Any Electronics Gurus On Here?

[RobinJI]

Hi guys, I'm in a bit of a pickle. I'm looking for a TIP120/121/122 transistor (datasheet here), and I could do with getting hold of it tomorrow, but I NEED it by Tuesday/Wednesday. Does anyone know of anywhere I can try to get one from? Maplins list a BD681, which seems to have a similar specification from what little they tell you, but it's not in stock anywhere. Is there any other stores that would keep this kind of thing in stock?

It's being used to control a PWM idle valve as part of my cars ECU, I've been told I need a TIP120/121/122, but I'm unsure what effect using a similar one would have, I'm guessing it's not set in stone, as they're giving 3 options in the first place.

It's being used to replace a ZTX450 (datasheet here) because apparently it can't handle the current from the type of idle valve I'm using. The thing that's got me confused is that the TIP120/121/122 is a Darlington transistor, not just a plain old normal one, and I'm not sure if this is relevant. I can get hold of a TIP3055 from Maplins, which seems to have similar specifications to the 120, but isn't a Darlington. I thiiinnk that as it seems to be being used purely for switching rather than amplification this shouldn't matter, but if anyone knows it'd be great?

It's being used in the bottom right circuit of this diagram, labelled as Q4:

Idle-1 being the output from a CPU, and IDL being an output controlling the Idle valve.

If the Darlington pair is important, then I could make one using the schematics in the datasheet for the TIP120, but I'm not sure what pair of transistors I should be using.

Thanks for any help guys, and sorry about the massively boring topic.

[David]

Do you have a schematic so I can spec it from there?

Edit: The circuit diagram won't load

Edited April 29, 2011 by David

[AndrewEH1]

Farnell, and RS are a good source of components.

Try this:

TIP120 Search

I think this would be the best match:

Multicomp TIP120 Darlington Transistor

[David]

Also your best bet would be www.farnell.com or http://uk.rs-online.com/web/ when it comes to purchasing components.

[RobinJI]

Cheers guys. The trouble with ordering one from on-line is that it won't be here until probably Wednesday thanks to the bank holidays, and I really could do with getting it earlier than that.

Here's the diagram:

The trouble is I don't know the info on the CPU output or the Idle valve.

[David]

Cheers guys. The trouble with ordering one from on-line is that it won't be here until probably Wednesday thanks to the bank holidays, and I really could do with getting it earlier than that.

Here's the diagram:

The trouble is I don't know the info on the CPU output or the Idle valve.

RS have trade counters you can collect from. Looks like the Bristol or Somerset store would be closest to you... http://uk.rs-online.com/web/generalDisplay.html?id=branchNetwork

Will have a look at the schematic in a sec.

[David]

I am happy to admit that I am being completely stupid but that schematic doesn't make sense to me... Do you have any more details?

[RobinJI]

Idle-1<< on the left is the output from the CPU, and >>IDL is connected to the idle valve which the circuit is controling. The circuit is there to provide a pulsing switched earth for the idle valve, so if I had to guess I'd be saying that the Idle-1 will be providing a small signal voltage, while IDL will be receiving either 5v or 12v from the idle valve. I'm not sure what sort of current draw the idle valve has, but I'm guessing it's more than a ZTX450 can handle, but less than a TIP120 can handle.

Looking at it I can't see any reason why simply a beefier NPN transistor couldn't deal with it, but it's just the fact that I've been told to use a TIP120, which is a Darlington pair that's got me thinking that maybe a standard transistor will need a higher Bass voltage than the CPU can provide in order to switch the circuit properly.

Oh yeah, Q4 is the transistor I've been told needs uprating.

Ah, cool, I didn't realise RS had stores you could collect from, although the local Maplins is 30 miles closer to me, so it would be a bonus if I could get something from there.

Edited April 29, 2011 by RobinJI

[casualjoe]

I'm no expert but I think the darlington transistors need a 2x higher base voltage to turn on due to the 2x Vbe junctions. Also the ouput signal from the CPU should be plenty enough to drive it, a HIGH is like 2 - 5V. You may be able to find a chunky tranny in an old radio, should be ok if switching speed isn't an issue.

Struggling with the whole subject myself at the moment, mad respect to those that get it

[RobinJI]

Cheers, I can get a TIP3055 from maplins tomorrow, which appears to be chunky enough to handle it, but I'm not sure if it'll mess anything else up, like you say, the speed might not be up there, I'm not sure how fast the pulses are for controlling the idle, there's no reason for them to be particularly fast, but it just depends what it was designed around in the first place. It might even be a setting I can fiddle with, in fact I'm off to play with the software and see if I can.

Edit, it is an adjustment I can make in the tuning software, but I'm unsure as to weather it needs to be matched to the hardware or if I can pick a slower speed if I want. The standard seems to be 50Hz

Edited April 29, 2011 by RobinJI

[casualjoe]

Your PWM won't be fast enough to require high speed transistors, probably runs in the kHz range, you should be fine.

[RobinJI]

Cheers, I've installed a TIP3055 for now, it seems to be easily beefy enough, so hopefully the worst that will happen is the idle valve wont work, which as it's only for fast idle when the engine's cold isn't the end of the world, then I can replace it with the correct part once the post's running again.