A quick question:
Is it reasonable to assume that most small general-purpose DC relays will not engage at 50% or the rated coil voltage? If not, how about 40%?
(This is *not* about the release voltage).
On 2024-08-23 12:37, Pimpom wrote:
A quick question:
Is it reasonable to assume that most small general-purpose DC relays
will not engage at 50% or the rated coil voltage? If not, how about 40%?
(This is *not* about the release voltage).
Not safe. If an external magnetic field is present (e.g. from nearby
relays) or a mechanical shock occurs it can activate and hold.
Been there, had that.
Arie
A quick question:
Is it reasonable to assume that most small general-purpose DC relays
will not engage at 50% or the rated coil voltage? If not, how about 40%? (This is *not* about the release voltage).
Pimpom <Pimpom@invalid.invalid> wrote:
A quick question:
Is it reasonable to assume that most small general-purpose DC relays
will not engage at 50% or the rated coil voltage? If not, how about 40%?
(This is *not* about the release voltage).
That roughly fits my observations but I don’t think it can be trusted , aging, vibration or temperatures outside room temp could make it pull in earlier. You might need to characterise the relays you have in mind.
A quick question:
Is it reasonable to assume that most small general-purpose DC relays
will not engage at 50% or the rated coil voltage? If not, how about 40%? >(This is *not* about the release voltage).
On 2024-08-23 4:59 a.m., Pimpom wrote:
On 23-08-2024 04:46 pm, piglet wrote:
Pimpom <Pimpom@invalid.invalid> wrote:
A quick question:
Is it reasonable to assume that most small general-purpose DC relays
will not engage at 50% or the rated coil voltage? If not, how about
40%?
(This is *not* about the release voltage).
That roughly fits my observations but I don’t think it can be trusted , >>> aging, vibration or temperatures outside room temp could make it pull in >>> earlier. You might need to characterise the relays you have in mind.
Thanks for the reply. Actually, my calculations indicate that less
than 25% of the nominal voltage will appear across the coil for some
tens of milliseconds when it should be disengaged. This is caused by a
decaying supply voltage, not an inductive spike.
I'd suggest using a power supply at the maximum decay voltage you expect
then bang the relay a bit to see if it will engage. I assume you have
the normal back diode across the coil for clamping the ringing voltage.
You could use the NO contacts in the relay to keep it energized (Hold)
if it momentarily closes during testing.
John :-#)#
I assume you have the normalIf you care about the life of the relay (and how well it "opens"),
back diode across the coil for clamping the ringing voltage.
On 2024-08-23 12:43 p.m., Don Y wrote:
On 8/23/2024 7:36 AM, John Robertson wrote:
I assume you have the normal back diode across the coil for clamping the >>> ringing voltage.If you care about the life of the relay (and how well it "opens"),
a better approach is to add a zener rated at the coil voltage in
series with that diode. The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing. The
zener speeds up this transition.
Don't you mean that one could use a zener diode in place of the back-EMF diode
- or parallel with it?
We've been using 1N400X (and 3A in some cases)diodes since the 70s for protecting pinball driver transistors - not too worried about the decay time so
haven't really looked deeper into it before.
Normal power supply decay will not release the relay quickly enough. So
I've added a section that turns the 12V relay off when the decaying PS
drops below 7.5V. Works fine in simulation.
However, due to interaction with other sections, the gate voltage of the transistor driving the relay rises again briefly to about 2.5V before
the shutdown process is complete. But by this time, the 12V supply has dropped to less than 3V.
On 8/23/2024 7:36 AM, John Robertson wrote:
I assume you have the normal back diode across the coil for clampingIf you care about the life of the relay (and how well it "opens"),
the ringing voltage.
a better approach is to add a zener rated at the coil voltage in
series with that diode. The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing. The
zener speeds up this transition.
Pimpom <Pimpom@invalid.invalid> wrote:
[...]
Normal power supply decay will not release the relay quickly enough. So
I've added a section that turns the 12V relay off when the decaying PS
drops below 7.5V. Works fine in simulation.
However, due to interaction with other sections, the gate voltage of the
transistor driving the relay rises again briefly to about 2.5V before
the shutdown process is complete. But by this time, the 12V supply has
dropped to less than 3V.
Add a few diode drops in the supply to the Base of the transistor and a pull-down resistor from Base to Earth. You could even use a 7.5v Zener
so the transistor switched directly off the PS and no other active
components were needed.
Pimpom <Pimpom@invalid.invalid> wrote:
[...]
Normal power supply decay will not release the relay quickly enough. So
I've added a section that turns the 12V relay off when the decaying PS
drops below 7.5V. Works fine in simulation.
However, due to interaction with other sections, the gate voltage of the
transistor driving the relay rises again briefly to about 2.5V before
the shutdown process is complete. But by this time, the 12V supply has
dropped to less than 3V.
Add a few diode drops in the supply to the Base of the transistor and a >pull-down resistor from Base to Earth. You could even use a 7.5v Zener
so the transistor switched directly off the PS and no other active
components were needed.
On Sat, 24 Aug 2024 11:11:19 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Pimpom <Pimpom@invalid.invalid> wrote:
[...]
Normal power supply decay will not release the relay quickly enough. So
I've added a section that turns the 12V relay off when the decaying PS
drops below 7.5V. Works fine in simulation.
However, due to interaction with other sections, the gate voltage of the >>> transistor driving the relay rises again briefly to about 2.5V before
the shutdown process is complete. But by this time, the 12V supply has
dropped to less than 3V.
Add a few diode drops in the supply to the Base of the transistor and a
pull-down resistor from Base to Earth. You could even use a 7.5v Zener
so the transistor switched directly off the PS and no other active
components were needed.
A 2-resistor voltage divider can reduce mosfet gate swing.
Or an R-C in his case of a transient gate drive spike.
Or both.
Good reading! It appears counterintuitive, but if I warp (!) my head around it
I should be able to sort it out.
On 8/24/2024 10:03 PM, John Robertson wrote:
Good reading! It appears counterintuitive, but if I warp (!) my head around >> it I should be able to sort it out.
Think about the extremes:
- with NO diode or snuber, there is nothing to keep the magnetic
 field intact when the switch opens. So, the spring force is the
 sole actor in play, pulling the contacts open
- with a diode, some current continues to "recirculate" in the coil
 thus keeping the magnetic field active -- so it is trying to hold
 the relay closed while the spring is trying to open it. *When*
 the recirculating current falls, the field collapses and the relay
 can open.
The zener lets the circuit look more like the "no diode" case.
On 2024-08-23 11:29 p.m., Don Y wrote:
On 8/23/2024 10:05 PM, John Robertson wrote:
On 2024-08-23 12:43 p.m., Don Y wrote:
On 8/23/2024 7:36 AM, John Robertson wrote:
I assume you have the normal back diode across the coil for clamping >>>>> the ringing voltage.If you care about the life of the relay (and how well it "opens"),
a better approach is to add a zener rated at the coil voltage in
series with that diode. The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing. The
zener speeds up this transition.
Don't you mean that one could use a zener diode in place of the back-
EMF diode - or parallel with it?
In series. It speeds up the decay of the magnetic field (and, thus, the
opening of the armature) by allowing a higher potential to exist across
the coil while it is "opening". The ideal condition is with NO catch diode >> (but that tends to fry solid state switches! :> ) You're making a
snubber, of sorts.
We've been using 1N400X (and 3A in some cases)diodes since the 70s for
protecting pinball driver transistors - not too worried about the
decay time so haven't really looked deeper into it before.
It really only matters if you are really concerned over the actual opening >> time (delay) of the relay and/or how long you want the relay (contacts) to >> last. (many of my designs have to have service lives of decades or more.) >>
<https://www.te.com/commerce/DocumentDelivery/DDEController?
Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN> will give
you a quick overview.
I have better notes from (relay) application engineers but I'm busy baking, >> tonight, so can only take quick pokes at my mail, etc.
Good reading! It appears counterintuitive, but if I warp (!) my head
around it I should be able to sort it out.
Thanks, that is very helpful.
John :-#)#
An even simpler coil catcher is a resistor. Many times the extra power consumption is not an issue and resistors are even cheaper and more
reliable than zener + diode.
On 8/25/2024 12:40 AM, piglet wrote:
An even simpler coil catcher is a resistor. Many times the extra power
consumption is not an issue and resistors are even cheaper and more
reliable than zener + diode.
A lot depends on the relay's (coil) duty cycle.
To limit the peak transient (to approximately the supply voltage) when
the coil opens, R needs to be on the order of the coil resistance.
If the this halves the load resistance that must be driven when the
coil is active.
Of course, if the duty cycle is low and 2X the driving current is
within the capabilities of the device you had already planned on
using for the switch, then this has minimal impact.
OTOH, if the coil current is higher, already taxing the capabilities
of the switch or the duty cycle has the coil energized more often
than not, then this can be a significant factor.
And, as R approaches the coil's resistance, the opening time/delay of
the relay *increases*. I.e., you want R to approach infinity to get the best opening transition but this also gives the highest switching
transient voltage -- often many times the supply voltage!
...The relay is likely to be a 12V 400Ω model so that the slow
turn off wouldn't unduly stress the transistor.
On 2024-08-23 12:43 p.m., Don Y wrote:
On 8/23/2024 7:36 AM, John Robertson wrote:
I assume you have the normal back diode across the coil for clampingIf you care about the life of the relay (and how well it "opens"),
the ringing voltage.
a better approach is to add a zener rated at the coil voltage in
series with that diode. The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing. The
zener speeds up this transition.
Hi Don,
Don't you mean that one could use a zener diode in place of the back-EMF >diode - or parallel with it?
We've been using 1N400X (and 3A in some cases)diodes since the 70s for >protecting pinball driver transistors - not too worried about the decay
time so haven't really looked deeper into it before.
Thanks,
John :-#)#
On 8/23/2024 10:05 PM, John Robertson wrote:
On 2024-08-23 12:43 p.m., Don Y wrote:
On 8/23/2024 7:36 AM, John Robertson wrote:
I assume you have the normal back diode across the coil for clamping the >>>> ringing voltage.If you care about the life of the relay (and how well it "opens"),
a better approach is to add a zener rated at the coil voltage in
series with that diode. The diode, by itself, delays opening of
the contacts and leads to more opportunities for arcing. The
zener speeds up this transition.
Don't you mean that one could use a zener diode in place of the back-EMF diode
- or parallel with it?
In series. It speeds up the decay of the magnetic field (and, thus, the >opening of the armature) by allowing a higher potential to exist across
the coil while it is "opening". The ideal condition is with NO catch diode >(but that tends to fry solid state switches! :> ) You're making a
snubber, of sorts.
We've been using 1N400X (and 3A in some cases)diodes since the 70s for
protecting pinball driver transistors - not too worried about the decay time so
haven't really looked deeper into it before.
It really only matters if you are really concerned over the actual opening >time (delay) of the relay and/or how long you want the relay (contacts) to >last. (many of my designs have to have service lives of decades or more.)
<https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN>
will give you a quick overview.
I have better notes from (relay) application engineers but I'm busy baking, >tonight, so can only take quick pokes at my mail, etc.
<https://www.te.com/commerce/DocumentDelivery/DDEController?
Action=srchrtrv&DocNm=13C3311_AppNote&DocType=CS&DocLang=EN>
Is there an index to these useful Application Notes?
I only found this when I searched for "application notes":
https://www.te.com/en/products/relays-and-contactors/relays/intersection/application-notes.html
And did you happen to write some of them by chance?
On 8/24/2024 11:21 PM, Don Y wrote:
On 8/24/2024 10:03 PM, John Robertson wrote:
Good reading! It appears counterintuitive, but if I warp (!) my head around >>> it I should be able to sort it out.
Think about the extremes:
- with NO diode or snuber, there is nothing to keep the magnetic
field intact when the switch opens. So, the spring force is the
sole actor in play, pulling the contacts open
- with a diode, some current continues to "recirculate" in the coil
thus keeping the magnetic field active -- so it is trying to hold
the relay closed while the spring is trying to open it. *When*
the recirculating current falls, the field collapses and the relay
can open.
The zener lets the circuit look more like the "no diode" case.
i.e., imagine that case as having an infinite voltage zener (open
circuit) in series with the diode -- NO diode!
Don't kill yourself worrying about this sort of thing for a pin table.
You're likely not as concerned with durability. The board's already
(likely) designed. And, it's already got an established (tolerable?)
level of EMI.
Actually the 2nd link you provided that speaks of Coil Suppression with DC relays pointed out that the zener and regular diode in series had a drop -out time of almost the same as an unprotected coil,
but the EMI was limited to the
zener's rating along with the diode voltage drop.
What is interesting is if you
have a regular diode across the coil it takes around 5 times as long to decay.
That may actually matter in pinball games - one could get snappier coil action
with the zener/diode combination (or zener across the driver transistor - skipping the regular diode as mentioned in the note) and folks may be able to
notice that. Hmm, 1.9ms vs 9.8ms - can humans detect that when playing considering that for the most part 100ms is considered 'instantaneous'? I may have to set up a game and see...
"Many engineers use a rectifier diode alone to provide the transient suppression for relay coils. While this is cost effective and fully eliminates
the transient voltage, its impact on relay performance can be devastating. Problems of unexplained, random "tack welding" frequently occur in these systems."
Rather, it should be seen as a counter to the "old saw" that you *just* use >> a recirculating diode without considering the consequences. Just like
considering how to *drive* the coil based on how it will be used.
[Remember the "pull in" coils and "end of stroke" (EOS) switches on flipper >> solenoids?]
Oh, yes, we constantly deal with them, and pitted contacts on the EOS switches
in our shop.
Would zener diodes across the contacts help reduce pitting?
Rather, it should be seen as a counter to the "old saw" that you *just* use
a recirculating diode without considering the consequences. Just like >>>> considering how to *drive* the coil based on how it will be used.
[Remember the "pull in" coils and "end of stroke" (EOS) switches on flipper
solenoids?]
Oh, yes, we constantly deal with them, and pitted contacts on the EOS
switches in our shop.
But, you have the advantage of being able to pull the machine off the floor, >> and burnish and regap the contacts. And, you get some idea of how likely >> the need base on how many plays it sees.
Pinball games are more service trouble than most operators want to deal with. Anything we can do to make them more reliable is part of our business model.
And I hate repeat service calls.
If the device in question is on a mountain in tibet... (yes! <frown>)
Would zener diodes across the contacts help reduce pitting?
An RC snubber might work better.
But, *maintaining* a pin table is half the fun! (unless, of course, you
are in a business to make money from them!)
Ah, but you see that is the point of doing repairs that are better than (improve upon) the original design.
Back in the 80s I figured out why one
manufacturer's games were blowing up their driver transistors randomly and published the answer in the trade journals of the day. It was a ground issue, where the commons were done through Molex pins and as the pins aged the ground
connections generated resistance, which led to transistors not fully biasing off and burning out. The fix was to beef up the ground connections and all subsequent games we serviced never blew the driver transistors any more and customers were happy!
Need I mention the factory never acknowledged the error and did NOT implement my recommendations.
By the 80s they gained a reputation for unreliability that plagued them until they finally closed shop in the mid-90s. Oddly enough the same company was considered the Rolls-Royce of pinball up to the Solid State machines. Their solid state games looked and played great until the ground issues started randomly occurring (fried coils and driver transistors) and then operators would get rid of that manufacturer's games and buy other brand's machines.
There are two cases I deal with - commercial and home clients. Both would rather see less of my shop then they have to and I am happy to oblige them because I hate fixing simple things that could be made more durable. Also we've
gained a bit of a reputation of machines lasting longer after being serviced by
us and I wish to improve that score.
So, reducing contact arcing is very important to me and my customers!
We only get more business the better we fix the games!!
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