Are relays polarized?

Thread Starter

xblitz

Joined May 2, 2021
17
Hello
I try to modelling a relay in ltspice and in my study I encounter a doubt. Consider a common PCB relais (like relais in arduino shields). These types of relais are polarized like diodes? The current through the inductor create a magnetic field that attract the switch but what happen if the current become negative? the switch will be attract or repulse? (of course in this case the relay can't change the state of the contacts).
 

ericgibbs

Joined Jan 29, 2010
18,766
hi xb,
The metal armature of the relay, [the piece that moves the contacts] is not a permanent magnet, so a North pole or South magnetic field will attract the armature onto the coil former.

So the relay is not polarized.

E

Footnote:
IF
the relay coil has an internal suppression Diode in parallel with the coil, then of course the relay is considered polarized, so the power to the relay must be of the correct polarity.
 
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bertus

Joined Apr 5, 2008
22,270
Hello,

It depends. When a back EMF diode is mounted inside the relays, you should take that in account.

Bertus
 

Thread Starter

xblitz

Joined May 2, 2021
17
thaks!
I know the use of snubber diode but in my question I exclude it. In these days I try to modelling some types of relais in LTSpice using current control switch\voltage control switch and play with their Rson and Rsoff but I can't obtain the correct behavior of NC throw in a SPDT relay: I found many modelling but they assume the polarization of relais - so it's possible obtain the correct behavior.
 

Thread Starter

xblitz

Joined May 2, 2021
17
The NO throw works correctly: the current in R2 flow when the absolute value of current across A and B is above the threshold. The NC throw don't. This happen because there are 2 path in parallel between the common and the contact NC and one or other have a low impedance. I try to use a deviator but LTSpice abort the simulation because it encounter problem to guess the potential of a node or other stuff that I don't remember.
 

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ericgibbs

Joined Jan 29, 2010
18,766
Hi xb,
I guess you know there is already a relay.lib in LTspice, is there a reason that you are trying to model your own relay.?

E
BTW: @crutschow has done some interesting work on LTS relays.
 

MisterBill2

Joined Jan 23, 2018
18,179
Hello,

It depends. When a back EMF diode is mounted inside the relays, you should take that in account.

Bertus
THIS IS VERY IMPORTANT!! connecting a relay with a hidden internal diode WILL cause all sorts of problems.. In addition, "Spice" is not the ultimate simulation program. It has quite a few limitations.
 

Alec_t

Joined Sep 17, 2013
14,280
A pulse of current one way through the coil latches the relay 'on'. A pulse of current the other way releases the relay to the 'off' state. External diodes or bidirectional TVS devices could suppress voltage spikes. Single-coil relays with such suppression components internally might exist, but I'm not aware of any.
 

crutschow

Joined Mar 14, 2008
34,285
You have a problem in you relay model, but I don't see what that is.

The standard LTspice relay model coils are polarized (unlike most standard relays without built-in suppression).
If you want to use them with a bipolar signal, a diode bridge can be added to control the relay (simulation below):
But normally bipolar control signals are not used to control a relay.

1638632088349.png
 

nsaspook

Joined Aug 27, 2009
13,086
Hello
I try to modelling a relay in ltspice and in my study I encounter a doubt. Consider a common PCB relais (like relais in arduino shields). These types of relais are polarized like diodes? The current through the inductor create a magnetic field that attract the switch but what happen if the current become negative? the switch will be attract or repulse? (of course in this case the relay can't change the state of the contacts).
Many small sensitive relays designed for low current/higher resistance coil (small 5v and 4.5v directly driven by a uC gpio types) pull-in are polarized independently from snubbers and diodes. It's best to actually check the specifications for each DC relay type. Common in miniature telecom relays, one of the more annoying layout screw-ups is getting the coil backwards (BTDT).

https://www.ia.omron.com/data_pdf/guide/36/generalrelay_tg_e_3_1_3-4(classifications).pdf

"Polarized Relays: Polarized relays use the magnetic flux of the permanent magnet in their electromagnetic sections. This means that the operating coil has polarity"

https://www.motionsolutions.com/store/pc/catalog/documents/omron/ElecCompCat_X301E1.pdf

Page 5: Moving Loop System
 
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