G
GerberMultit00l
First of all let me say thank you for the sudden overwhelming burst of
support. I'll try to address all responses if it makes sense to do so.
1.) Rich Grise Wrote
Unfortunately this is not an option. I am confined to the space
allocated to me in our product packaging. I have an area of
approximately 26mm wide x 26mm long x 30mm high to fit a relay
solution. This device also needs to be PCB mounted. And finally, it
must be able to function in the temperature range from -40C to 72C. It
must survive without damage (non operational) from -50C to 125C.
I am using the chosen device near its rated limit but still within a
reasonable safety margin of 15% below published limit (which the
manufacture says is very conservative). I would like to see a larger
safety margin and am working on that.
My problems start to occur when system voltage increases out of the
specified limits for sustained durations of time (as will occur in any
vehicle from time to time). I tried implementing a new over voltage
protection scheme that will shut the relay off and protect the contacts
long before system voltage rises to a level that will be damaging to
the relay.
Take a look at the relay specs for yourself here:
http://relays.tycoelectronics.com/datasheets/134J-VF7.pdf
Tim also wrote: "
True, I'm playing it close. However, these relays have been used in
rougher environments than what I am attempting and seem to survive.
I'm confident that with a little tweaking I can get this to work.
I do not believe I have a big problem with inductance. I say this
because if the load did have a large amount of inductance I should see
a fairly large negative voltage spike (at least a couple hundred volt)
when turning off the load. I see a negative going voltage spike of
about 8V, not even enough to go below ground from the 28V starting
point.
Loop inductance due to wiring harness may be an issue with my test
setup. My product is inside an environmenal chamber and the power
supply is outside the chamber. The power lead going to the product is
approximately 3m and so is the ground return line. Wire gage is 8AWG.
I do see a small bump in voltage across the main power terminals of my
product (about 10V for 1us) when it makes or breaks the load. This
bump decreases as I lower load current so it must be inductance.
The in-vehicle application might be different. The power lead will be
restricted to below 2m and the ground return will be tied to the
vehicle chasis local to the product.
Now Spehro, how about this Hybrid Switch you were talking about?
The manuacture offers a number of contact materials for this device. I
was contemplating trying out Silver Cadmium Oxide, Silver Tin Indium
Oxide, and Silver Copper Nickel. However, the manufacture sticks by
their guns saying that fine grain Silver is still the best material for
my application.
I don't know if I agree with what you say in this instance. I am
increasing contact gap by filing down the armature stop of the relay so
the entire armature mechanism can swing away from the opposing contact
a little more. I am not affecting the mechanism the contact is mounted
to or its spring tension. You would have to see the internals of the
relay to understand what I say. I wish I could post pictures. The
manufactures images of this relay only show it with a cover, none
without.
Think of this relay design being like a door with a door stop behind it
preventing the knob from hitting the wall. A spring pulls the door
open and towards the door stop. I'm just grinding the door stop down a
little. The spring is under a certain amount of tension when the
armature is held 0.4mm from the opposing contact (stock relay). The
spring wants to pull the contact back to rest but it can't because of
the doorstop. By grinding the doorstop down I allow the door to swing
further back towards rest. The spring is still under tension, just a
little less since it is closer to rest position.
Now say someone wants to close the door. It takes a larger magnetic
force to get it started on its way back to close because of the
increase in distance (equivalent to a higher pull-in voltage
requirement). Once the door passes the 0.4mm mark on its way towards
being closed (the opposing contact) it is just like a stock unit. The
hold in force is the same whether the door was originally opened 0.4mm,
0.8mm, or 1.2mm
Drawing the armature in from a further distance will requires more
pull-in force. But, I have more than adequate pull in force now. A
stock relay pulls in at about 14.4V at 25C. I do not need to activate
the relay unless system voltage is above 25V. So, if the pull in
voltage of the relay increased to 16V because of my modification it
still does not affect me.
This discussion about weak contact pressure brings up an important
point. I might have found something that contributed to my recent
failures. I was driving the relay coil with a specific high voltage
NPN small signal transistor. I found that the relay coils resistance
was about 100 ohm lower than what was published. So, the required
drive current for this coil went up. The transistor did not have
enough DC current gain at higher temperatures to keep up. Therefore
the transistor was not turning on all the way. There was only 14V
across the coil! This was barely enough to pull it in, and in some
instances it would not. Since then I replaced this transistor with one
that has more than adequate current gain. The armature now snaps into
place at any temperature.
I agree. I chose this relay for one reason. It is the only one I
could find that fits my packaging requirements that can operate at the
capacity I need it to. It just so happens that the stock unit cannot
handle those rare instances where system voltage ramps up out of spec.
However, lucky for me this relay is also highly customizable. I can
have it built for what I need. I just need help in determining what is
good enough.
What do my competitors use? I could not tell you. This is a new
product to the market and we do not have competitors. That, and I
was asked to design a device for a market I am not familiar with.
Everything I learned about 24V vehicle systems so far either from a
lacking SAE specification on the subject or just hear say. So, I'm
venturing into a new frontier. Not the best situation to be in, but I
am there nevertheless.
Yep, I looked into this. Not practical for the small outline of this
relay. I asked the manufacture to try and find a solution.
I also considered this design approach. It may still be feasible and I
intend on using it as a last ditch effort. However, there are a few
obstacles to overcome.
Since I am switching my load with a high side drive I would need a
P-Channel FET across the contacts. You just can't find high voltage,
high current P-CH FETS with a low enough on resistance to keep internal
power dissipation down while passing up to 30A. That and I have no
room to heat sink the device. My total allocated space for relay and
FET is 26mm x 26mm x 30mm. Not that heat sinking would do any good
since my electronics are sealed inside a small glass-filled nylon
enclosure. I would need to hold the device on for 50ms to 100ms at a
time. It would get hot after a few activations. This might be
managable if ambient temperatures could be held to a reasonable level.
However, my product will be mounted under the hood of heavy trucks and
utility vehicles. Temperatures could easily approach 50C.
Then there is the issue of time. My management wants to release this
product later this spring. I need time to come to a solution and then
do adequate testing before I let it hit the street. I've passed the
point of no return with my current design approach.
7.) Fred Bloggs proposed a clever design idea. If I don't get
anywhere with my current design I'll surely try this. Heck, I might
just do it anyway for piece of mind. It performs a similar function to
the FET solution I was contemplating but will probably be cheaper to
implement.
I would have to reduce the size of my main relay in order to fit a
small secondary relay in my packaging space. I know of a few smaller
main relays that can easily carry the current levels I am working with.
What was killing me is making and breaking the contacts at high
voltage. Fred's solution would correct that with a modest increase in
component count.
Question though, what causes the secondary relay to drop out. Doesn't
it hold itself in after the main relay (K1) drops out?
John Fields also brought this up. I tried to voice an arguement
against this above. Maybe this was just my ignorance. Mind to
ellaborate on why you say this? Just trying to learn. I went into
this project taking relay technology for granted. I never intended to
get so far in depth with it but I am very quickly finding out
otherwise.
I tried the diode trick but found it had no effect. I believe this is
because I am not experiencing problems due to inductance. I think I'm
just over stressing the relay contacts at high voltage extremes.
Something similar to that
.
I would love to use a solid state solution. I wanted to do this in a
bad way. Problem though. I can't keep a FET cool enough. My product
must operate up to 70C ambient. The FET would be passing up to 30A for
2.5 minutes. Even if I could use a ultra low RDSON part of say 4 mohm
it would still be dissipating 3.6W. Stack a few in parallel and I
still won't be able to remove the heat. My electronics are embedded in
a small sealed glass filled Nylon box (sealed from the elements that
is). Even if I could fit a heat sink inside my packaging space I still
don't have a way to get the hot air out and cool air in. We cannot
re-tool the housing to accomodate a heat sink at this point.
That and then there is the issue of reverse battery connection and the
internal body diode of the FET(s). I would need to add a rather large
diode in series with the load to prevent damaging reverse current flow.
This diode would take up even more space.
To put it bluntly, I'm try to cram 20 pounds of sh__ in a 5 pound bag.
Well, that's all for now. Hopefully you all won't fall asleep reading
through all this. Sorry for dragging this on forever but I wanted to
spell out more details to clue you in on my specific situation.
Thanks a bunch
George "Gerb" Marutz II
support. I'll try to address all responses if it makes sense to do so.
1.) Rich Grise Wrote
Unfortunately this is not an option. I am confined to the space
allocated to me in our product packaging. I have an area of
approximately 26mm wide x 26mm long x 30mm high to fit a relay
solution. This device also needs to be PCB mounted. And finally, it
must be able to function in the temperature range from -40C to 72C. It
must survive without damage (non operational) from -50C to 125C.
I am using the chosen device near its rated limit but still within a
reasonable safety margin of 15% below published limit (which the
manufacture says is very conservative). I would like to see a larger
safety margin and am working on that.
My problems start to occur when system voltage increases out of the
specified limits for sustained durations of time (as will occur in any
vehicle from time to time). I tried implementing a new over voltage
protection scheme that will shut the relay off and protect the contacts
long before system voltage rises to a level that will be damaging to
the relay.
2.) Tim Shoppa said:
Take a look at the relay specs for yourself here:
http://relays.tycoelectronics.com/datasheets/134J-VF7.pdf
Tim also wrote: "
True, I'm playing it close. However, these relays have been used in
rougher environments than what I am attempting and seem to survive.
I'm confident that with a little tweaking I can get this to work.
resistors, for example.
high-current >DC circuits just love to arc away anyhow.
I do not believe I have a big problem with inductance. I say this
because if the load did have a large amount of inductance I should see
a fairly large negative voltage spike (at least a couple hundred volt)
when turning off the load. I see a negative going voltage spike of
about 8V, not even enough to go below ground from the 28V starting
point.
Loop inductance due to wiring harness may be an issue with my test
setup. My product is inside an environmenal chamber and the power
supply is outside the chamber. The power lead going to the product is
approximately 3m and so is the ground return line. Wire gage is 8AWG.
I do see a small bump in voltage across the main power terminals of my
product (about 10V for 1us) when it makes or breaks the load. This
bump decreases as I lower load current so it must be inductance.
The in-vehicle application might be different. The power lead will be
restricted to below 2m and the ground return will be tied to the
vehicle chasis local to the product.
Now Spehro, how about this Hybrid Switch you were talking about?
4.) John Fields said:
The manuacture offers a number of contact materials for this device. I
was contemplating trying out Silver Cadmium Oxide, Silver Tin Indium
Oxide, and Silver Copper Nickel. However, the manufacture sticks by
their guns saying that fine grain Silver is still the best material for
my application.
John said:
quickly as they did before, which will give the arc a chance to live
mechanicals it doesn't usually end up serendipetously.
I don't know if I agree with what you say in this instance. I am
increasing contact gap by filing down the armature stop of the relay so
the entire armature mechanism can swing away from the opposing contact
a little more. I am not affecting the mechanism the contact is mounted
to or its spring tension. You would have to see the internals of the
relay to understand what I say. I wish I could post pictures. The
manufactures images of this relay only show it with a cover, none
without.
Think of this relay design being like a door with a door stop behind it
preventing the knob from hitting the wall. A spring pulls the door
open and towards the door stop. I'm just grinding the door stop down a
little. The spring is under a certain amount of tension when the
armature is held 0.4mm from the opposing contact (stock relay). The
spring wants to pull the contact back to rest but it can't because of
the doorstop. By grinding the doorstop down I allow the door to swing
further back towards rest. The spring is still under tension, just a
little less since it is closer to rest position.
Now say someone wants to close the door. It takes a larger magnetic
force to get it started on its way back to close because of the
increase in distance (equivalent to a higher pull-in voltage
requirement). Once the door passes the 0.4mm mark on its way towards
being closed (the opposing contact) it is just like a stock unit. The
hold in force is the same whether the door was originally opened 0.4mm,
0.8mm, or 1.2mm
Drawing the armature in from a further distance will requires more
pull-in force. But, I have more than adequate pull in force now. A
stock relay pulls in at about 14.4V at 25C. I do not need to activate
the relay unless system voltage is above 25V. So, if the pull in
voltage of the relay increased to 16V because of my modification it
still does not affect me.
This discussion about weak contact pressure brings up an important
point. I might have found something that contributed to my recent
failures. I was driving the relay coil with a specific high voltage
NPN small signal transistor. I found that the relay coils resistance
was about 100 ohm lower than what was published. So, the required
drive current for this coil went up. The transistor did not have
enough DC current gain at higher temperatures to keep up. Therefore
the transistor was not turning on all the way. There was only 14V
across the coil! This was barely enough to pull it in, and in some
instances it would not. Since then I replaced this transistor with one
that has more than adequate current gain. The armature now snaps into
place at any temperature.
I agree. I chose this relay for one reason. It is the only one I
could find that fits my packaging requirements that can operate at the
capacity I need it to. It just so happens that the stock unit cannot
handle those rare instances where system voltage ramps up out of spec.
However, lucky for me this relay is also highly customizable. I can
have it built for what I need. I just need help in determining what is
good enough.
What do my competitors use? I could not tell you. This is a new
product to the market and we do not have competitors. That, and I
was asked to design a device for a market I am not familiar with.
Everything I learned about 24V vehicle systems so far either from a
lacking SAE specification on the subject or just hear say. So, I'm
venturing into a new frontier. Not the best situation to be in, but I
am there nevertheless.
5.) Don Murray said:
Yep, I looked into this. Not practical for the small outline of this
relay. I asked the manufacture to try and find a solution.
6.) Bjarne Bäckström said:
I also considered this design approach. It may still be feasible and I
intend on using it as a last ditch effort. However, there are a few
obstacles to overcome.
Since I am switching my load with a high side drive I would need a
P-Channel FET across the contacts. You just can't find high voltage,
high current P-CH FETS with a low enough on resistance to keep internal
power dissipation down while passing up to 30A. That and I have no
room to heat sink the device. My total allocated space for relay and
FET is 26mm x 26mm x 30mm. Not that heat sinking would do any good
since my electronics are sealed inside a small glass-filled nylon
enclosure. I would need to hold the device on for 50ms to 100ms at a
time. It would get hot after a few activations. This might be
managable if ambient temperatures could be held to a reasonable level.
However, my product will be mounted under the hood of heavy trucks and
utility vehicles. Temperatures could easily approach 50C.
Then there is the issue of time. My management wants to release this
product later this spring. I need time to come to a solution and then
do adequate testing before I let it hit the street. I've passed the
point of no return with my current design approach.
7.) Fred Bloggs proposed a clever design idea. If I don't get
anywhere with my current design I'll surely try this. Heck, I might
just do it anyway for piece of mind. It performs a similar function to
the FET solution I was contemplating but will probably be cheaper to
implement.
I would have to reduce the size of my main relay in order to fit a
small secondary relay in my packaging space. I know of a few smaller
main relays that can easily carry the current levels I am working with.
What was killing me is making and breaking the contacts at high
voltage. Fred's solution would correct that with a modest increase in
component count.
Question though, what causes the secondary relay to drop out. Doesn't
it hold itself in after the main relay (K1) drops out?
John Fields also brought this up. I tried to voice an arguement
against this above. Maybe this was just my ignorance. Mind to
ellaborate on why you say this? Just trying to learn. I went into
this project taking relay technology for granted. I never intended to
get so far in depth with it but I am very quickly finding out
otherwise.
8.) Tony Williams said:
I tried the diode trick but found it had no effect. I believe this is
because I am not experiencing problems due to inductance. I think I'm
just over stressing the relay contacts at high voltage extremes.
On a truck. What the heck is this guy doing? Icemelting? Space9.) Tim Shoppa said:
Something similar to that
.10.) Winfield Hill said:
I would love to use a solid state solution. I wanted to do this in a
bad way. Problem though. I can't keep a FET cool enough. My product
must operate up to 70C ambient. The FET would be passing up to 30A for
2.5 minutes. Even if I could use a ultra low RDSON part of say 4 mohm
it would still be dissipating 3.6W. Stack a few in parallel and I
still won't be able to remove the heat. My electronics are embedded in
a small sealed glass filled Nylon box (sealed from the elements that
is). Even if I could fit a heat sink inside my packaging space I still
don't have a way to get the hot air out and cool air in. We cannot
re-tool the housing to accomodate a heat sink at this point.
That and then there is the issue of reverse battery connection and the
internal body diode of the FET(s). I would need to add a rather large
diode in series with the load to prevent damaging reverse current flow.
This diode would take up even more space.
To put it bluntly, I'm try to cram 20 pounds of sh__ in a 5 pound bag.
Well, that's all for now. Hopefully you all won't fall asleep reading
through all this. Sorry for dragging this on forever but I wanted to
spell out more details to clue you in on my specific situation.
Thanks a bunch
George "Gerb" Marutz II
