All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > General Electronics Chat

Notices

General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

Reply   Post New Thread
 
Thread Tools Display Modes
  #11  
Old 04-27-2013, 06:12 PM
k7elp60 k7elp60 is offline
Senior Member
 
Join Date: Nov 2008
Location: Utah, USA
Posts: 334
Default

Quote:
Originally Posted by strantor View Post
on the inside, a relay is like a spring loaded light switch with an electromagnet attached to it. as you increase the voltage to the coil, the current in the electromagnet increases and the magnetic field gets stronger, which brings the contacts closer to touching eachother. At some point (7V apparently) the electromagnetism has increased sufficiently to juuuust barely make the contacts start to touch eachother, and you start to detect current flowing through the contacts. This is not a great place to stop. you need to continue all the way to 12V. You need to have a strong magnetism to hold the contacts firmly together. If you do not (if you leave it at 7V) then you will have sparking and arcing inside between the contacts which will cause them to wear prematurely or possibly even weld themselves together if your load draws enough current.

You can observe this with a household light switch. If you try to hold the switch in the center position and move it back and forth in tiny increments you can hear the arcing inside; if you remove the cover, on some switches, you may even be able to see the sparks.
I agree with strantor. If the relay doesn't have the normal operating voltage on the coil the contacts are not fully making contact and the contact resistance can be affected. This could cause more voltage drop across the contacts than anticipated.
Reply With Quote
  #12  
Old 04-27-2013, 06:31 PM
ErnieM's Avatar
ErnieM ErnieM is offline
Senior Member
 
Join Date: Apr 2011
Location: Lon Guyland, Noo Yawk
Posts: 4,723
Default

A relay coil has various voltage ratings:

Code:
 - Nominal voltage, or the "design" midpoint. 
    5V, 9V, 12V, 15V, 24V are commonly found.

 - maximum voltage (so you don't burn up the coil)
 
 - pick up (pull in) voltage: the minimum guaranteed voltage to 
    make the contact close. Obviously, it is designed to work at a 
    lower voltage to insure making the spec.
 
 - release voltage: the maximum voltage a closed contact will release.  
   Again, it is designed to release at a voltage greater 
   to meet this spec.
Select All
I've seen engineers select a 15V relay for a 12V system where they are depending on the maximum pull in voltage to be met by their driver, and also are using the higher coil resistance of the higher voltage rated part to lower the current consumption.

"Nominal voltage" is only a suggestion. It is the other numbers that make or break the design.
__________________
“Freedom is never more than one generation away from extinction. We didn’t pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children’s children what it was once like in the United States where men were free.”
Reply With Quote
The Following User Says Thank You to ErnieM For This Useful Post:
Tajiknomi (04-28-2013)
  #13  
Old 04-28-2013, 01:44 AM
THE_RB's Avatar
THE_RB THE_RB is offline
Senior Member
 
Join Date: Feb 2008
Posts: 4,341
Default

Quote:
Originally Posted by k7elp60 View Post
I agree with strantor. If the relay doesn't have the normal operating voltage on the coil the contacts are not fully making contact and the contact resistance can be affected. This could cause more voltage drop across the contacts than anticipated.
I'll agree with ErnieM and disagree with you two.

With relays the voltage is needed primarily for pull-in, because the least efficient magnetic path for the armature is at the start.

Once the relay pulls-in, the metals parts of the amrature are touching, or very close to touching, and the magnetic path is so much more efficient they only need a fraction of the current to remain closed and keep the contacts closed.

So operating a relay from a reduced voltage would first start to give the symptom of failing to close altogether, long before you got the symptom of "increased contact resistance".

Example; 12v relay;
12v = 100mA coil current
pull-in current might be 70mA
hold current might be 30mA
__________________
Roman Black - PICs and electronics. Author of BTc PIC-sound encoder, Shift1-LCD project, the TalkBotBrain talking PIC controller, LiniStepper open-source microstepping motor driver, the Black Regulator 2-transistor SMPS, and probably some other stuff; www.RomanBlack.com
Reply With Quote
The Following User Says Thank You to THE_RB For This Useful Post:
strantor (04-28-2013)
  #14  
Old 04-28-2013, 02:10 AM
strantor's Avatar
strantor strantor is offline
Senior Member
 
Join Date: Oct 2010
Location: Houston tx
Posts: 3,531
Default

Quote:
Originally Posted by THE_RB View Post
I'll agree with ErnieM and disagree with you two.

With relays the voltage is needed primarily for pull-in, because the least efficient magnetic path for the armature is at the start.

Once the relay pulls-in, the metals parts of the amrature are touching, or very close to touching, and the magnetic path is so much more efficient they only need a fraction of the current to remain closed and keep the contacts closed.

So operating a relay from a reduced voltage would first start to give the symptom of failing to close altogether, long before you got the symptom of "increased contact resistance".

Example; 12v relay;
12v = 100mA coil current
pull-in current might be 70mA
hold current might be 30mA
I'll disagree with myself, and agree with you. At least on the theory part about the efficiency of the magnetic path and actuation current vs. holding current. But I stick to my story about using the proper voltage. In practice, I have witness the things that I described (chatter, arcing, welded contacts) and correlated them with low supply voltage. If I could go back in time I might find though, that the problems were caused by fluctuating supply voltage and not just low supply voltage.
Reply With Quote
  #15  
Old 04-28-2013, 11:12 AM
Tajiknomi Tajiknomi is offline
Junior Member
 
Join Date: Mar 2010
Posts: 13
Send a message via Yahoo to Tajiknomi
Default

@Strantor : I got that point now, Coz i have tried another Relay , Which is 6Volts but it active on 3.5V (According to my DMM) But as you have said ,it isn't firm Contact.. So one should make a relay working on its Described voltage, not below that voltage Coz of Arcing..
Thanks to all for your kind and helpful replies
Reply With Quote
Reply   Post New Thread

Tags
, , , ,


Similar Threads
Thread Thread Starter Forum Replies Last Post
Need circuit for spdt action from spst relay codehead The Projects Forum 13 12-03-2011 01:43 AM
12v DC relay wiring & control circuit, Help gturnbull The Projects Forum 14 06-21-2008 12:47 AM
UK supplier for 12v, 5a relay with <200mw coil greg123 General Electronics Chat 5 03-24-2008 08:04 AM
12V relay to 24V output thegreat General Electronics Chat 3 03-14-2008 03:40 AM
EFI Fuel Relay Problem akajee The Projects Forum 4 01-26-2008 08:18 PM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 06:06 AM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.