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Question about designing DC relays in DC application

G

Ge0rge Marutz

Jan 1, 1970
0
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Thanks,

George
 
G

Ge0rge Marutz

Jan 1, 1970
0
Oh boy... I just noticed I butchered this posts title. I must have
been distracted and didn't finish my thought. It should read "
Question about designing WITH relays in a DC application"

Forgive me?

George
 
T

Tom Bruhns

Jan 1, 1970
0
Ge0rge said:
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Thanks,

George

I believe you will find that exactly what happens when the contacts
opens is more important than which side is which. I think if you can
make sure the relay opens quickly, that will work best to prolong the
life of the contacts, given your resistive load at low voltage. To
that end, let the relay coil voltage fly back to as high a voltage as
you reasonably can, so the coil current decays as quickly as possible.
To a lesser degree, it's good to cause the relay to close as crisply as
you reasonably can. So a coil driver that over-voltages the coil
initially for a few milliseconds, and then goes to a lower voltage to
hold the relay in, can work well.

This is the sort of thing is something the relay manufacturer should be
able to help with, through either ap notes on their web site or
assistance from an aps engineer.

Cheers,
Tom
 
J

John Popelish

Jan 1, 1970
0
Ge0rge said:
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Unless this relay has magnetic blow outs on the contacts,
the polarity of the current through the contacts probably
makes no difference. My worry would be using a relay with
so little extra current rating at such a high temperature.
Is the coil rated to operate at 100C? Have you a derating
curve for the contact current rating versus temperature?
 
T

tlbs101

Jan 1, 1970
0
Ge0rge said:
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Thanks,

George

For your application, I have never heard or seen that it matters where
the hot side and the load side are connected, nor can I think of any
safety issues that would make a difference (unless this is an "open"
relay -- not encased in plastic).

I have read about, and used relays in other applications (i.e.
low-level mercury-wetted relays) where it seems to make a difference
where the "hot" side is connected to the relay contacts, because of the
way the armature "wets" the stator, but this doesn't apply in your
application.

It sounds like you are using the proper relay in the proper
application. To increase reliability make sure the relay is rated for
a temperature much higher than 100C. If you can find a relay with
contacts rated for higher current that would help, also.
 
G

Ge0rge Marutz

Jan 1, 1970
0
John said:
Unless this relay has magnetic blow outs on the contacts,
the polarity of the current through the contacts probably
makes no difference. My worry would be using a relay with
so little extra current rating at such a high temperature.
Is the coil rated to operate at 100C? Have you a derating
curve for the contact current rating versus temperature?

No magnetic blow outs (to extinguish arcs I assume). Just a simple
sealed relay.

Coil is rated to operate at 125C.

I have worked with the manufacture extensively regarding this relay.
It is not printed, but they guarantee performance of this relay up to
40A at 125C. I had them run a "test to failure" on 20 samples given my
worse case conditions. I need about 100,000 cycles out of the product
during its lifetime. Their testing revealed the first dropped out of
spec (although still functional) at 230,000 cycles. The better of the
batch went over 300,000 cycles before falling out of spec.

I am also running a similar test in parallel to confirm their results.
I feel fairly confident I have the right device for the job.

Thanks,

George
 
E

ehsjr

Jan 1, 1970
0
Ge0rge said:
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Thanks,

George

Swapping = no problem. Why not use a mosfet? Then you
don't need to worry about burning up the contacts.

Ed
 
S

scada

Jan 1, 1970
0
Ge0rge Marutz said:
I hopefully have a quick and simple one.

I use a form A relay to switch battery voltage to a grounded resistive
load. Does it matter which contact is always hot and which one ties to
the load? I have been assigning the common contact to VBAT and the
N.O. contact to the load side. However, it would be much cleaner and
more convenient to swap the two contacts in a new board layout. Would
this pose any problems?

I'm running this device at 13.5VDC, 25A. The part is rated for 16V,
30A. Ambient temperature in my application will reach approximately
100C. I will do anything I can to prolong its life expectancy. One
would think swapping contacts would be a no-brainer. However, there
are always got-cha's.

Thanks,

George

We use allot of high current contactors in DC applications. I can't think of
an instance that does not connect the load (to neg return) to the movable
contact and the source (pos) to the stationary contact. Of course, all our
contactors have magnetic blow out arc chutes and that alone depicts this
contact arrangement to clear the arc upwards. If the contacts were reversed
the arc would deflect downwards, possibly burning up the contacts, or coil.
I'm not certain if it matters with your relay, but why not configure it that
way to be safe! Also you can buy relays with magnetic blow outs for that
small of current.
 
G

Ge0rge Marutz

Jan 1, 1970
0
ehsjr said:
Swapping = no problem. Why not use a mosfet? Then you
don't need to worry about burning up the contacts.

Ed

Three reasons:

1.) Cost - a FET may be cheaper than a relay, but with support
circuitry and mechanical mods it is a more expensive solution.

2.) Heat dissipation - there is no room in my package space to add a
heat sink. I could modify the package space, but the heat sink
assembly and the modification adds to the cost.

3.) The potential for reverse voltage condition - current will conduct
through the body diode of a FET when power wires are connected
backwards (by mistake of course). This is not desireable in my
application. There are ways fix this, but they add to #1 COST.

A big thank you to everyone for participating in my request for help,

George
 
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