why so hard to find stdp relay?

I ordered a relay harness for a new HID system and the relay harness didn't come with a relay    . No problem I thought, I'd just figure out what I need a take quick run to the automotive store. So, by doing a bit of research, I determined that the relay I need is called a STDP looks like (although I'd prefer Tyco if I can find it):

That quick run to the store turned into an adventure visiting eight different stores and still coming back empty handed. I find SPST, DPDT, and DTSP So, I get back online to find more information, and I really don't see much mention of STDP automotive relays, except in reference to HID systems. I checked TYCOs website and a few sellers who are referenced on this site and other places and don't see anything. So, am I even looking for the right thing? Or can something else be made to work? I don't know much about relays, but don't think a SPST or DTSP would work. Can a DPDT be used and just not use the 5th terminal?

Thanks!!!

STDP may be a little tricky to locate.  But you should have no trouble finding a SPDT relay.   You should be able to find one at any stereo shop.   A DPDT relay will have 8 terminals.  If the relay simply turns the HIDs on and does not control any other lamps, a SPST relay will do that.  It just does not have the 5th terminal.

So I can use 1 spst for two lights? I thought maybe the reason for stdp was because its being used for two ballasts...but just a guess. So yes, the harness is being only used for two HID ballasts. I'll give a SPST a try.

Thank you!!!

and just for reference, that relay pictured is a SPST relay, not a STDP.

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Kevin Pierson

Traditionally the number of poles is listed first, and then the number of throws. So a double pole, single throw relay would be DPST.
Kevin's right, the one pictured is only a SPST, since it has only one common terminal, going to only one contact.
If you really needed a DPST, you could just use two relays with the coils wired together.
The only things you need to be concerned with is that whatever you're controlling them with is able to handle two relays, and the relays are rated to handle the current you need to switch.

Just use 2 relays

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John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !

I am brand new here, just found it today researching relay pop. Anyways, I usually help out in designing relay harnesses over at HIDplanet, so I am familiar with this topic.


99% of the time, unless you are doing something complex, you only need a SPST relay for a relay harness. On the harness I use, I use TWO relays, simply because I am controlling a bixenon headlight and used the second relay to control the bixenon solenoids. The first relay by itself powers my ballasts (Denso's) perfectly fine.

As someone else mentioned the relay shown is a SPST. Like I said before, pin 87a is almost NEVER used in relay harnesses and I see no need for seperate lines for each ballast.

Did you get a solution for your relay pop?

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Let's Go Brandon Brown. Congratulations on your first Xfinity Series Win. LGBFJB

Yeah, I read about using a diode to take care of the coil field collapse. That section actually was what drew me here. It was well detailed and easy to understand. Hopefully, using that diode will work (need to get to my relays now, which are hidden in a waterproof box in my bumper).

I have started reading here more to get a more in depth understanding of electronics and must say this site is almost overwhelming in the amount of knowledge.

You can install the diodes at the switch or whatever triggers the relay.  It may work a touch better if they were at the relay, but if it is that much of a problem to get to the relays, you can try at the switch.

I may need to get to the relays anyways, as yesterday, my high beams stopped working. I haven't had time to check if it is a blown fuse (worrisome) or a dead relay (almost as worrisome since the box is supposed to be water proof). Also, the relays plug into my stock H4 socket, so there is no real switch.

Ahhh, the fun of doing HID retrofits, beats the heck out of a kit though.

Thank you also for your follow up and advice, it really reflects well on this community. I'll let you know in the next few days what I end up doing. Hopefully, as time goes on I will be able to learn a lot more about things like mosfets and transistors and the like (I know NOTHING about them and really would like to learn how to design simple circuits like that). I have seen a LOT of circuits here that use those and feel that is the NEXT step to start learning, now that I have designed some REALLY complex circuits using just relays (atleast they are complex to me).

Pedro etc. your comment about 87a is basicly correct when using a relay as a powe (current) amplification which; but in other applications, such as imobilsers and changeovers inc. locking applications, the NC 87a is indispensible. Although I do agree that if you look carefully a 4 pole has a higher contact rating.  For your info there is also an 87b where 87 and 87b are both switched but diode separated.

Yeah, I know for other situations pin 87a is very very very useful. The OP was asking about an HID harness, which seldom uses the 87a pin.

Also to the OP, my local radioshack had some Tyco 5 pin relays in the parts section.