ShopTalk Forums | Aircooled VW Tech & Restoration Help

SUbuggy wrote: very nice pic, but all relays in diagram are SPST.

Both types of relays have terminals 85/86 "coil" or trigger circuit and terminal 30, the main power feed.

the key difference is a SPST has termainal 87 and 87a. a SPDT has TWO 87 terminals and NO 87a terminal. So what makes them different? when you apply your main power feed (single pole circuit) to the relay at terminal at terminal 30, you have a normally closed (N.C.) connection at 87a (ie 87a will become hot). when you energize the relay via +12 and ground across 85 and 86 terminals, the normally open (N.O.)terminal 87 becomes "hot".
but a SPDT relay has TWO OUTPUT terminals (87). the idea is to terminate power for two different devices using a single feed....

Wrong. Be sure you know the correct answer before you give a "quiz".
SPDT has two outputs, 87 and 87a. Depending upon the state of the relay (energized or not) one or the other is "hot" (closed with 30).
SPST may have a single 87 or a dual 87, closed to 30 when the relay is energized. When the relay is not energized, there is no "hot" output. If there happens to be a second Term 87, it's there for connection convenience - electrically it's identical to the other (if it were not, it would not have the same terminal number). Another type of SPST relay has one Term 87 and one Term 87b - each having its own set of contacts which work in unison (close to Term 30 when energized). Externally you would not otherwise be able to tell it apart from a SPST relay with dual 87 terminals (unless one set of contacts were to go bad) but each set of contacts can handle the rated load rather than having just one set for both outputs.

DesertGuy has it right. "DT" means double throw, not dual terminal.

Marc wrote:DesertGuy has it right.

Thanks for the clarification Marc... and here I was ready to call BOSCH and let them know that have it wrong

when i get back to my shop i will post the two part numbers of the bosch SPST and SPDT relays.

There are many, many more than two part numbers. You need to do some more research anyway, perhaps you'll come across some of them in the process.

To preclude the following post… In no way, do I wish to discredit anyone! I merely wish to set the record straight… to the best of my knowledge! If anyone should believe that I have made a mistake, please by all means feel free to give constructive criticism.

SUbuggy wrote:…BOTH STYLES OF RELAYS have FIVE terminals…

There are some SPST relays out there with four terminals.

SUbuggy wrote:…the key difference is a SPST has termainal 87 and 87a. a SPDT has TWO 87 terminals and …

As Marc has already stated, you are incorrect.

Pole – The number of inputs to a switch or relay.
Throw – The number of output circuits from a switch or relay.

DesertGuy wrote:…I hear that it is a good idea to use a diode between the coil (85, 86) to prevent spiking of electrical current when the item is turned off. Is this a requirement or just good practice?

SUbuggy wrote:…in my experience, i have never had to run a resistor across 85/86 to prevent or shunt any sort of "noise"…

In an older (non-computerized) vehicle, the need for a clamping diode or bleed resistor probably isn’t really that critical. Now on a computer controlled circuit, where as a computer “driver” or transistor is switching the relay, the voltage surge or spike from the collapsing magnetic field could indeed do damage. Some computers are more sensitive than others though.

SUbuggy wrote:…furthermore the relay is not polarity sensitive. you can trigger with +12 on 85 and have 86 grounded all the time, or you can trigger with ground and leave 85 hot all the time, you can even reverse+12 and ground across 85/86 and it will work either way.

If the relay has no clamping diode, regardless of whether it has a bleed resistor or not, then yes, you can put power or ground to either the 85 or 86. If it has a clamping diode, then you must put positive to the 86 and negative to the 85 or you will pop the diode like a fuse rendering it useless!

MNAirHead wrote:Can someone please add "RELAY" to the glossary sticky and give it a quick definition?

http://www.shoptalkforums.com/viewtopic.php?t=106095

Relay – An electromechanical device that uses low current to control a high-current circuit.