Help Choosing Correct Transistor

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello,

I am attempting to hack my cars brake lights. There are three bulbs with the below stats that need to switch on and off using the Arduino for prototyping.

Each bulb has two filaments and 4 contacts:
12.8V -- 2.23A
14V -- 0.48A

I assume that one rail is the always on when the headlights are on and the other is the brighter one that kicks in when the brake is pressed or the blinker is on.

So I'm going to have to make some sort of power supply to drop the 12V car system to 5V for the ardy. I'm pretty sure I can figure that out. And I know I'm going to have to use transistors to flick these bulbs on and off.

Something I've never had to do before though is find the right transistor for a specific application. I did some searching and located the following...

URL GOES HERE

I just want some of you uber brains out there to check my work... Did I find the correct one here?

Thanks for your help in advance,
-Patrick

2

Here's the URL it wouldnt let me post in the previous message due to the fact that it was my first message on these boards.

http://www.alliedelec.com/search/productdetail.aspx?SKU=5680545#tab=Specs

3

Each bulb has two filaments and 4 contacts:
12.8V -- 2.23A
14V -- 0.48A

What kind of car and what kind of bulb? The only ones I'm familiar with have 3 contacts - 2 hot (both 12 V) and ground. One line is hot when the brake or turn signal is in use use. The other is hot when the lights are on.

Where do you live? There are pretty strict rules in most places concerning brake and tail lights.

4

Heres the specs on the bulbs, line #18:

http://www.sylvaniaautocatalog.com/new_sylvania/ProductBrowse.asp?Batchid=9&FigNumber=27

Theres two filaments with a hot and ground for each on these. No big deal though. I did some poking around with my multimeter today and it is as you say... Ones for the brakes / blinkers and the other for the running lights.

I've done my research on the laws and no problems there. The mods I'm going to make will make the car more noticable if anything.

Now back to the question at hand... For some reason I wasnt able to test the amperage with my multimeter... Every time I tried the light would go out until (for example) we let off the brake and re-applied. So, I'm not sure on that.

Surly though the amps wont be over the rating shown in the above data sheet.

So do you think I have located the correct transistor for this project? Or am I way off base? Am I testing for the amperage incorrectly? I placed the probes in the same locations as for when I tested for the voltage...

Thanks so much for your help.

-Patrick

5

yes you are testing amperage incorrectly, the probes need to be in series with the circuit, also watch out most multimeters have 2 options

1 is using the same sockets you normally use, this is fused protected and its a tiny amount

the other is NOT fused protected and typically has a MAX rating of 10 amps, going beyond that will kill your meter as the current is actually flowing through the device

6

Maybe you'd be better off with relays? It'd sure save all the trouble of finding a suitable transistor. Relays can be found in high ratings for relatively cheap, and then you don't have to worry about burning it out (most of the time).

Just what I'd do..

7

Oh, I'm an idiot. lol I knew that.

Thanks for the help Osgeld. I'll post back when I get the amps.

As for relays, I hadnt thought of that. I'll look around and see what I can find. Do you know of any good places to look for relays that will work well with Arduinos or for this project?

Thanks a ton,
-Patrick

8

I would suggest DigiKey, or other major electronic suppliers.

You can find such a broad range of things there, that I'm sure you'll find something suitable.

Keep in mind that you'll probably also need a transistor to power the relay, but these will (probably) be much low power transistors than the ones you were looking at.

[edit]Unless I'm very much mistaken, this one should work for your purposes. Like I suggested, you would need a small transistor to switch it.[/edit]

9

Did I find the correct one here?

Probably not: car lights usually work by switching the high side of the circuit, with the ground return through the frame. So you probably want to use a PNP transistor, or a P-channel MOSFET.

As long as you're poking around the socket, you can confirm that your lights are also wired in the normal fashion.

Ran

10

Thanks for that. Wow you guys are awesome.

Another quick question on testing for amps. Is there a method to test without breaking the existing circuit? I dont want to have to slice my brake wires back together just to do the testing.

Questions on that relay you suggested TchnclFl:

  1. I dont know what the difference is between Coil Voltage (12V) and Contact Rating (5A @ 28VDC). I know this thing will handle the amps, but when I checked the cars circuits it was running at 13.5V. Thats over this things coil voltage. I dont know if that matters.

  2. The Control On voltage is 8.4V and Off is 0.6V. Does that mean that anything over 0.6V will trigger the relay? So the Arudinos 5V out of the pin will be enough? Can you explain the need for the transistor please? I've never dealt with relays before, sorry for the newbness.

As for the wiring heres a pic of the wires out of the bulb for those that were interested. Yes the ground goes to the chassis.
http://img27.imageshack.us/img27/308/img2371smallq.jpg

Thanks again,
-Patrick

11

I don't claim to be an expert, but here are my thoughts:

A relay's coil voltage is the voltage required to activate the coil (and close the connection). This has nothing to do with the Contact Rating.

The Contact Rating is the amount of electricity that can safely pass through the relay (the part that will complete the circuit for the light). The contacts will handle 28VDC (should be fine for a car), and 5A (which also should be fine).

And as for having to cut the wires, you will have to unless you can find a "plug" connector where the light plugs into the power source.

I'm not sure what the Control on voltage is, you'll have to ask someone else.

Even if the relay would trigger with 5V, the current draw is too high (nearly 40 mA) for an Arduino pin, thus you will have to use a transistor to switch on the relay (sounds redundant, I know, but that's how it works).

That's pretty much all I can say about that :P.

I don't claim to be an expert, but here are my thoughts:

A relay's coil voltage is the voltage required to activate the coil (and close the connection). This has nothing to do with the Contact Rating.

The Contact Rating is the amount of electricity that can safely pass through the relay (the part that will complete the circuit for the light). The contacts will handle 28VDC (should be fine for a car), and 5A (which also should be fine).

I'm not sure what the Control on voltage is, you'll have to ask someone else.

Even if the relay would trigger with 5V, the current draw is too high (nearly 40 mA) for an Arduino pin, thus you will have to use a transistor to switch on the relay (sounds redundant, I know, but that's how it works).

That's pretty much all I can say about that :P.

And as for having to cut the wires, you will have to unless you can find a "plug" connector where the light plugs into the power source.

12

Perfect. Looks like I'm all set then.

Thank you all so much, you rock.

I'll post a close out video when I get this working.

Thanks again!
-Patrick

13

Alright, sounds good.

Good luck!

14

Thanks Richard,

So in the case of this relay that TchnclFl suggested:

should I use the 5V that I regulate for the Arudino or the 12V from the car to energize the coil?

-Patrick

15

The coil voltage appears to be 12v and 38ma current, so you will probably need to power it from the 12v supply.

16

Thats what I was thinking... Though I was worried because my car for whatever reason stays up around 13.5V.

I was worried that going over the specified 12V might be a problem. What do ya think?

-Thanks,
-Patrick

17

Do a google search for "transistor relay schematic" to see how to operate a 12v relay using a 5v control. Pretty simple. There are other recent threads here on using optical isolators to do the same.

18

And as for having to cut the wires, you will have to unless you can find a "plug" connector where the light plugs into the power source.

Nah: there are "tap" crimp-on connectors intended to connect an additional wire in parallel with an existing circuit. They do insulation displacement, just like computer ribbon cable connectors. Since Patrick is adding a function, and not intending to interrupt the existing connection, there's no need to cut.

19

my car for whatever reason stays up around 13.5V

It's supposed to: the nominal "12V" battery is actually supposed to charge up to about 12.6V (some details on wikipedia). Car alternators run about 13.5-14V because you need a voltage higher than the battery's to charge it. Older alternators, without the benefit of solid-state charging controls, will peak even higher as the RPMs go up.

Relays are designed to tolerate a range of voltages around their nominal rating. Unless you're planning to have it on for hours at a time, I wouldn't worry about even a relay not designed for "automotive" use suffering from the higher voltage.

Since you're in TX, I'd suggest shopping at either BG Micro or Goldmine for surplus "automotive" relays: you'll probably save enough from retail just on the relays to pay for the shipping. Plus you'll find other goodies you'll want for future projects.

20

Thanks again for your help guys.

Quick question as to an alternative to a relay for this... Would it be possible to use a darlington pair such as the TIP-120 instead of the relay?

Here's a datasheet for one that I found.
http://www.st.com/stonline/books/pdf/docs/4128.pdf

If this works it would allow me to switch the lamps at a much faster rate than the relays with a less complicated circuit...

Got the idea from this page:

What do you guys think?

-Patrick