Using AC relays with DC power

J

Jason Dugas

Guest
I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when I
de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on the
fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason
 
On Thu, 2 Sep 2004 23:27:13 -0500, the renowned "Jason Dugas"
<kb5urq@hotmail.com> wrote:

I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when I
de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on the
fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason
Click to expand...
You'll have problems at that voltage/current, especially with an
inductive load. What's the nature of the load?

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"Jason Dugas" <kb5urq@hotmail.com> wrote in message
news:10jfskk6c4js445@corp.supernews.com...
I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when
I de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on
the fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason
Click to expand...
The load we were using was resistive. The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc)
then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.
 
Jason Dugas wrote:
[snip]

The load we were using was resistive.
Click to expand...
Any significant inductance in the DC supply?

The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc)
then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.
Click to expand...
--
Paul Hovnanian mailto:paul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
100 buckets of bits on the bus
100 buckets of bits
You take one down,
and short it to ground
FF buckets of bits on the bus
 
No significant inductance that I know of-- its a Sorenson switching supply.
We were running 72Vdc @ 5A though if that makes a difference.
"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message
news:4138777E.4DDF88AE@Hovnanian.com...
Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off
the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the
arc)
then puffs of smoke coming from the relay. We tried a snubber circuit--
1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.

--
Paul Hovnanian mailto:paul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
100 buckets of bits on the bus
100 buckets of bits
You take one down,
and short it to ground
FF buckets of bits on the bus
Click to expand...
 
On Fri, 3 Sep 2004 08:44:30 -0500, the renowned "Jason Dugas"
<kb5urq@hotmail.com> wrote:

"Jason Dugas" <kb5urq@hotmail.com> wrote in message
news:10jfskk6c4js445@corp.supernews.com...
I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when
I de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on
the fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason


The load we were using was resistive. The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc)
then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.
Click to expand...
The model you are using has a contact-to-contact breakdown of only
1,000 VAC. Here is perhaps a more suitable type of relay- one that has
curves showing the maximum switching capicty (Fig 1)

http://rocky.digikey.com/WebLib/Aromat/Web%20Data/JC-Relays.pdf

Note that this '15A' relay has a switching capacity of between 1A and
2A for inductive *or* resistive loads at ~70VDC. There is always some
inductance in the wiring, of course.

If you don't need isolation, a MOSFET would be the obvious way to
handle this. For example, an IRFB61N15D would not need much of a heat
sink.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Paul Hovnanian P.E. wrote:

Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance
Click to expand...
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Cheers!
Rich
 
Rich Grise wrote:

Paul Hovnanian P.E. wrote:


Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?


That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Cheers!
Rich


That same one, and some others:
Click to expand...

http://205.243.100.155/frames/longarc.htm


--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
 
Rich Grise wrote...
Paul Hovnanian P.E. wrote:

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614
Click to expand...
Is there a way to save that clip?


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)
 
Winfield Hill <Winfield_member@newsguy.com> wrote:


http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Is there a way to save that clip?
Click to expand...
http://www.ourshanty.com/video/LugoSWR.swf
 
In article <U2p_c.2257$vI2.275@trnddc02>,
Rich Grise <null@example.net> wrote:

That's kind of irrelevant, since you don't need inductance to
make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between
the atoms breaks down. When the contacts are one air molecule
apart, they're close enough to break down the air, forming
plasma, which continues to conduct until either the gap gets too
big to sustain the arc, or something else interrupts the current.
Click to expand...
Or until the voltage across the arc falls below
about 22V (the arc sustaining voltage at ntp)?

I have a huge contactor, rated to break 600Vdc at 15A.
There is nothing fancy about it, like magnetic blowout.
It merely consists of about 2 dozen contacts in series,
arranged in a circle, with all contacts simultaneously
switched by a central rotary actuator.

I have always assumed that 600/24 is about 25V per contact
and the simultaneous switching means that the first contact
to open (and arc) only has to withstand the arc for a very
short time, perhaps something less than a mS or so.

--
Tony Williams.
 
Rich Grise wrote:
Paul Hovnanian P.E. wrote:

Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.
Click to expand...
That effect is the same for either an AC or DC circuit, with or without
series inductance. Its the inductance that provides additional e.m.f. to
sustain the arc's duration beyond the point at which a theoretical
purely resistive circuit arc would be extinguished.

--
Paul Hovnanian mailto:paul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
professor; n, One who talks in someone else's sleep.
 
"nospam" <nospam@nospam.invalid> wrote in message
news:sjqkj05g9db49fptkvqu4u5qs6fijv4n9c@4ax.com...
Winfield Hill <Winfield_member@newsguy.com> wrote:
Click to expand...

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Is there a way to save that clip?

http://www.ourshanty.com/video/LugoSWR.swf
Click to expand...
You don't _have_ to save it. It is on another URL, called
www.teslamania.com, along with a couple other spectacular arcs!.
 
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