spst relay, current?
Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=119158
Printed Date: April 28, 2024 at 3:39 PM
Topic: spst relay, current?
Posted By: clapclapngo
Subject: spst relay, current?
Date Posted: January 06, 2010 at 8:47 PM
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?
Replies:
Posted By: i am an idiot
Date Posted: January 06, 2010 at 9:20 PM
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. https://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.
Posted By: clapclapngo
Date Posted: January 06, 2010 at 9:56 PM
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!
Posted By: 91stt
Date Posted: January 06, 2010 at 10:25 PM
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.
Posted By: clapclapngo
Date Posted: January 06, 2010 at 11:01 PM
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?
Posted By: i am an idiot
Date Posted: January 06, 2010 at 11:31 PM
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.
Posted By: oldspark
Date Posted: January 06, 2010 at 11:33 PM
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).
Posted By: howie ll
Date Posted: January 07, 2010 at 10:42 AM
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.
Posted By: oldspark
Date Posted: January 07, 2010 at 5:11 PM
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.
Posted By: oldspark
Date Posted: January 07, 2010 at 5:30 PM
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....
Posted By: i am an idiot
Date Posted: January 07, 2010 at 5:52 PM
A graphic demonstration just for Mr. Howie. Solder a red wire to terminal 86 of a relay. Solder a black wire to terminal 85 of the relay. Now hold your index finger on one terminal and your thumb on the other. Have a friend touch the other end of those wires to the respective terminals of your battery. Now have him remove and replace one of those wires. He may have to do it several times in a quick manner for you to get the full effect. The pain that you feel is the voltage spike that occurs anytime an electromagnet is turned off. As you could imagine that voltage spike is not liked by a lot of the electrical equipment in your vehicle. The diode eliminates this spike. It doesn't hurt real bad. Go ahead try it. No really it doesn't. It's only 12 volts how could it shock you?
Posted By: oldspark
Date Posted: January 07, 2010 at 6:21 PM
One forgets the simple demonstrations!
It reminds me of manually opening ignition points etc to clean them (with IGN on).
But that is why any automotive accessory should have its own protection against such spikes - at least on its Power Supply (not it's inputs or outputs unless relays are intended).
A good related test is to feel that spike at any distance from the coil/solenoid - I have felt relay spikes across the battery terminals (yeah - a battery is a BIG cap isn't it; does it seem stupid adding a tiny ceramic cap across it?).
But the simplest test tools are often the best.
I thought Howie was referring to some nice CRO pic or something. But this demo method is so simple could have been easily published with an Audio dB meter reading.
Reminds me of using the hand as a test for cable sizing. Or aging switches. (IE - heat.)
Posted By: howie ll
Date Posted: January 07, 2010 at 6:37 PM
Yeah, my thumb is 0 gauge, and as for, let's not go there. Any one have a conversion table for US to Metric on wire gauges?
I started out on the old Lucas/Ripaults stuff 14/28 where it meant current capacity/wire strands, now it's Metric everywhere bar the US and while I have no problems with spanner sizes, this stuff gives me problems.
Mr. I how about wetting your fingers first? I got caught that way about 35 years ago, you don't forget it easily.
Or stick a new PP3 battery to your tongue, I have a pal who tests the that way, what a masochist!
What's a reactive load (simply please)? I thought that was a speaker or relay coil yes I know they are the same.
Posted By: i am an idiot
Date Posted: January 07, 2010 at 6:51 PM
Posted By: oldspark
Date Posted: January 08, 2010 at 7:08 PM
Those 9V batteries should be banned!
A few years ago, they killed 3 people here in Aus. (That's how the stats reported it.)
Maybe we need to lower the HV-DC definition? Think of the income for HVDC licence holders - car fuses, car audio, and fact any non-6V vehicle electrics!
Geez - I'm so good for the recovering economy!
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