[EE] Substitute for mechanical contacts

[Van Horn, David]

Is there yet a device that will substitute for real relay contacts in an AC circuit?

I have a test fixture in mind, which is complicated, but the interesting part is discharging a low impedance capacitor of about 0.1uF at 1kV into a coil, and allowing the current to ring down.
Spice tells me to expect currents in the kA range, and a prototype built with a relay contact seems to confirm this. Tungsten contacts take a lot of abuse, and they also generate a lot of heat.

Minimal voltage drop, ability to trigger "closure" and "opening" at any point in time, huge currents, and bidirectional currents without turning off are all requirements.





--
David VanHorn
Lead Hardware Engineer

Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO 80301 USA
phone: [phone] x110
email: ***@backcountryaccess.com<mailto:***@backcountryaccess.com>

[AB, Pearce - UKRI STFC]

Presumably you are really only interested in three things: -

1. Being able to sustain around 1kV while the contacts are open.
2. Being able to handle high current on contact closure.
3. Low contact resistance when closed.
4. Are not worried about a voltage spec on contacts opening (is the recharge voltage turned on separately?)?

Its ages ago (like back when I was an apprentice) but an RT unit I used to deal with used a relay with contacts about 3/16" diameter (they were lightly domed) to handle large currents for operating a 75W transmitter off 12V or 24V. I remember the contacts as being silver plated brass (don't recall coming across tungsten contacts), and I suspect you are after something similar. I cannot recall if they would open far enough to take 1kV in air, but removal of the back contact would allow them to open wider. These were a large (about 2" x 3" footprint open frame relay without a cover, and I would have thought something similar would suit your purpose. I'm afraid I don't have any info on manufacturer or model, but at the time I always thought they would be ideal relays to use for switching driving lights on a car ...

Post by Van Horn, David
Is there yet a device that will substitute for real relay contacts in an AC circuit?
I have a test fixture in mind, which is complicated, but the interesting part is discharging a low
impedance capacitor of about 0.1uF at 1kV into a coil, and allowing the current to ring down.
Spice tells me to expect currents in the kA range, and a prototype built with a relay contact seems
to confirm this. Tungsten contacts take a lot of abuse, and they also generate a lot of heat.
Minimal voltage drop, ability to trigger "closure" and "opening" at any point in time, huge currents,
and bidirectional currents without turning off are all requirements.

[Van Horn, David]

1. Being able to sustain around 1kV while the contacts are open.

At least 1kV before the initial contact closure. After that it gets hard to calculate since there may be residual energy circulating.

2. Being able to handle high current on contact closure.

I'm closing a very low inductance capacitor into a small (10's of uH) inductance. Current will ramp up FAST.

3. Low contact resistance when closed.

And low voltage drop, though mechanical contacts may be actually worse than I think in real life. Still lower loss is better.


4. Are not worried about a voltage spec on contacts opening (is the recharge voltage turned on separately?)?

Recharge happens as the contacts open, from a different inductor which has been charging while the contacts were closed.
Current there is <1A, but the inductor is substantial. One of the nasty things I want to reduce is the losses as the mech contacts open and close to arcs across the contacts.
The original solution with tungsten contacts work, but the contacts die (no surprise)


-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of AB, Pearce - UKRI STFC
Sent: Thursday, April 5, 2018 9:32 AM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: RE: [EE] Substitute for mechanical contacts

Presumably you are really only interested in three things: -

1. Being able to sustain around 1kV while the contacts are open.
2. Being able to handle high current on contact closure.
3. Low contact resistance when closed.
4. Are not worried about a voltage spec on contacts opening (is the recharge voltage turned on separately?)?

Its ages ago (like back when I was an apprentice) but an RT unit I used to deal with used a relay with contacts about 3/16" diameter (they were lightly domed) to handle large currents for operating a 75W transmitter off 12V or 24V. I remember the contacts as being silver plated brass (don't recall coming across tungsten contacts), and I suspect you are after something similar. I cannot recall if they would open far enough to take 1kV in air, but removal of the back contact would allow them to open wider. These were a large (about 2" x 3" footprint open frame relay without a cover, and I would have thought something similar would suit your purpose. I'm afraid I don't have any info on manufacturer or model, but at the time I always thought they would be ideal relays to use for switching driving lights on a car ...

Post by Van Horn, David
Is there yet a device that will substitute for real relay contacts in an AC circuit?
I have a test fixture in mind, which is complicated, but the
interesting part is discharging a low impedance capacitor of about 0.1uF at 1kV into a coil, and allowing the current to ring down.
Spice tells me to expect currents in the kA range, and a prototype
built with a relay contact seems to confirm this. Tungsten contacts take a lot of abuse, and they also generate a lot of heat.
Minimal voltage drop, ability to trigger "closure" and "opening" at
any point in time, huge currents, and bidirectional currents without turning off are all requirements.

--
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[Denny Esterline]

High energy capacitive discharge is fairly well trodden ground. See if any
of these give you some food for thought.
http://www.electricstuff.co.uk/destructotron.html
http://www.capturedlightning.com/frames/shrinkergallery.html
https://hackaday.com/2012/09/28/how-a-quarter-shrinker-works/

-Denny


On Thu, Apr 5, 2018 at 9:01 AM, Van Horn, David <

Post by AB, Pearce - UKRI STFC
1. Being able to sustain around 1kV while the contacts are open.
At least 1kV before the initial contact closure. After that it gets hard
to calculate since there may be residual energy circulating.
2. Being able to handle high current on contact closure.
I'm closing a very low inductance capacitor into a small (10's of uH)
inductance. Current will ramp up FAST.
3. Low contact resistance when closed.
And low voltage drop, though mechanical contacts may be actually worse
than I think in real life. Still lower loss is better.
4. Are not worried about a voltage spec on contacts opening (is the
recharge voltage turned on separately?)?
Recharge happens as the contacts open, from a different inductor which has
been charging while the contacts were closed.
Current there is <1A, but the inductor is substantial. One of the nasty
things I want to reduce is the losses as the mech contacts open and close
to arcs across the contacts.
The original solution with tungsten contacts work, but the contacts die (no surprise)
-----Original Message-----
Sent: Thursday, April 5, 2018 9:32 AM
Subject: RE: [EE] Substitute for mechanical contacts
Presumably you are really only interested in three things: -
1. Being able to sustain around 1kV while the contacts are open.
2. Being able to handle high current on contact closure.
3. Low contact resistance when closed.
4. Are not worried about a voltage spec on contacts opening (is the
recharge voltage turned on separately?)?
Its ages ago (like back when I was an apprentice) but an RT unit I used to
deal with used a relay with contacts about 3/16" diameter (they were
lightly domed) to handle large currents for operating a 75W transmitter off
12V or 24V. I remember the contacts as being silver plated brass (don't
recall coming across tungsten contacts), and I suspect you are after
something similar. I cannot recall if they would open far enough to take
1kV in air, but removal of the back contact would allow them to open wider.
These were a large (about 2" x 3" footprint open frame relay without a
cover, and I would have thought something similar would suit your purpose.
I'm afraid I don't have any info on manufacturer or model, but at the time
I always thought they would be ideal relays to use for switching driving
lights on a car ...

Post by Van Horn, David
Is there yet a device that will substitute for real relay contacts in an

AC circuit?

Post by Van Horn, David
I have a test fixture in mind, which is complicated, but the
interesting part is discharging a low impedance capacitor of about 0.1uF

at 1kV into a coil, and allowing the current to ring down.

Post by Van Horn, David
Spice tells me to expect currents in the kA range, and a prototype
built with a relay contact seems to confirm this. Tungsten contacts

take a lot of abuse, and they also generate a lot of heat.

Post by Van Horn, David
Minimal voltage drop, ability to trigger "closure" and "opening" at
any point in time, huge currents, and bidirectional currents without

turning off are all requirements.
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mailman/listinfo/piclist
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[Van Horn, David]

http://www.capturedlightning.com/frames/shrinkergallery.html


I have a few of Bert's larger "shockfossils". At home is my prize, one of the few trees he's made.
Loading Image...
Not that one, but almost identical.

[Van Horn, David]

High energy capacitive discharge is fairly well trodden ground. See if any of these give you some food for thought.
http://www.electricstuff.co.uk/destructotron.html
http://www.capturedlightning.com/frames/shrinkergallery.html
https://hackaday.com/2012/09/28/how-a-quarter-shrinker-works/


These are primarily "one shot" applications, I need tens of pulses per second.

[Denny Esterline]

Post by Van Horn, David
These are primarily "one shot" applications, I need tens of pulses per second.
Ok, that's a bit more energy. :-)

It's been a few years since I looked at capacitive discharge "toys"... But
I'm nearly
positive that some of them were using semiconductor switching. Beefy
hockey-puck
IGBTs if memory serves.

-Denny

[Adam Field]

On Thu, Apr 5, 2018 at 1:47 PM, Van Horn, David <

Post by Denny Esterline
High energy capacitive discharge is fairly well trodden ground. See if any
of these give you some food for thought.
http://www.electricstuff.co.uk/destructotron.html
http://www.capturedlightning.com/frames/shrinkergallery.html
https://hackaday.com/2012/09/28/how-a-quarter-shrinker-works/
These are primarily "one shot" applications, I need tens of pulses per second.

I designed and built a few of these for my day job (larger capacitor maker
you would recognize). They are "hockey puck" SCRs with multiple diodes in
parallel to catch the ring back. I used single SCRs as it made the design
much easier. You need to know how much di/dt you want to handle. I started
with these (there are probably better parts on the market now):

https://www.vishay.com/thyristors/list/product-94414/

These have good voltage handling and are capable of 1000 A/us of current
rise. To get the max di/dt out of an SCR you need to drive the gate as hard
as possible. For the Vishay SCR I used a microchip mosfet driver at around
20V if I recall correctly. To manage di/dt I use inductance in the circuit,
which is usually just extra wire. Using v=L(di/dt) you can work out how
much inductance you'll need to protect the SCR. To measure wire inductance
I just back calculated the ringing frequency with my known capacitance
after firing the circuit with a low voltage.

The device sheets always specify "non repetitive" and so far I'm not sure
what that means exactly. They are happy with large currents at 5 to 15Hz
with some factor of safety in the di/dt limit. The largest di/dt happens at
the beginning of the sinusoidal discharge.

This is an LT spice circuit that track really well with "real life"

Loading Image...

For timing the discharges I used a PIC that's controlled by UART.

Interesting SCR stuff:

http://educypedia.karadimov.info/library/an1008.pdf
https://www.dynexsemi.com/assets/downloads/dnx_an4999.pdf


Also, SBE (not my employer) is using a physical discharge contact for their
large capacitor work:

http://www.sbelectronics.com/wp-content/uploads/2009/11/PMC2006-presentation.pdf

[Sean Breheny]

Really interesting, Adam! I've built a few can crushers myself, using very
similar techniques to yours. My largest has a 15mF (0.015 Farad) 1200V
capacitor bank and needs a minimum inductance of about 3 uH to protect the
SCR. Typical peak current is 40kA. The capacitor bank is constructed as
three 400V 45mF stages in series. Each on of those has three paralleled
~1kA stud-mounted diodes to prevent reverse-charging the caps. I started
with a single one per stage but they failed after a few shots.

As far as I understand, "non-repetitive" means that sufficient time must
elapse between events that there is no build-up of heat between pulses. In
other words, the thermal initial conditions of each pulse are the same.

SCRs are fairly slow to turn on and they turn on by propagation of a "wave"
of charge carriers which travels much slower than the speed of light
(roughly about 2 mm per microsecond or 2 km per second - about the same
speed as a high explosive detonation wave). The need to limit dI/dT during
turn-on comes from the fact that the current handling ability of the SCR is
proportional to the fraction of the device which is conducting. If the
current rises too fast, you end up carrying almost full current before the
entire SCR junction area is conducting. Some large SCRs can only handle
about 100 A/microsecond dI/dT

I experimented with using a saturable reactor to limit dI/dT until the SCR
turned on fully. I found that a very large core was needed to hold off
saturation until the SCR was fully on. The reason I did this was because
high dI/dT is very beneficial in a can crusher, so ideally I wanted the
extra inductance to "go away" once the SCR was fully on.

To David's original question, though: 100nF at 1kV is not much energy. I
wouldn't expect too much difficulty in switching that, except that most
means of switching it are likely to be rather lossy since the cap will be
mostly discharged by the time that the switch is fully ON. This is probably
true even for a mechanical switch since the arc will start and complete
before the contacts actually touch.

Sean

Post by Adam Field
On Thu, Apr 5, 2018 at 1:47 PM, Van Horn, David <

Post by Denny Esterline
High energy capacitive discharge is fairly well trodden ground. See if

any

Post by Denny Esterline
of these give you some food for thought.
http://www.electricstuff.co.uk/destructotron.html
http://www.capturedlightning.com/frames/shrinkergallery.html
https://hackaday.com/2012/09/28/how-a-quarter-shrinker-works/
These are primarily "one shot" applications, I need tens of pulses per second.

I designed and built a few of these for my day job (larger capacitor maker
you would recognize). They are "hockey puck" SCRs with multiple diodes in
parallel to catch the ring back. I used single SCRs as it made the design
much easier. You need to know how much di/dt you want to handle. I started
https://www.vishay.com/thyristors/list/product-94414/
These have good voltage handling and are capable of 1000 A/us of current
rise. To get the max di/dt out of an SCR you need to drive the gate as hard
as possible. For the Vishay SCR I used a microchip mosfet driver at around
20V if I recall correctly. To manage di/dt I use inductance in the circuit,
which is usually just extra wire. Using v=L(di/dt) you can work out how
much inductance you'll need to protect the SCR. To measure wire inductance
I just back calculated the ringing frequency with my known capacitance
after firing the circuit with a low voltage.
The device sheets always specify "non repetitive" and so far I'm not sure
what that means exactly. They are happy with large currents at 5 to 15Hz
with some factor of safety in the di/dt limit. The largest di/dt happens at
the beginning of the sinusoidal discharge.
This is an LT spice circuit that track really well with "real life"
https://i.imgur.com/bWBy5N6.png
For timing the discharges I used a PIC that's controlled by UART.
http://educypedia.karadimov.info/library/an1008.pdf
https://www.dynexsemi.com/assets/downloads/dnx_an4999.pdf
Also, SBE (not my employer) is using a physical discharge contact for their
http://www.sbelectronics.com/wp-content/uploads/2009/11/
PMC2006-presentation.pdf
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http://mailman.mit.edu/mailman/listinfo/piclist

[Sean Breheny]

High speed (4000 fps) video of can crushing coil explosion from magnetic
forces (this was done with the unit I just described - 0.015F at 1200V)

[Van Horn, David]

To David's original question, though: 100nF at 1kV is not much energy. I wouldn't expect too much difficulty in switching that, except that most means of switching it are likely to be rather lossy since the cap will be mostly discharged by the time that the switch is fully ON. This is probably true even for a mechanical switch since the arc will start and complete before the contacts actually touch.


Yup. That's one effect I want to get rid of.

[Richard Prosser]

Have you looked into operating a bipolar transistor in avalanche breakdown
mode?
We use this to trigger a laser diode as it gives a very fast risetime (~2
to 5 nS) for a rangefinder application.

But ours work at about 200V. Maybe series them and trigger one - the others
will auto trigger. There doesn't appear much of a range available. Standard
high voltage bipolars may work, but may vary considerably in trigger
sensitivity etc. Peak current might be a problem also, we're only running
at about 20A.

RP

On 7 April 2018 at 02:53, Van Horn, David <

Post by Sean Breheny
To David's original question, though: 100nF at 1kV is not much energy. I
wouldn't expect too much difficulty in switching that, except that most
means of switching it are likely to be rather lossy since the cap will be
mostly discharged by the time that the switch is fully ON. This is probably
true even for a mechanical switch since the arc will start and complete
before the contacts actually touch.
Yup. That's one effect I want to get rid of.
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View/change your membership options at
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[Van Horn, David]

It needs to conduct bidirectionally while on.


-----Original Message-----
From: piclist-***@mit.edu <piclist-***@mit.edu> On Behalf Of Richard Prosser
Sent: Saturday, April 7, 2018 1:50 AM
To: Microcontroller discussion list - Public. <***@mit.edu>
Subject: Re: [EE] Substitute for mechanical contacts

Have you looked into operating a bipolar transistor in avalanche breakdown mode?
We use this to trigger a laser diode as it gives a very fast risetime (~2 to 5 nS) for a rangefinder application.

But ours work at about 200V. Maybe series them and trigger one - the others will auto trigger. There doesn't appear much of a range available. Standard high voltage bipolars may work, but may vary considerably in trigger sensitivity etc. Peak current might be a problem also, we're only running at about 20A.

RP

Post by Sean Breheny
To David's original question, though: 100nF at 1kV is not much energy.
I wouldn't expect too much difficulty in switching that, except that
most means of switching it are likely to be rather lossy since the cap
will be mostly discharged by the time that the switch is fully ON.
This is probably true even for a mechanical switch since the arc will
start and complete before the contacts actually touch.
Yup. That's one effect I want to get rid of.
--
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View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist

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[RussellMc]

On 6 April 2018 at 02:47, Van Horn, David <

Post by Van Horn, David
Is there yet a device that will substitute for real relay contacts in an AC circuit?

Out of head / only minor practical experience comments:

​Lots of good answers already. Any semiconductor device is going to have
at least one forward biased junction voltage drop when on. ​

​Some have two (+).
If junction drop is acceptable then "there are fully SS solutions". Doing
it - see other answers.

I assume you are well aware of, but, just in case:
If a junction drop is not acceptable ​the "usual trick" is to use a
physical contact in parallel with a semiconductor switch.
The semiconductor can be designed to handle to on and off edges and the
contact takes over as soon as the unacceptable transients have been
'tamed'.
Then it's "just engineering" :-).

Russell

[Isaac M. Bavaresco]

Capacitor Discharge Ignition modules use an SCR to discharge a polyester
capacitor with approximately 400 V into the primary of the ignition
transformer.

I think there should be SCRs that can handle your requirements of
voltage and current.

Cheers,

Isaac

Post by Van Horn, David
Is there yet a device that will substitute for real relay contacts in an AC circuit?
I have a test fixture in mind, which is complicated, but the interesting part is discharging a low impedance capacitor of about 0.1uF at 1kV into a coil, and allowing the current to ring down.
Spice tells me to expect currents in the kA range, and a prototype built with a relay contact seems to confirm this. Tungsten contacts take a lot of abuse, and they also generate a lot of heat.
Minimal voltage drop, ability to trigger "closure" and "opening" at any point in time, huge currents, and bidirectional currents without turning off are all requirements.
--
David VanHorn
Lead Hardware Engineer
Backcountry Access, Inc.
2820 Wilderness Pl, Unit H
Boulder, CO 80301 USA
phone: [phone] x110

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