QUIZ: bosch relays

[SUbuggy]

here is my .02

the common bosch relays you see available evrywhere actually come in two forms:
SPST
SPDT

what is the difference, how can you tell them apart, and when would you choose to use one ove the other?

[DesertGuy]

Hmmm... OK, I'll bite.

(As I understand It)

A relay is a way to A) isolate one circuit from another; and/or B) to trigger a larger current draw by using a smaller current trigger.

SPST: Single Pole Single Throw
SPDT: Single Pole Double Throw

(Corrected: SPST can have 4 or 5 terminals SPDT has 5 terminals) . A SPST will only supply current when the coil is triggered. However, the SPDT will supply current when triggered, but can also supply current on contact 87a when non-energized.

As far as when to use one over the other... I would "guess" it would be determined by what you are trying to achieve by the circuit's design. I imagine that either could be used to "trigger" a circuit path... but couldn't you use the SPDT in more elaborate circuits, such as mode/circuit path switching...

This is only my guess, and I can definately be wrong

I hope I at least pass... (remember C's gets degrees )

[DesertGuy]

I added a pic of a SPDT in the hopes I can get extra credit

But this brings up a question that I have:

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? Do I run a high risk of damaging things if I opt to not use one...Also, if I did use one, how is it attached considering that I have power to one side and a ground to the other (basically, how do I make it all fit, what is the "right" way)?

[SUbuggy]

OK the answers are
SPST does mean single pole single throw
SPDT is single pole double throw.

BOTH STYLES OF RELAYS have FIVE terminals.

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. example: if you wanted to run TWO offroad lights, you would wire the +12 feed going to the first light to the one of the 87 terminals and the second light to the second 87 terminal (assuming the load of both lights is not greater than your fused feed on 30, or the 20/30 amp rating of the relay.)

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?

in my experience, i have never had to run a resistor across 85/86 to prevent or shunt any sort of "noise". 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 gound and leave 85 hot all the time, you can even reverse+12 and ground across 85/86 and it will work either way.

*tip
POLARITY REVERSING

ever need to control a DC motor where you need to reverse polarity of the + and - feeds that come off the motor? a very slick and easy way to do it is with TWO SPST relays.

RELAY 1
30= ground, gauged heavy enough to support whatever your load is.
87a= motor positive connection
87= fused +12 connection, gauged to support your motor load
85=trigger up +12 volt from your switch or triggering device (assuming you wish to posite trigger
86 ground

Relay 2
30= ground, gauged heavy enough to support whatever your load is.
87a= motor negative connection
85=trigger down +12 volt from your switch or triggering device (assuming you wish to posite trigger)
86 ground


IMPORTANT THINGS TO REMEBER

ALWAYS fuse your main power feed (30)
NEVER excede the amperage limit of your relay (this style is avaible in 20 or 30 amp)
DON'T over-complictae things, a relay is just a switch (terms 85/86) that activates a higher current capabilty switch (terms 30/87).

[DesertGuy]

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. example: if you wanted to run TWO offroad lights, you would wire the +12 feed going to the first light to the one of the 87 terminals and the second light to the second 87 terminal (assuming the load of both lights is not greater than your fused feed on 30, or the 20/30 amp rating of the relay.)

Holy Guacamole... I thought these were the complete opposite...

[MNAirHead]

This is important stuff as there are SOOOOO many customized dubs out there now.

I've installed them on my light arrays in our offroad cars...



Questions.
1-When do I know I need to use a relay?

2-Are the terminals always marked the same way?

3-Sizing... for the average guy.. how do I know how big a relay to get? OR.. is it like house AC wiring.. if we just use 4 guage wire on the question mark stuff we're fine (over simplification).

[MNAirHead]

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

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

[SUbuggy]

When do I know I need to use a relay?

you use a relay when ever the current of the device you are running exceeds the limit of your triggering device.

example.......you want to run two big offroad lights. but you have a nice illuminated toggle switch that is only rated for 5 amps. if you ran 20amps of lighting through the switch you would eventually fry the switch. ever notice your switches getting warm? a sure bet you are running it over the limit of what it is designed for.
another good reason to use a relay is because you do not have to run heavy gauge power wire to your switch then back to your lights. it only takes about 1oo milliamp to trigger the relay, so you can run tiny 20ga wire from your switch to a relay near your lights and keep your heavy power runs short.

Are the terminals always marked the same way

I believe these markings are a GERMAN standard. you will find on german car diagrams these are standardized markings. you will not find the same markings on asian products.

Sizing... for the average guy.. how do I know how big a relay to get?

i beleive the bosch relays and the cheap (yes the parts hous noc-offs are substandard, always look for the bosch brand)imitations are all 20 or 30 amp. usually, right on the top it tells the amperage rating. I stick with 30 amp MAX (10 gauge power feed) and if i need more power i just use additional relays on seperate circuits. there are availble heavy current relays (can you say solenoid) but then you need to run larger gauge wire to feed it.

[bigbugman]

You can buy relays that already have the diode wired internally. It is not a neccesity, it would just keep the voltage spike that occurs when you de-energize the relay ( much like an ignition coil ) from eventually damaging the switch that turns it on.

[DesertGuy]

Thanks alot for the info on relays so far... However, I am a little fuzzy on the naming conventions regarding SPST and SPDT relays. Considering the two can have specific uses, I want to make sure I get it correct.

I thought the above pic was illustrating a SPDT. However, when I read the above definitions, it seems the pic is a SPST, per definition. Then when I look in the Bosch guide, I see this...

I will be ordering some relays very soon (online) and want to make sure I order the correct ones. Am I missing something here?

[sideshow]

Are the terminals always marked the same way...I believe these markings are a GERMAN standard.

Lets start a rumor that is DIN standard so most of Europe is wired by number this way.

[SUbuggy]

your diagram has the correct schematics, but the names are backwards.

double throw means TWO OUTPUTS ie to 87 terminals.

for the record, ihave installed hundreds if not thousands of relays without any protection across the coil terminals. this includes sensitive circuits from things such as EFI ecu's that have an output to control things like fuel pumps and have NEVER had an issue.

[DesertGuy]

double throw means TWO OUTPUTS ie to 87 terminals.

Hmmm, I thought that was a SPST (Dual).

[2088 bob]

I will admit there are some really cheap partshouse cube type relays out there .but by the same token if you use them as much as i do there are sources for good quality cheaper priced relays out there as well

one source that comes to mind is delcity ...... www.delcity.net

i also don't run a diode across the coil contacts ,never had issues with this ,allthough one would surmise if you were using a relay that had an activation coil that had required more current this may or maynot become an issue

[DesertGuy]

Concerning the designation of throw in regards to relays, I was under the impression that throw refers to the "unique" directions that a circuit can be constructed, not necessarily the number of "output" terminals that the relay has.

(Simplified example)

For instance, I could use a SPDT relay to construct a circuit that controls a high/low beam. (unique) Whereas, I could use a SPST (Dual) to control two fog lights (same, but independent).

When I refer to "official" BOSCH literature concerning SPST and SPDT relays schematics, I find the following...

Am I missing something?