Extremely basic relay question
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=11490
Printed Date: August 29, 2025 at 2:08 AM
Topic: Extremely basic relay question
Posted By: OwenZ
Subject: Extremely basic relay question
Date Posted: March 26, 2003 at 5:44 PM
Hi, I'm trying to understand the different types of relays, SPST, SPDT, etc, etc. I guess what I really need to know is, what are the differences in these types of relays? The number of outputs needed? I've used relays without ever understanding, "Is this a single or double throw?" or "Why is a double pole used and not a single pole?" If anyone can explain in simple terms or point me to a website, I'd appreciate it. Thanks, Owen
Replies:
Posted By: TomEllis
Date Posted: March 31, 2003 at 1:10 PM
Let me see if I can answer your question.First, the SPST stands for Single Pole Single Throw.The Single Throw is the the contacts are either normally open or normally closed and change when the relay is activated. The Single Pole means that there are only one set of relay contacts.The SPDT stands for Single Pole Double Throw. The Single Pole means one set of contacts. The Double Throw means that there are two sets of output contacts with one common contact. One set of contacts being normally closed, then changing to normally open when the relay is activated, with the other contacts being normally open and changing to normally closed when activated.I hope this helps. The type of situation would determine what kind of relay you would need in the circuit.
Please let me know if you need more information.
Tom
Posted By: OwenZ
Date Posted: March 31, 2003 at 1:21 PM
????? The dizziness is setting in....I need to get myself to the library or something! That sorta made sense, N/O to N/C and N/C to N/O...but I'm still lost. Oh well, I'm not designing anything, just trying to learn. Thanks! Owen
Posted By: GoZpascal
Date Posted: March 31, 2003 at 4:06 PM
Single Pole Single throw.. Normally open.. You have the relay connected to a light. The light is off until you energize the relay. This closes the circuit and the light comes on. Single Pole Single Throw.. Normally closed. You have the relay connected to a light. The light is always on until you energize the relay. This opens the circuit and the light goes off. Single Pole Double Throw.. You have two lights connected. One is off, one is on. When you energize the relay they swap. one goes off, the other comes on. Keep turning the relay on and off you will flash the lights back and forth. Does that help any? -mike
Posted By: OwenZ
Date Posted: March 31, 2003 at 4:10 PM
Ahhhh! (light goes on), that really does help! Thanks! Owen
Posted By: GoZpascal
Date Posted: March 31, 2003 at 4:13 PM
forgot the why part. You can use SPDT in place of a SPST but you're essentially wasting part of the relay. If you only connect to one pole, and switch the relay it connects to the other pole but there's nothing there to use it. However, the cost difference between them is so small it's easier to just have the SPDT on hand and not worry about it. The relay section of this site does an excellent job of both explaining how they work and showing real-life uses. https://www.the12volt.com/relays/relays.asp -mike
Posted By: OwenZ
Date Posted: March 31, 2003 at 4:32 PM
Yeah, I looked thru that section first and understood the inner workings of the relays, but couldn't really grasp the light on - light off concept. Thanks, Owen
Posted By: teknoid
Date Posted: April 01, 2003 at 10:12 PM
https://www.bonevil.com/relay/ By far, the best car relay info I've found.
Posted By: OwenZ
Date Posted: April 02, 2003 at 9:48 AM
Posted By: MCubed45
Date Posted: April 02, 2003 at 6:19 PM
that sight's pretty good. the relays section of the12volt's got some better special apps of relays though. for instance it IS possible to construct a latching relay from SPDT relays and only a SINGLE output from an alarm. it on the special apps page. that setup's really useful if your alarm doesn't have a latching output and you wanted to turn on your radio/lights/etc by remote and also be able to turn them off using the same channel. seems like real wast of an extra channel if you use two.
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