all the time, causing a flood. > If across the valve coil, it will blow something, hopefully not circuit > traces off the board. > > So, I'm looking for practical suggestions on how to choose the supressor > type (Zener or MOV) style, trigger voltage and where to put it in the > circuit. > > Thanks, > > Jon
Can you use a solid state relay? Maybe off the board? Another "designed by monkeys" consumer product.
Jon Elson wrote:

> We have one of those $1K high efficiency washing machines. It does work, and > clearly reduced our water and gas bill, so I'm not complaining. > > Lately, it has been getting "slow fill" errors, and I tried a bunch > of typical stuff before finding the REAL problem. They have a bank > of electromechanical relays to operate a bank of solenoid water valves. > There is no arc suppression on these relays, and the cold water valve > relay is totally shot. I have replacement relays on order, but want to > add arc suppression to prolong the life of the relays. > > The coils are 120 V AC, the relays are TINY things. I ordered some > Zener-type transient suppressors which indicate a 193 V trip voltage, > bipolar. > I'm not clear about whether this is the level at which all units will > start to conduct or the level at which none will conduct. > > Also, would it be better to put the suppressor across the relay contacts > or across the valve solenoid? If the device fails to short when across the > contacts, it will just turn the valve on all the time, causing a flood. > If across the valve coil, it will blow something, hopefully not circuit > traces off the board. > > So, I'm looking for practical suggestions on how to choose the supressor > type (Zener or MOV) style, trigger voltage and where to put it in the > circuit. > > Thanks, > > Jon
Get some DC controlled SSR's (solid state relays), that can operate from 120 DC Or AC, diode them from the coils supply, place a cap on the control terminal of the SSR. The M1, M2 terminals will go across the contacts. What this does is the SSR first comes on before the contacts close, there by removing the load. when the contacts close, the SSR will not conduct and thus it won't be doing any work. The cap on the control terminal allows for a slight time off delay, this allows for the relay to open and the SSR will be conducting for a very short time afterwards.. Actually, I think there are add on products that already do this.. Not sure. Jamie.
tm wrote:


> Can you use a solid state relay? Maybe off the board?
There are 8 of these. If I can find a drop-in SSR that matches the pinout, I sure will do this the next time it needs to be opened up. Off the board won't work, there really is limited space in the "console" that sits above the the washer top/rear. If it was only one relay, it would have already gotten hacked as you suggest.
> Another "designed by monkeys" consumer product.
Yeah, they must sell a LOT of these $275 controller boards. Jon
Jamie wrote:

> Jon Elson wrote: > >> We have one of those $1K high efficiency washing machines. It does work, >> and clearly reduced our water and gas bill, so I'm not complaining. >> >> Lately, it has been getting "slow fill" errors, and I tried a bunch >> of typical stuff before finding the REAL problem. They have a bank >> of electromechanical relays to operate a bank of solenoid water valves. >> There is no arc suppression on these relays, and the cold water valve >> relay is totally shot. I have replacement relays on order, but want to >> add arc suppression to prolong the life of the relays. >> >> The coils are 120 V AC, the relays are TINY things. I ordered some >> Zener-type transient suppressors which indicate a 193 V trip voltage, >> bipolar. >> I'm not clear about whether this is the level at which all units will >> start to conduct or the level at which none will conduct. >> >> Also, would it be better to put the suppressor across the relay contacts >> or across the valve solenoid? If the device fails to short when across >> the contacts, it will just turn the valve on all the time, causing a >> flood. If across the valve coil, it will blow something, hopefully not >> circuit traces off the board. >> >> So, I'm looking for practical suggestions on how to choose the supressor >> type (Zener or MOV) style, trigger voltage and where to put it in the >> circuit. >> >> Thanks, >> >> Jon > > Get some DC controlled SSR's (solid state relays), that can operate > from 120 DC Or AC, diode them from the coils supply, place a cap on the > control terminal of the SSR. The M1, M2 terminals will go across the > contacts. > What this does is the SSR first comes on before the contacts close, > there by removing the load. when the contacts close, the SSR will not > conduct and thus it won't be doing any work. > > The cap on the control terminal allows for a slight time off delay, > this allows for the relay to open and the SSR will be conducting for a > very short time afterwards.. > > Actually, I think there are add on products that already do this.. Not > sure.
No room for double relays. I'm not sure your double relay scheme will fix the problem, anyway, it occurs on shut-off! The SSR would shut off first, then the mechanical relay would drop out later, and still arc (although the off SSR might have an internal snubber or avalanche mode that would reduce the arc.) First, there are EIGHT of these in a tiny space, so adding 8 more relays will never work. But, why use TWO relays when one good one will do it? A cap big enough to delay the turnoff of the SSR could damage the relay driver chip, and would also keep the magnetic relay on. Jon
"Jon Elson" <jmelson@wustl.edu> wrote in message 
news:a_OdnUzCt-COI_HNnZ2dnUVZ_rSdnZ2d@giganews.com...
> tm wrote: > > >> Can you use a solid state relay? Maybe off the board? > > There are 8 of these. If I can find a drop-in SSR that matches the > pinout, I sure will do this the next time it needs to be opened up. > Off the board won't work, there really is limited space in the "console" > that sits above the the washer top/rear. If it was only one relay, > it would have already gotten hacked as you suggest. >> Another "designed by monkeys" consumer product. > Yeah, they must sell a LOT of these $275 controller boards. > > Jon
I guess the relay switches 120 volts AC, right? What drives the relay? Is it 5 volts, other? Is there an optoisolated triac arrangement available that could mount to the solenoids and wire to the board directly? Can you observe the relay contacts to see if your snubber idea is effective? You might try something like an X rated 0.068 uF and 10 ohms in series across the contacts. You want to suppress the high voltage from the solenoid coil. Almost any amount will help. What is the make of the washer? I would want to avoid it for sure.