# dpdt relay question

Discussion in 'Electronic Basics' started by Ryan Kremser, Oct 1, 2003.

1. ### Ryan KremserGuest

Ok the ratings on a relay that i have are 1A @ 24 DC & 1A @ 120v AC..
are these values per side of the relay (it being a dpdt has two -I thought)
or are these totals as in .5A per side?

2. ### Ryan KremserGuest

ahh forgot to mention it is a hrs2h-s-dc5v from HUIGANG

3. ### Rein WiehlerGuest

those numbers are the contact ratings in that relay.
You can switch 1A at 24VDC (24W DC load) or 1A at 120VAC (120W AC load).
Looks like you need 5VDC to operate the relay.
rw

4. ### Ryan KremserGuest

ok but there are 2 sets of contacts it being a dpdt relay, are the values
per set or cumulative

5. ### Walter HarleyGuest

Per pole. That is, per "side" - pole is the 'p' in "dpdt".

6. ### Rein WiehlerGuest

thats the rating for each set
rw

7. ### David HarmonGuest

If you wire the contacts in parallel, is it OK to
double the current rating?

8. ### Walter HarleyGuest

No, because you have no way of ensuring the current is equally split between
the contacts.

9. ### Ryan KremserGuest

ok good, so correct me if i'm wrong but reading your posts its my
understanding that i can run 1 amp through each pole allowing me to switch a
2 amp device if I place both poles in parallel?

10. ### bemwGuest

No. At the moment of closing or opening, one pole will react quicker/slower
than the other, resulting in the single pole briefly carrying the total
load. Arcing will occur and the contact surfaces may be damaged or welded
together.

11. ### Guest

No, for reasons others have stated. But in addition,
you should under rate the circuit so that it does not
use the full current rating of the relay contacts.
That means that if you are controlling 1 amp of
current, a relay rated at 1 amp is a poor choice.
Use a relay with higher rated contacts.

On top of that, you should add elements to the circuit
as needed to prevent or minimize arcing of the contacts.

12. ### JeffMGuest

Walter has already told you NO.

13. ### Ryan KremserGuest

Thanks, sorry that was so hard, I missed the first set of posts which had

14. ### John FieldsGuest

---
I think the answers you got were bogus.

In the first place, even if the resistances of the two contacts were
different the difference would be in milliohms (or, more likely
microohms) so the "extra" heating the lower resistance one would suffer
would be negligible. Just for comparison, think of a couple of
resistors in parallel with a difference in resistance of 1%. For any
current flowing through the pair, the smaller resistor will only carry
about 1% more current than if they were equal. In the second place, the
temperature coefficient of resistance of metals is generally positive,
so even if one set of contacts did conduct substantially more current
than the other one, and got hotter, its resitance would then go up and
the current causing it to heat would be shunted to the other set of
contacts until a thermal balance was achieved.

There _are_ other considerations which have to do with the type of load
you want to switch, whether you want to do cold or hot switching, and
whether you're switching AC or DC, but a blanket condemnation of
paralling contacts to increase the current switched isn't warranted.

15. ### Walter HarleyGuest

I wouldn't say I gave him a blanket condemnation (though others may have); I
just said that in this instance, 0.5 + 0.5 doesn't equal 1.0. I'll stand by
that math

But John, I'm not sure you're attending to the right aspects. You're
talking about overheating during normal operation. I don't think that's the
main thing relays are current-rated for, is it? I mean, a piece of 22-gauge
wire will carry 3A comfortably, and I've seen relays with contacts and
internal wiring that are physically larger than that rated for less.

I am *NOT* an expert on relays, so read this with considerable skepticism,
but:

I think the current rating of a relay has more to do with the amount of
current that it can safely interrupt, without damage to the terminals from
arcing. That is, the rating is concerned with the situation where the relay
is closed, current is flowing, and then the relay opens. The two contacts
of a DPDT relay must open at slightly different times (perhaps 0.1msec
apart); so, if they're paralleled, one contact is always getting opened with
the full current on it. It is precisely that brief period of opening that
matters, because if there's too much current to interrupt, the contacts will
bond and fail to open.

More realistically, since we're only talking about a factor of two overload,
the contacts will still successfully open; but over time, the effect will be
exactly the same as if the relay were being run at twice its rating, because
it will always be the case that _at the most critical moment_, all the
current is flowing through one contact. Little micro-welds will form and be
physically pulled apart by the relay spring, but they'll be twice as big as
they're supposed to be.

16. ### Guest

Exactly. It's interesting to put a set of relay contacts
through repetitive make/break cycles, and scope the current
through the contacts. You'll see plenty of bouncing with
typical relays. If those contacts are paralleled for increased
current, the transfer and the bounces would have to be precisely
in sync to avoid the scenario you mentioned.

17. ### John FieldsGuest

---
The current rating of relay contacts is based on the voltage drop across
the contacts with a certain current through them. The voltage drop is
due to the contact resistance, and putting more that the rated current
through the contact resistance will result in the contacts heating more
than with the rated current flowing through them, increasing the contact
resistance, increasing the power dissipation of the contacts, raising
the temperature of the contacts...
---

---
There are two specifications to consider here: One with the current a
relay is specified to carry with the contacts closed and another with
the relay "hot-switching" the current.

Here's a decent link: http://www.aromat.com/pcsd/rti.pdf
---
Relay contacts can't weld on break, unless they bounce, since the plasma
will only occur after the contacts have separated. They can (and do)
weld on make, however, since bounce almost always accompanies make.
---
---
Again, this only occurs when the contacts close, and if they bounce when
they open. What _does_ happen when the contacts open (and when they
bounce) is that the arc/plasma which is created when the contacts open
causes metal to migrate from one contact to the other. It's this
phenomenon which causes the contacts to weld closed, since when the
contacts finally come to rest they can be sitting in a pool of molten
metal. Especially switching DC, since when the bouncing decays a time
will come when the arc won't extinguish between bounces because the
contact separation won't be enough to extinguish the arc.

Also to be considered is that when one set of contacts closes and
bounces, the other set will not be far behind and will quench the arc
created by the first set as soon as the second set closes. Of course if
the second set bounces before the first set comes to rest another arc
will be generated by the second set bouncing, but then when the first
set makes again the arc will be extinguish. Since this arcing will be
shared by both sets of contacts and will exist for a shorter time than
if it occurred with only one set of contacts, it may well be possible
that (on make) the life of the contacts in parallel will be longer than
twice the life of either, making 0.5 + 0.5 = 1.1

18. ### Walter HarleyGuest

That is indeed a great link! Very informative; thanks. Color me eddicated