the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
icon

spst relay, current?


Post ReplyPost New Topic
< Prev Topic Next Topic >
clapclapngo 
Member - Posts: 7
Member spacespace
Joined: January 03, 2010
Location: New York, United States
Posted: January 06, 2010 at 8:47 PM / IP Logged  
I have a dumb question. If you were to hook up a SPST relay with (-)500mA @ 86 and (+)12v @ 85, how is it the current from the (+)12v doesn't travel into the component supplying the (-)500mA making it toast? Is it because the flow of current from each side stops @ the coil?
i am an idiot 
Platinum - Posts: 13,666
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: January 06, 2010 at 9:20 PM / IP Logged  

85 and 86 are connected to the coil.  The coil is simply an electromagnet.  With no voltage on the coil, terminal 30 is connected to terminal 87A.  When the coil is energized, terminal 30 is now connected to terminal 87.  Terminals 87 and 87A get their power from terminal 30. 

https://www.the12volt.com/relays/relays.asp  Diagrams on this page are a physical layout of a relay on the left  The middle diagram shows a relay in it's at rest state.  Notice that 30 is connected to 87A.  The diagram on the right shows a relay that has power across the coil, notice that 30 is now connected to terminal 87.

http://www.bcae1.com/relays.htm  The first picture on this page is probably the easiest of them to understand.  Notice the coil is drawn as a magnet below the contacts of the switching part of the relay.  The relay pictured here is a single pole single throw device.

clapclapngo 
Member - Posts: 7
Member spacespace
Joined: January 03, 2010
Location: New York, United States
Posted: January 06, 2010 at 9:56 PM / IP Logged  
Ohhhh! Thanks " i am an idiot " LoL! That was very helpful and i think its great seeing how much advice you've given in these forums. I've been researching and "studying" up on installing a remote starter in my wifes car and found this site very informative and I've seen your name alot, thats great. But you described a SPDT relay and my question pertained to a SPST relay with no normally closed circut or 87a terminal. See im learning. You ansered my question just the same and thanks!
91stt 
Silver - Posts: 564
Silver spaceThis member consistently provides reliable informationspace
Joined: May 24, 2006
Location: New Jersey, United States
Posted: January 06, 2010 at 10:25 PM / IP Logged  
Terminals 85 and 86 are connected to the relay coils as stated by Mr Idiot. Although the coil is only wire, it does have a resistance hence it is a load when it is part of a circuit. As a load it will limit current flow. A typical Bosch type relay draws from about 90 mA to 200 mA, depending on the brand you use. So getting back to your question, the -500 mA output doesn't fail because the coil does not draw in excess of the rated output current.
clapclapngo 
Member - Posts: 7
Member spacespace
Joined: January 03, 2010
Location: New York, United States
Posted: January 06, 2010 at 11:01 PM / IP Logged  

91stt,

Are you saying that the coil is drawing 90-200mA from the 12v 85 terminal so only that much travels to the (-)500mA 86 terminal? And like "im an idiot" there is no or little voltage on the coil?

i am an idiot 
Platinum - Posts: 13,666
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: January 06, 2010 at 11:31 PM / IP Logged  
There is 12 volts across the coil anytime the alarm puts ground on the ground side of the coil. The coil on an actual Bosch/Now Tyco relay draws 160 Milliamps. That is all the current it will pull.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 06, 2010 at 11:33 PM / IP Logged  
V=IR.
If the coil had current through it, it will have a voltage across it.
If a coil is 120 Ohms, its current at 12V will be (from V=IR) I=V/R = 12V/120 Ohms = 0.1 Amps = 100mA.
(These are typical figures as also suggested by 91stt. I find most "typical" automotive relays are about 68 Ohms upwards - but "heavy" relay coils/solenoids can be much lower resistance (hence higher current).)
The coil/solenoid voltage and current is TOTALLY separate to contact current ratings. (Unless they are connected together, or shorted due to moisture etc, or arc across because of high voltages - like a few hundred volts on a low-voltage (contact) rated relay.)
Contact voltage ratings are usually an indication of voltage breaking capacity, but let's not go there.... I just mention that because it may be different to its "safety" rating - ie, non-electrocution, though its safety rating should (obviously?) be higher than its rated switching voltage - eg, max 28V DC or 130VAC, but "enclosure safe" to 500VAC etc. Damn - I went there anyhow...!
Does that help? (Except for the last paragraph? LOL)
So a relay can be a "current amplifier", and an isolator between different voltages.
EG - 12V @ 100mA into the solenoid (85 & 86) can switch many Amps through the heavy contacts (30 to 87 when an energised SPST; maybe from its NC (de-energised Normally Closed) 30 to 87a contacts if SPDT, etc. The switched current can be from a different supply (another battery, voltage, AC etc).
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
Location: United Kingdom
Posted: January 07, 2010 at 10:42 AM / IP Logged  
Yes guys and I know where this question's going.
Leaving that relay energised constantly will drain your battery overnight.the internal coil resistance (or impedance if I don't want Oldspark jumping down my throat) is enough to stop a short.
Mr. Idiot once gave a graphic demonstration of why you should place a diode across the coil contacts.
If our first poster has to ask this question maybe he shouldn't attempt the install.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 07, 2010 at 5:11 PM / IP Logged  
Good addition!
But I'll jump down your throat (LOL!) and say resistance/impedance IMO big deal - especially here where it is DC & on/off situation. (You usually see a relay coil's resistance given in its specs.)
I may use "impedance" to prevent other pedantics or to highlight that there is an AC component (no pun) involved (eg - stiffening caps & Amp-PSU impedances!!)
And good point about the spike quenching reverse biased diode ACROSS the coil (85, 86) - I recently wrote something about that (like how I dislike relays with INBUILT diodes, and how a relay withstands its own spike).
Alas I just stuck to the basis OP issue. After-all, I'm not one to ramble on about related issues. (big-LOL!)
Not that I answered too well.
I probably should have posted a simple diagram.
And stated it's not "current in #86" & "+12V @ #85", but BECAUSE there is 12V ACROSS 85 & 86, current (say 500mA) flows THROUGH 85 & 86 - ie, in one and out the other.
(BTW - if the relay has an inbuilt diode, 86 should be the +ve end & #85 the ground or -ve end!.)
Now, where have I seen tutorials on something like V=IR - is it called Oh'm's Law ("Oh My" Law)? That's right - various panels & widgets when I visit the12volt.com!
Ah yes, good old Ohm's Law. So tricky to gel, but oh so easy to apply thereafter (albeit maybe with some classes in correct application, clear thinking, or circuit modeling of real situations (ie battery internal resistance, cable resistance etc).
But let's not ramble.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 07, 2010 at 5:30 PM / IP Logged  
PS (albeit as a new post)...
I just clicked "Ohm's Law" and saw the pretty Ohms/Power wheel.
I might be thinking of another tut that shows current in-through-out etc.
But there are good sites with tech-yourself diagrams etc. I dislike reinventing the wheel... (pun intended).
As to the "formula" wheel, I thought it was real neat when I first saw it - albeit a simple Ohm's Law only version (not the Power stuff).
But I found it was useless when I left it at home....
So I just remember V=IR & P=VI. (in english - the Voltage across something equals its Resistance time the Current (I) going through it. And its Power is its Voltage x Current (I). All units are Volts, Amps, Ohms & Watts. Power is "power used" - whether converted into other energy or work or lost as heat etc)
From "simple" maths R=V/I etc should be easy.
Then a bit more maths.... (eg):
V=IR
hence I=V/R (equation #1)
P=VI (equation #2)
hence P=Vx(V/R) (equ #1 into #2)
hence P=(VxV)/R
   = V-squared divided by R etc.
FYI:
The latter is a popular derivation for audio buffs ... eg, what is the MAXIMUM power I can get from a 1-Ohm speaker driven at 12V? It is 12x12/1 = 144W (RMS). So how can a 12 Volt amplifier put out more than 150W (or 210W @ 14.5V) into a single 1-Ohm speaker??!
But I'm not one to digress nor ramble...
Though I have been known to plant the odd seed or two....
Page of 2

  Printable version Printable version Post ReplyPost New Topic
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot delete your posts in this forum
You cannot edit your posts in this forum
You cannot create polls in this forum
You cannot vote in polls in this forum

  •  
Search the12volt.com
Follow the12volt.com Follow the12volt.com on Facebook
Thursday, March 28, 2024 • Copyright © 1999-2024 the12volt.com, All Rights Reserved Privacy Policy & Use of Cookies
Disclaimer: *All information on this site ( the12volt.com ) is provided "as is" without any warranty of any kind, either expressed or implied, including but not limited to fitness for a particular use. Any user assumes the entire risk as to the accuracy and use of this information. Please verify all wire colors and diagrams before applying any information.

Secured by Sectigo
the12volt.com spacer
the12volt.com spacer
the12volt.com spacer
Support the12volt.com
Top
the12volt.com spacer
the12volt.com spacer