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Searching for engineer experienced in 24V vehicle electrical design.

Hello,

My company decided to port our 12V automotive product design to 24V for
sale in the European truck market. I am experiencing a problem with
the transistion and was wondering if you, or someone you may know,
would be willing to help. I am attempting to either prevent, or quickly
extinguish, the arcing effect across a set of relay contacts.

Reply to this post if you think you can help and I'll supply more
details.

Gerb
 
Because this is an area that I'm interested in, I googled this a bit
further and came up with instances of people using a MOV for preventing
this sort of thing. In general, the aim is to prevent the current surge
associated with switching inductive and to a certain extent
capacitative loads. The MOV, diode idea and transient voltage
suppressors are all candidates. Google any of the above, or "relay arc"
for more info.

I'd be curious to know what you come up with.

Chris
 
Because this is an area that I'm interested in, I googled this a bit
further and came up with instances of people using a MOV for preventing
this sort of thing. In general, the aim is to prevent the current surge
associated with switching inductive and to a certain extent
capacitative loads. The MOV, diode idea and transient voltage
suppressors are all candidates. Google any of the above, or "relay arc"
for more info.

I'd be curious to know what you come up with.

Chris

I am switching a wire wound resistor using a 24V high current relay.
The relay acts as a high side driver between the battery and grounded
load. Wire wound resistors have some inductance but it is very low (I
believe below 1uH). I do not think this is a contributing factor to my
problem. I may be wrong.

SAE says 24V automotive systems typically sit at 28V while charging.
They can drift up to 32V under normal operating conditions. I am
driving a 1 ohm load. My product seems to have no problem passing 28A
at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct.

I have tried all recommended approaches. RC network across relay
contacts, RC network across load, diode across load, MOV. No
suppression network seems to work. The only thing I found that works
is to increase the gap between relay contacts. I have not had a chance
to do extensive testing on modified relays yet to see if it is the
cure.

My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....

Thanks for your input Chris. I welcome you, and others, to join in for
more trouble shooting.

Gerb
 
R

Rich Grise

Jan 1, 1970
0
...at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct. ....
My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

Use a car starter solenoid for the relay. Better yet, truck. They handle
hundreds of amps, albeit intermittently. You could use both - the starter
solenoid closes a few tens or hundreds of millisecs before the main one,
and stays closed for awhile when the main one opens; but you might not
need this.

Obviously, you are running this thing right on the hairy edge of its
spec - you just need a beefier part.

Good Luck!
Rich
 
G

Gregory L. Hansen

Jan 1, 1970
0
I am switching a wire wound resistor using a 24V high current relay.

Completely off the subject, but I suddenly wondered whether wire wound
resistors introduce significant inductance. I've always thought of them
as just resistors that were manufactured a different way.
 
T

Tim Shoppa

Jan 1, 1970
0
My stock relays have fine grain silver contacts and are
gapped 0.4mm apart. The relays I modified are gapped
to approx 0.85mm. They seem to start arcing at about 36V

What are your "stock relays" rated at? 0.4mm seems like a really small
gap for even a 12V power relay.
They seem to start arcing at about 36V which
is outside the normal operation range of the vehicle
and should be good enough for me.

Way too close for me! Especially in an environment with extremely
broad temperature and humidity requirements...

Tim.
 
S

Spehro Pefhany

Jan 1, 1970
0
On Wed, 9 Mar 2005 01:28:38 +0000 (UTC), the renowned
Completely off the subject, but I suddenly wondered whether wire wound
resistors introduce significant inductance. I've always thought of them
as just resistors that were manufactured a different way.

Yes, some most certainly do. It can cause problems with current sense
resistors, for example.

Of course in this case, you've got the inductance of the resistor plus
the inductance related to the loop area of the wiring. And
high-current DC circuits just love to arc away anyhow.

One way around it would be to use a hybrid switch.


Best regards,
Spehro Pefhany
 
R

Rich Grise

Jan 1, 1970
0
What are your "stock relays" rated at? 0.4mm seems like a really small
gap for even a 12V power relay.


Way too close for me! Especially in an environment with extremely
broad temperature and humidity requirements...
Maybe he should be looking for a "Contactor".

Cheers!
Rich
 
S

Spehro Pefhany

Jan 1, 1970
0
Maybe he should be looking for a "Contactor".

Cheers!
Rich

Or hire a sub-contactor.



Best regards,
Spehro Pefhany
 
T

Tim Shoppa

Jan 1, 1970
0
Maybe he should be looking for a "Contactor".

For a 1A resistive load? This is well within the capabilities of any
automotive relay and they're made by the billions. They also have this
tendency to fail shorted, especially if they're a horn relay :)

Tim.
 
J

John Fields

Jan 1, 1970
0
For a 1A resistive load? This is well within the capabilities of any
automotive relay and they're made by the billions. They also have this
tendency to fail shorted, especially if they're a horn relay :)
 
J

John Fields

Jan 1, 1970
0
I am switching a wire wound resistor using a 24V high current relay.
The relay acts as a high side driver between the battery and grounded
load. Wire wound resistors have some inductance but it is very low (I
believe below 1uH). I do not think this is a contributing factor to my
problem. I may be wrong.

SAE says 24V automotive systems typically sit at 28V while charging.
They can drift up to 32V under normal operating conditions. I am
driving a 1 ohm load. My product seems to have no problem passing 28A
at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct.

I have tried all recommended approaches. RC network across relay
contacts, RC network across load, diode across load, MOV. No
suppression network seems to work. The only thing I found that works
is to increase the gap between relay contacts. I have not had a chance
to do extensive testing on modified relays yet to see if it is the
cure.

My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....

---
I think the most important things to do when trying to quench an arc
are to get the contacts as far apart as possible as quickly as
possible, and if you know that you're going to be using them in that
kind of service to get them made with the proper contact material.

Opening up the gap between the contacts on relays you already have
might help, but it may be that lowering the contact pressure by doing
that will hurt you in the long run by allowing bounce to last for a
longer time and changing the thermal characteristics of the contacts.
With less pressure their resistance will increase slightly, causing
them to heat up slightly, increasing their resistance a little more...
Also, because of the reduced pressure, the contacts won't separate as
quickly as they did before, which will give the arc a chance to live
longer. It's probably not _that_ serious, but they were designed the
way they were for a reason and when you start fooling around with the
mechanicals it doesn't usually end up serendipetously.

I think you're going to wind up in trouble unless you get the right
relays for the job. That is, _manufactured_ with the proper contact
spacing, current rating and contact materials. What does your
European competition use?
 
D

Don Murray

Jan 1, 1970
0
I am switching a wire wound resistor using a 24V high current relay.
The relay acts as a high side driver between the battery and grounded
load. Wire wound resistors have some inductance but it is very low (I
believe below 1uH). I do not think this is a contributing factor to my
problem. I may be wrong.

SAE says 24V automotive systems typically sit at 28V while charging.
They can drift up to 32V under normal operating conditions. I am
driving a 1 ohm load. My product seems to have no problem passing 28A
at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct.

I have tried all recommended approaches. RC network across relay
contacts, RC network across load, diode across load, MOV. No
suppression network seems to work. The only thing I found that works
is to increase the gap between relay contacts. I have not had a chance
to do extensive testing on modified relays yet to see if it is the
cure.

My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....

Thanks for your input Chris. I welcome you, and others, to join in for
more trouble shooting.

Gerb

Hi,
your problem is an old one when switching high currents and the
practical solution to this is a relay with blowout magnets at the
contacts.These extinguish the arc quickly by forming a magnetic field
thst
repels the plasma field .hope this helps
 
B

Bjarne Bäckström

Jan 1, 1970
0
John Fields said:
On 8 Mar 2005 15:03:20 -0800, [email protected] wrote: [...]
My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....

---
I think the most important things to do when trying to quench an arc
are to get the contacts as far apart as possible as quickly as
possible, and if you know that you're going to be using them in that
kind of service to get them made with the proper contact material.
[...]

One solution, that I've used sometimes, is to simply put a (FET)
transistor across the contacts. Turn the transistor on shortly before
the contacts open, and turn it off when they are fully open. It's a
simple solution -- and often cheaper than the alternatives.
 
F

Fred Bloggs

Jan 1, 1970
0
I am switching a wire wound resistor using a 24V high current relay.
The relay acts as a high side driver between the battery and grounded
load. Wire wound resistors have some inductance but it is very low (I
believe below 1uH). I do not think this is a contributing factor to my
problem. I may be wrong.

SAE says 24V automotive systems typically sit at 28V while charging.
They can drift up to 32V under normal operating conditions. I am
driving a 1 ohm load. My product seems to have no problem passing 28A
at 28V. A pretty good spark is generated when the relay contacts
separate, but this is manageable. My problem occurs when system
voltage rises to 32A at 32V. When the relay contacts separate a
sustained arc of approx 2-3 seconds is created. This arc is severely
damaging to the relay contacts. It melts the contact acting as anode.
One or two of these arcs kills the relay. The anode contact melts away
and the relay can no longer conduct.

I have tried all recommended approaches. RC network across relay
contacts, RC network across load, diode across load, MOV. No
suppression network seems to work. The only thing I found that works
is to increase the gap between relay contacts. I have not had a chance
to do extensive testing on modified relays yet to see if it is the
cure.

My stock relays have fine grain silver contacts and are gapped 0.4mm
apart. The relays I modified are gapped to approx 0.85mm. They seem
to start arcing at about 36V which is outside the normal operation
range of the vehicle and should be good enough for me.

I'm looking for an expert opinion from someone who has been down this
road before. Am I missing something? What does it take to extinguish
an arc? Am I on the right path, etc....

Thanks for your input Chris. I welcome you, and others, to join in for
more trouble shooting.

Gerb

Right- at these current levels, any passive type suppression network
will be huge. The problem is that contact voltage vs current curve
exceeds the sustained arc threshold for your relay. You can use a
smaller auxiliary relay to keep this in check with a voltage divider. At
32V the main relay opens on a 22V open circuit voltage, and the smaller
aux relay then drops out after delay and breaks the circuit completely
at a much reduced current 11 amps. K2 would be a 12V relay coil.
View in a fixed-width font such as Courier.
 
F

Fred Bloggs

Jan 1, 1970
0
John said:
---
I think the most important things to do when trying to quench an arc
are to get the contacts as far apart as possible as quickly as
possible, and if you know that you're going to be using them in that
kind of service to get them made with the proper contact material.

Opening up the gap between the contacts on relays you already have
might help, but it may be that lowering the contact pressure by doing
that will hurt you in the long run by allowing bounce to last for a
longer time and changing the thermal characteristics of the contacts.
With less pressure their resistance will increase slightly, causing
them to heat up slightly, increasing their resistance a little more...
Also, because of the reduced pressure, the contacts won't separate as
quickly as they did before, which will give the arc a chance to live
longer. It's probably not _that_ serious, but they were designed the
way they were for a reason and when you start fooling around with the
mechanicals it doesn't usually end up serendipetously.

I think you're going to wind up in trouble unless you get the right
relays for the job. That is, _manufactured_ with the proper contact
spacing, current rating and contact materials. What does your
European competition use?

The current rating of the contacts is a strong function of the force
with which they are held closed. He most certainly has reduced this
rating significantly by doubling the spacing- maybe halved it. Fiddling
with the armature travel is a dumb thing to do.
 
T

Tony Williams

Jan 1, 1970
0
. (bulk ceramic)
. 2.2 OHM
. .---||--/\/\--.
. | K2 |
. | |
. | |
. BATT>--+---||--------+---->>---to 1 ohm load
. K1 |
. ---/
. // \ 12V
. --- zener
. CNTL>--+ |
. | |
. K1|| K2||
. K1|| K2||
. K1|| K2||
. | |
. --- ---
. /// ///

Hmm.... my solution was going to involve a timed
MOSFET, but I don't think I'll bother now.

Maybe add a diode from the 1-ohm to Ground though.
This clamps any negative HV spike from the cabling
inductance.
 
T

Tim Shoppa

Jan 1, 1970
0
LOL, it's a one _ohm_ load...

My god, the mind boggles. 28V * 28A = 784Watts. Into a resistor. On
a truck. What the heck is this guy doing? Icemelting? Space heating?

Tim.
 
W

Winfield Hill

Jan 1, 1970
0
Tony Williams wrote...
Hmm.... my solution was going to involve a timed MOSFET, but
I don't think I'll bother now.

Maybe add a diode from the 1-ohm to Ground though. This
clamps any negative HV spike from the cabling inductance.

Nah. Just plop an Infineon PROFET Smart SIPMOS switch in there.
Get raw performance, short-circuit and thermal protection, and
an error indication, all at a low cost. For example, use a
BTS443, BTS432, BTS442, etc.

At first they called these a "Sense Highside Switch," and later,
protected-POWER "Smart High Side Switches," to help your searching.
http://www.infineon.com/cgi/ecrm.dll/ecrm/scripts/prod_cat.jsp?oid=-8171
Poke around a bit... BTW, DigiKey has a nice in-stock selection.
http://www.infineon.com/cmc_upload/migrated_files/document_files/Application_Notes/36341.pdf
 
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