K
krem
Guest
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
still be limited to 1 amp if i'm only switching 30vac?
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J
John Popelish
Guest
krem wrote:
based on the continuous rating of the contacts (current) or on the
ability of the contacts to handle the flash when the current is
interrupted. I have seen both cases.
--
John Popelish
Maybe yes, maybe no. It depends on whether the current rating isI have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
based on the continuous rating of the contacts (current) or on the
ability of the contacts to handle the flash when the current is
interrupted. I have seen both cases.
--
John Popelish
G
gcd
Guest
Hi,
John is correct, there is a rating for switching current and one for maximum
continuous current.
for simplicity, as most resellers don't have data sheets for their relays -
1) the rated current is the maximum switching current.
2) the rated voltage cannot support the rated current, the voltage rating is
simply the maximum voltage allowable across the contact tips.
3) you need the maximum power rating as well which then provides a load line
for which you can determine the safe switching operating area.
In detail.
Typically ( ie reputable manufactures / resellers) will give the rating for
a non arcing switchover. ie the arc quenches well within the time taken for
the contact to swap over.
The voltage rating for the relay is that voltage that can be withstood in
opening or closing the contact
You put these two points on a V vs I curve and join the two points (V,0) and
(0,I)
this then is a simplistic power curve for the contact. The contact will
most likely support higher power levels but unless given, this errs on the
side of caution.
If you know the power rating, you draw a load line of that value and where
it intercepts a vertical line for the I and a horizontal line for the V that
then defines the safe operating area.
A relay can switch higher currents. You change from a Limit curve type 1( no
arc - extinguishes during transition time) to a Limit curve type 2 ( safe
braking , no stationary arc) in which the arc will extinguish but lasts
longer than type 1.
Ideally the data sheet will also specify both a maximum and minimum power
rating, as you can get thermal EMFs generated, and the values depend on the
contact materials. (typically in the order of 10 - 100uV)
Sorry for the waffle, but relays are not as simple as they may seem. If you
can find them, siemens used to make good application notes for relays.
Greg
"John Popelish" <jpopelish@rica.net> wrote in message
news:3FCD7D26.C5E4E90@rica.net...
John is correct, there is a rating for switching current and one for maximum
continuous current.
for simplicity, as most resellers don't have data sheets for their relays -
1) the rated current is the maximum switching current.
2) the rated voltage cannot support the rated current, the voltage rating is
simply the maximum voltage allowable across the contact tips.
3) you need the maximum power rating as well which then provides a load line
for which you can determine the safe switching operating area.
In detail.
Typically ( ie reputable manufactures / resellers) will give the rating for
a non arcing switchover. ie the arc quenches well within the time taken for
the contact to swap over.
The voltage rating for the relay is that voltage that can be withstood in
opening or closing the contact
You put these two points on a V vs I curve and join the two points (V,0) and
(0,I)
this then is a simplistic power curve for the contact. The contact will
most likely support higher power levels but unless given, this errs on the
side of caution.
If you know the power rating, you draw a load line of that value and where
it intercepts a vertical line for the I and a horizontal line for the V that
then defines the safe operating area.
A relay can switch higher currents. You change from a Limit curve type 1( no
arc - extinguishes during transition time) to a Limit curve type 2 ( safe
braking , no stationary arc) in which the arc will extinguish but lasts
longer than type 1.
Ideally the data sheet will also specify both a maximum and minimum power
rating, as you can get thermal EMFs generated, and the values depend on the
contact materials. (typically in the order of 10 - 100uV)
Sorry for the waffle, but relays are not as simple as they may seem. If you
can find them, siemens used to make good application notes for relays.
Greg
"John Popelish" <jpopelish@rica.net> wrote in message
news:3FCD7D26.C5E4E90@rica.net...
krem wrote:
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
Maybe yes, maybe no. It depends on whether the current rating is
based on the continuous rating of the contacts (current) or on the
ability of the contacts to handle the flash when the current is
interrupted. I have seen both cases.
--
John Popelish
J
Jerry G.
Guest
The ratings of the contacts can be fairly complex, and involve a number of
factors. Basically the contacts should be able to support 1 Amp of
continuous current, not including any peaks upon closure (latching) of the
contacts.
I would normally rate the relay contacts to at least double or more of the
amount of current I want it to handle. This way, a high reliability factor
will be introduced.
--
Greetings,
Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================
"krem" <news@rkremser.endjunk.com> wrote in message
news:ecfc146d046eb5664db4e8f8d6d86315@news.teranews.com...
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
factors. Basically the contacts should be able to support 1 Amp of
continuous current, not including any peaks upon closure (latching) of the
contacts.
I would normally rate the relay contacts to at least double or more of the
amount of current I want it to handle. This way, a high reliability factor
will be introduced.
--
Greetings,
Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================
"krem" <news@rkremser.endjunk.com> wrote in message
news:ecfc146d046eb5664db4e8f8d6d86315@news.teranews.com...
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
Guest
krem wrote:
1) If your relay is really rated to *SWITCH* 1A at 120 vac,
then it can handle more at 30 vac, all other conditions being
equal.
2) A rating to switch 1A *should* limit your design to
switching less than that. It is not a good practice to
run components at their rated maximum.
Essentially, if a hobbyist has a question about a relay's
ability to handle the current in the circuit, he should use
a bigger (higher rated) relay - one *well* in excess of the
maximum expected current.
decision,
A couple of points:I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
1) If your relay is really rated to *SWITCH* 1A at 120 vac,
then it can handle more at 30 vac, all other conditions being
equal.
2) A rating to switch 1A *should* limit your design to
switching less than that. It is not a good practice to
run components at their rated maximum.
Essentially, if a hobbyist has a question about a relay's
ability to handle the current in the circuit, he should use
a bigger (higher rated) relay - one *well* in excess of the
maximum expected current.
decision,
K
krem
Guest
<ehsjr@bellatlantic.net> wrote in message
news:3FD36BD6.27F28169@bellatlantic.net...
Thanks, the only reason i'm not just going out an buying the higer rated
relays is that i already have 100 of the 1A 120vac around the house and need
to use about 28 for the project i'm working on so its an expensive
proposition. Unless some one knows of a cheep source
news:3FD36BD6.27F28169@bellatlantic.net...
krem wrote:
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
A couple of points:
1) If your relay is really rated to *SWITCH* 1A at 120 vac,
then it can handle more at 30 vac, all other conditions being
equal.
2) A rating to switch 1A *should* limit your design to
switching less than that. It is not a good practice to
run components at their rated maximum.
Essentially, if a hobbyist has a question about a relay's
ability to handle the current in the circuit, he should use
a bigger (higher rated) relay - one *well* in excess of the
maximum expected current.
decision,
Thanks, the only reason i'm not just going out an buying the higer rated
relays is that i already have 100 of the 1A 120vac around the house and need
to use about 28 for the project i'm working on so its an expensive
proposition. Unless some one knows of a cheep source
J
John Popelish
Guest
krem wrote:
designed contact snubbers, but that might cost a significant fraction
of what better relays cost.
Do all the contacts need to carry more than 1 amp, or just a few of
them?
How much current must the contacts carry (worst case) and what sort of
load is being driven?
--
John Popelish
You might improve the contact life of the ones you have with wellThanks, the only reason i'm not just going out an buying the higer rated
relays is that i already have 100 of the 1A 120vac around the house and need
to use about 28 for the project i'm working on so its an expensive
proposition. Unless some one knows of a cheep source
designed contact snubbers, but that might cost a significant fraction
of what better relays cost.
Do all the contacts need to carry more than 1 amp, or just a few of
them?
How much current must the contacts carry (worst case) and what sort of
load is being driven?
--
John Popelish
M
Michael
Guest
"krem" <news@rkremser.endjunk.com> wrote in message news:<6d852bc158dc5abad55665b2bf22abc6@news.teranews.com>...
Relays can dissappate heat through the pins, so if you are making
your own pcb for them , try to have a lot of copper around the contact
pins.
Simple, Try one, see how hot it gets.ehsjr@bellatlantic.net> wrote in message
news:3FD36BD6.27F28169@bellatlantic.net...
krem wrote:
I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
A couple of points:
1) If your relay is really rated to *SWITCH* 1A at 120 vac,
then it can handle more at 30 vac, all other conditions being
equal.
2) A rating to switch 1A *should* limit your design to
switching less than that. It is not a good practice to
run components at their rated maximum.
Essentially, if a hobbyist has a question about a relay's
ability to handle the current in the circuit, he should use
a bigger (higher rated) relay - one *well* in excess of the
maximum expected current.
decision,
Thanks, the only reason i'm not just going out an buying the higer rated
relays is that i already have 100 of the 1A 120vac around the house and need
to use about 28 for the project i'm working on so its an expensive
proposition. Unless some one knows of a cheep source
Relays can dissappate heat through the pins, so if you are making
your own pcb for them , try to have a lot of copper around the contact
pins.
R
Rich Grise
Guest
How much current are you trying to switch? If you have
100, and only need 38, put two in parallel for each
circuit; that'll give you 2A.
But frankly, I wouldn't depend on it being good for
much more than the rating. I've seen relays where
the contacts were rated for one current at 240VAC,
and _less_ current at 28VDC, but that's one of the
differences between switching AC and DC.
Good Luck!
Rich
Michael wrote:
100, and only need 38, put two in parallel for each
circuit; that'll give you 2A.
But frankly, I wouldn't depend on it being good for
much more than the rating. I've seen relays where
the contacts were rated for one current at 240VAC,
and _less_ current at 28VDC, but that's one of the
differences between switching AC and DC.
Good Luck!
Rich
Michael wrote:
...
Thanks, the only reason i'm not just going out an buying the higer rated
relays is that i already have 100 of the 1A 120vac around the house and need
to use about 28 for the project i'm working on so its an expensive
proposition. Unless some one knows of a cheep source
Simple, Try one, see how hot it gets.
Relays can dissappate heat through the pins, so if you are making
your own pcb for them , try to have a lot of copper around the contact
pins.
S
Spudley
Guest
If your relay has multiple contacts then I would advise running them
in parallel to double their contact rating.
Else get relays with higher contact current rating.
Perhaps you could divide the loads across multiple relays to reduce
the current imposed on each relay.
krem wrote:
in parallel to double their contact rating.
Else get relays with higher contact current rating.
Perhaps you could divide the loads across multiple relays to reduce
the current imposed on each relay.
krem wrote:
I have a relay which is labed to be able to switch 120vac @ 1A.
Will
i still be limited to 1 amp if i'm only switching 30vac?
K
KLR
Guest
On Wed, 03 Dec 2003 05:14:07 GMT, "krem" <news@rkremser.endjunk.com>
wrote:
Other factors in such a situation are
-What is the load exactly
-how often does it operate
-how important is the application
Depending on the answers to these - you may be wise to even derate the
thing to less than the actual ratings.
AC is a lot kinder to contacts than DC
If in doubt - specify a higher rated relay
wrote:
take it much higher - specially if there is much arcing.I have a relay which is labed to be able to switch 120vac @ 1A. Will i
still be limited to 1 amp if i'm only switching 30vac?
I would say yes - you are. I certainly wouldn't
Other factors in such a situation are
-What is the load exactly
-how often does it operate
-how important is the application
Depending on the answers to these - you may be wise to even derate the
thing to less than the actual ratings.
AC is a lot kinder to contacts than DC
If in doubt - specify a higher rated relay