Relay will not open (but will close)

Hi Folks-

I'm hoping some experienced EEs in the group can help me with a problem 
I'm having with a circuit using Kemet EC2-12NU miniature signal relays, 
a low voltage relay.  The switch will reliably close when the coil is 
energized.  The switch will open about 50% of time when the coil is 
de-energized.  There is no delay effect where it might open shortly 
after the coil is de-energized.  If it won't open, it will never open 
until I energize the coil and then de-energize again, at which point it 
might open.

We have three of these relays on our board all of which are under 
microcontroller control.  The failing one is different from the others 
because it switches the 12V supply voltage to a 12V->500V DC power 
supply.  The 500V power supply consumes up to 650 mA under full load. 
My testing is unloaded so I don't know what the no-load current is.  The 
relay behaves the same whether the 500V power supply is loaded or not. 
The contacts are rated at 2A.

The relay coil drives an internal DPDT switch.  I can clearly hear the 
switch click a little muffled when it won't switch.  I presume only half 
of the internal switching hardware is slamming when it fails to switch.

I believe there is some sort of external interaction that is causing the 
relay switch to refuse to open.  The relay functions 100% reliably when 
it's removed from the circuit.  The other relays in the circuit work 
reliably.

Can anyone offer any possible reasons why I'm seeing the relay fail to 
reliably open?

Thanks - JJS

On Tue, 20 Sep 2016 11:42:01 -0700, John Speth wrote:

> Hi Folks-
> 
> I'm hoping some experienced EEs in the group can help me with a problem
> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
> a low voltage relay.  The switch will reliably close when the coil is
> energized.  The switch will open about 50% of time when the coil is
> de-energized.  There is no delay effect where it might open shortly
> after the coil is de-energized.  If it won't open, it will never open
> until I energize the coil and then de-energize again, at which point it
> might open.
> 
> We have three of these relays on our board all of which are under
> microcontroller control.  The failing one is different from the others
> because it switches the 12V supply voltage to a 12V->500V DC power
> supply.  The 500V power supply consumes up to 650 mA under full load. My
> testing is unloaded so I don't know what the no-load current is.  The
> relay behaves the same whether the 500V power supply is loaded or not.
> The contacts are rated at 2A.
> 
> The relay coil drives an internal DPDT switch.  I can clearly hear the
> switch click a little muffled when it won't switch.  I presume only half
> of the internal switching hardware is slamming when it fails to switch.
> 
> I believe there is some sort of external interaction that is causing the
> relay switch to refuse to open.  The relay functions 100% reliably when
> it's removed from the circuit.  The other relays in the circuit work
> reliably.
> 
> Can anyone offer any possible reasons why I'm seeing the relay fail to
> reliably open?
> 
> Thanks - JJS

Maybe you're driving a capacitive load?  If it's a capacitive load then 
the contacts may be welding on make (because of the current surge).  Then 
not releasing on break because -- well, they've been welded.

There's a slight chance that the load is heavily inductive and the 
contacts are getting crapped up on break (because of sparking), but I'm 
much less sure of that.

A far less likely chance is that there's a permanent magnet lurking near 
that one relay, that's "helping" it stay closed.

If it's welding or sparking then a teardown of the relay, and of a brand 
new one for comparison, should show some significant differences in the 
contacts.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

John Speth wrote:

> Hi Folks-
>
> I'm hoping some experienced EEs in the group can help me with a problem 
> I'm having with a circuit using Kemet EC2-12NU miniature signal relays, 
> a low voltage relay.  The switch will reliably close when the coil is 
> energized.  The switch will open about 50% of time when the coil is 
> de-energized.  There is no delay effect where it might open shortly 
> after the coil is de-energized.  If it won't open, it will never open 
> until I energize the coil and then de-energize again, at which point it 
> might open.
>
> We have three of these relays on our board all of which are under 
> microcontroller control.  The failing one is different from the others 
> because it switches the 12V supply voltage to a 12V->500V DC power 
> supply.  The 500V power supply consumes up to 650 mA under full load. 
> My testing is unloaded so I don't know what the no-load current is.  The 
> relay behaves the same whether the 500V power supply is loaded or not. 
> The contacts are rated at 2A.
>
> The relay coil drives an internal DPDT switch.  I can clearly hear the 
> switch click a little muffled when it won't switch.  I presume only half 
> of the internal switching hardware is slamming when it fails to switch.
>
> I believe there is some sort of external interaction that is causing the 
> relay switch to refuse to open.  The relay functions 100% reliably when 
> it's removed from the circuit.  The other relays in the circuit work 
> reliably.
>
> Can anyone offer any possible reasons why I'm seeing the relay fail to 
> reliably open?
>
> Thanks - JJS

I'm wondering about damaging the coil driver circuit; some of those
relay coils can have a ton of inductive kickback.  How are you driving
the coil, and do you have an explicit clamp?

-- 
Rob Gaddi, Highland Technology -- www.highlandtechnology.com

Email address domain is currently out of order.  See above to fix.

On 9/20/2016 2:42 PM, John Speth wrote:
> Hi Folks-
>
> I'm hoping some experienced EEs in the group can help me with a problem
> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
> a low voltage relay.  The switch will reliably close when the coil is
> energized.  The switch will open about 50% of time when the coil is
> de-energized.  There is no delay effect where it might open shortly
> after the coil is de-energized.  If it won't open, it will never open
> until I energize the coil and then de-energize again, at which point it
> might open.
>
> We have three of these relays on our board all of which are under
> microcontroller control.  The failing one is different from the others
> because it switches the 12V supply voltage to a 12V->500V DC power
> supply.  The 500V power supply consumes up to 650 mA under full load. My
> testing is unloaded so I don't know what the no-load current is.  The
> relay behaves the same whether the 500V power supply is loaded or not.
> The contacts are rated at 2A.
>
> The relay coil drives an internal DPDT switch.  I can clearly hear the
> switch click a little muffled when it won't switch.  I presume only half
> of the internal switching hardware is slamming when it fails to switch.
>
> I believe there is some sort of external interaction that is causing the
> relay switch to refuse to open.  The relay functions 100% reliably when
> it's removed from the circuit.  The other relays in the circuit work
> reliably.
>
> Can anyone offer any possible reasons why I'm seeing the relay fail to
> reliably open?

You seem to be focusing on the contact side while the coil has not be 
analyzed so much.  A relay coil has make and break specs.  To energize 
the relay the minimum Pull-In Voltage must be met.  For the relay to 
de-energize the maximum Drop-Out Voltage must be met.  If your driver 
circuit has any leakage current it might be enough to keep the relay 
energized.

This is a 12 volt coil relay.  What is your driving circuit?

-- 

Rick C

On 20/09/2016 19:42, John Speth wrote:
> Hi Folks-
>
> I'm hoping some experienced EEs in the group can help me with a problem
> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
> a low voltage relay.  The switch will reliably close when the coil is
> energized.  The switch will open about 50% of time when the coil is
> de-energized.  There is no delay effect where it might open shortly
> after the coil is de-energized.  If it won't open, it will never open
> until I energize the coil and then de-energize again, at which point it
> might open.
>
> We have three of these relays on our board all of which are under
> microcontroller control.  The failing one is different from the others
> because it switches the 12V supply voltage to a 12V->500V DC power
> supply.  The 500V power supply consumes up to 650 mA under full load. My
> testing is unloaded so I don't know what the no-load current is.  The
> relay behaves the same whether the 500V power supply is loaded or not.
> The contacts are rated at 2A.
>
> The relay coil drives an internal DPDT switch.  I can clearly hear the
> switch click a little muffled when it won't switch.  I presume only half
> of the internal switching hardware is slamming when it fails to switch.
>
> I believe there is some sort of external interaction that is causing the
> relay switch to refuse to open.  The relay functions 100% reliably when
> it's removed from the circuit.  The other relays in the circuit work
> reliably.
>
> Can anyone offer any possible reasons why I'm seeing the relay fail to
> reliably open?

The Kemet EC2-12NU relay seems to have a polarised coil. Are you driving 
with the correct polarity?

Yes I know its a non-latching type but have experience when you drive 
some relays the wrong way that things didn't work out well!

That might not be the cause, in which case I'm also wondering if you're 
switching a capacitive load?

-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

On 9/20/2016 2:36 PM, rickman wrote:
> On 9/20/2016 2:42 PM, John Speth wrote:
>> Hi Folks-
>>
>> I'm hoping some experienced EEs in the group can help me with a problem
>> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
>> a low voltage relay.  The switch will reliably close when the coil is
>> energized.  The switch will open about 50% of time when the coil is
>> de-energized.  There is no delay effect where it might open shortly
>> after the coil is de-energized.  If it won't open, it will never open
>> until I energize the coil and then de-energize again, at which point it
>> might open.
>>
>> We have three of these relays on our board all of which are under
>> microcontroller control.  The failing one is different from the others
>> because it switches the 12V supply voltage to a 12V->500V DC power
>> supply.  The 500V power supply consumes up to 650 mA under full load. My
>> testing is unloaded so I don't know what the no-load current is.  The
>> relay behaves the same whether the 500V power supply is loaded or not.
>> The contacts are rated at 2A.
>>
>> The relay coil drives an internal DPDT switch.  I can clearly hear the
>> switch click a little muffled when it won't switch.  I presume only half
>> of the internal switching hardware is slamming when it fails to switch.
>>
>> I believe there is some sort of external interaction that is causing the
>> relay switch to refuse to open.  The relay functions 100% reliably when
>> it's removed from the circuit.  The other relays in the circuit work
>> reliably.
>>
>> Can anyone offer any possible reasons why I'm seeing the relay fail to
>> reliably open?
>
> You seem to be focusing on the contact side while the coil has not be
> analyzed so much.  A relay coil has make and break specs.  To energize
> the relay the minimum Pull-In Voltage must be met.  For the relay to
> de-energize the maximum Drop-Out Voltage must be met.  If your driver
> circuit has any leakage current it might be enough to keep the relay
> energized.

I can hear the coil slapping the contacts so I assume the coil is 
actuating adequately.  The two other relays in the circuit are behaving 
nicely.

> This is a 12 volt coil relay.  What is your driving circuit?

The driver is a FQD13N10L N-Channel MOSFET.  We have a clamping diode on 
all relay coils.  The gate is driven by a 3.3V push-pull digital output 
from a National Instruments uSB-6001 DAQ card.  The coil high side is 
connected to the 12V rail.  The MOSFET is swithing on the low side.

JJS

On 9/20/2016 3:10 PM, Mike Perkins wrote:
> On 20/09/2016 19:42, John Speth wrote:
>> Hi Folks-
>>
>> I'm hoping some experienced EEs in the group can help me with a problem
>> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
>> a low voltage relay.  The switch will reliably close when the coil is
>> energized.  The switch will open about 50% of time when the coil is
>> de-energized.  There is no delay effect where it might open shortly
>> after the coil is de-energized.  If it won't open, it will never open
>> until I energize the coil and then de-energize again, at which point it
>> might open.
>>
>> We have three of these relays on our board all of which are under
>> microcontroller control.  The failing one is different from the others
>> because it switches the 12V supply voltage to a 12V->500V DC power
>> supply.  The 500V power supply consumes up to 650 mA under full load. My
>> testing is unloaded so I don't know what the no-load current is.  The
>> relay behaves the same whether the 500V power supply is loaded or not.
>> The contacts are rated at 2A.
>>
>> The relay coil drives an internal DPDT switch.  I can clearly hear the
>> switch click a little muffled when it won't switch.  I presume only half
>> of the internal switching hardware is slamming when it fails to switch.
>>
>> I believe there is some sort of external interaction that is causing the
>> relay switch to refuse to open.  The relay functions 100% reliably when
>> it's removed from the circuit.  The other relays in the circuit work
>> reliably.
>>
>> Can anyone offer any possible reasons why I'm seeing the relay fail to
>> reliably open?
>
> The Kemet EC2-12NU relay seems to have a polarised coil. Are you driving
> with the correct polarity?

I double checked that.  I even wired one on a bench breadboard to 
confirm our circuit application of the part.

> Yes I know its a non-latching type but have experience when you drive
> some relays the wrong way that things didn't work out well!
>
> That might not be the cause, in which case I'm also wondering if you're
> switching a capacitive load?

I'm wondering about the same thing.  The HV power supply is a Matsusada 
RD6-0.5P-LU.  The data sheet is kinda lacking in characterization data 
for extreme and transient conditions.  It's all typical steady state data.

JJS

On 9/20/2016 12:28 PM, Rob Gaddi wrote:
> John Speth wrote:
>
>> Hi Folks-
>>
>> I'm hoping some experienced EEs in the group can help me with a problem
>> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
>> a low voltage relay.  The switch will reliably close when the coil is
>> energized.  The switch will open about 50% of time when the coil is
>> de-energized.  There is no delay effect where it might open shortly
>> after the coil is de-energized.  If it won't open, it will never open
>> until I energize the coil and then de-energize again, at which point it
>> might open.
>>
>> We have three of these relays on our board all of which are under
>> microcontroller control.  The failing one is different from the others
>> because it switches the 12V supply voltage to a 12V->500V DC power
>> supply.  The 500V power supply consumes up to 650 mA under full load.
>> My testing is unloaded so I don't know what the no-load current is.  The
>> relay behaves the same whether the 500V power supply is loaded or not.
>> The contacts are rated at 2A.
>>
>> The relay coil drives an internal DPDT switch.  I can clearly hear the
>> switch click a little muffled when it won't switch.  I presume only half
>> of the internal switching hardware is slamming when it fails to switch.
>>
>> I believe there is some sort of external interaction that is causing the
>> relay switch to refuse to open.  The relay functions 100% reliably when
>> it's removed from the circuit.  The other relays in the circuit work
>> reliably.
>>
>> Can anyone offer any possible reasons why I'm seeing the relay fail to
>> reliably open?
>>
>> Thanks - JJS
>
> I'm wondering about damaging the coil driver circuit; some of those
> relay coils can have a ton of inductive kickback.  How are you driving
> the coil, and do you have an explicit clamp?

You could be right about that but the fact that there are two other 
relays being driven identically makes me doubt that the coil could could 
be the culprit.  The relay data sheet makes a vague statement that 
holding the coil energized can heat the coil and emit an organic gas 
that can destroy the contacts.  There was no quantifiable information 
the accompany the statement.

My conclusion: Don't ignore the possibility of the coil being part of 
the problem.

JJS

On 9/20/2016 11:56 AM, Tim Wescott wrote:
> On Tue, 20 Sep 2016 11:42:01 -0700, John Speth wrote:
>
>> Hi Folks-
>>
>> I'm hoping some experienced EEs in the group can help me with a problem
>> I'm having with a circuit using Kemet EC2-12NU miniature signal relays,
>> a low voltage relay.  The switch will reliably close when the coil is
>> energized.  The switch will open about 50% of time when the coil is
>> de-energized.  There is no delay effect where it might open shortly
>> after the coil is de-energized.  If it won't open, it will never open
>> until I energize the coil and then de-energize again, at which point it
>> might open.
>>
>> We have three of these relays on our board all of which are under
>> microcontroller control.  The failing one is different from the others
>> because it switches the 12V supply voltage to a 12V->500V DC power
>> supply.  The 500V power supply consumes up to 650 mA under full load. My
>> testing is unloaded so I don't know what the no-load current is.  The
>> relay behaves the same whether the 500V power supply is loaded or not.
>> The contacts are rated at 2A.
>>
>> The relay coil drives an internal DPDT switch.  I can clearly hear the
>> switch click a little muffled when it won't switch.  I presume only half
>> of the internal switching hardware is slamming when it fails to switch.
>>
>> I believe there is some sort of external interaction that is causing the
>> relay switch to refuse to open.  The relay functions 100% reliably when
>> it's removed from the circuit.  The other relays in the circuit work
>> reliably.
>>
>> Can anyone offer any possible reasons why I'm seeing the relay fail to
>> reliably open?
>>
>> Thanks - JJS
>
> Maybe you're driving a capacitive load?  If it's a capacitive load then
> the contacts may be welding on make (because of the current surge).  Then
> not releasing on break because -- well, they've been welded.
>
> There's a slight chance that the load is heavily inductive and the
> contacts are getting crapped up on break (because of sparking), but I'm
> much less sure of that.
>
> A far less likely chance is that there's a permanent magnet lurking near
> that one relay, that's "helping" it stay closed.
>
> If it's welding or sparking then a teardown of the relay, and of a brand
> new one for comparison, should show some significant differences in the
> contacts.

Tim, I believe you might have given me the next line of investigation. 
I need to address the capacitive load possibility next.  I added a clamp 
diode across the switch "output" to ground (cathode at 12V, anode to 
ground) believing it would clamp turn off inductive transients.  It 
didn't help but it wasn't incorrect for the inductive load possibility. 
I need to get a current probe.  My senior EE associate will take over 
the problem soon.

JJS

I humbly thank all respondents for their help.  There is certainly a 
wealth of hard-earned knowledge here considering how small the news 
group has gotten.

In a nutshell, I think the contacts are welding shut due to arcing. 
That's because we have little engineering information about the load 
that's being switched.  We will characterize the power supply and take 
steps to cor3ect deficiencies.

JJS