Anyone know how NPN transistors work? - Page 2 - Fuelly Forums

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Old 10-07-2008, 06:00 AM   #11
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Junkyards here suck too.

Can you write it up in a more organized format with drawings?
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Old 10-07-2008, 06:31 AM   #12
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I guess I'm kinda fortunate. There's a huge U-Pull-It yard about 20 miles from my parent's house. Its nice because you can "learn" the easy way to do the job on a car that isn't yours and if there's any collateral damage you know not to do it the same way on your car.

-Jay
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Old 10-07-2008, 10:25 AM   #13
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Jay we had an awesome pick and pull by my parents, but that's over an hour away and mostly domestic stuff. It was a blast to just take stuff apart for the hell of it. Near me there's only one wrecking yard I know of and they remove it themselves. Their prices are reasonable but their selection sucks, plus they got rid of a lot of cars when scrap metal was high.

Cow: I had a 30 pack of NPN transistors from Radio Shack already, I think they were like $5 or something. They're cheap. Anyway attached is a Paint picture, woo!

It's pretty straight forward, with the flat side of the transistor facing you, left to right is collector, base, emitter, I tried to make this more clear with the 2nd view to the right. The X's mean you cut and splice here.

I said the base can be hook to any signal to turn on the relay and this is true. You could hook it to a Photosensor and use a PNP transistor instead to have headlights only come on when it's dark and there were plans online to use a microphone through another transistor to amplify the current and use that to control a transistor. So then you could turn on a relay based on noise.

It mostly suited my needs because everything I need to do was in the steering column and easily fit inside it, seemless install.
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Old 10-07-2008, 05:20 PM   #14
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i thought most cars had some sort of key in ignition/headlight buzzer from the factory?

every car weve owned (heck even my chevette had them )

i took them out tho because it got annoying listening to them beeeeeep while listening to the radio with the door open.

u sure noone did that to your car? or the buzzer may have just gotten bad.
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Old 10-07-2008, 05:38 PM   #15
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The Buick, and the beast have headlight on alarms from the factory. My 86 Chevy Silverado does not. I did pull one from a 1980 Century and it plugged right in though, so Rusty has it (now). I imagine it would have worked on my 74 Chevy truck as well, and definately my 1980 Bonneville wagon. I did rip the buzzer out of the AMC Spirit we had because the switch went bad in the seatbelt buckle, and when you started the car the darn buzzer would not shut off for almost a minute. That early 80's Buick module worked well, and was easily adaptable to other vehicles.

-Jay
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Old 10-07-2008, 09:36 PM   #16
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Yeah all my Chevys had them stock, though one had the buzzer removed. I'm not sure if there is one or not. Either way I've ignored buzzers and chimes before and I prefer my headlights to be off if the key is off so this is better than stock now.
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Old 10-08-2008, 07:21 AM   #17
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You figured it out already but yes the base on an NPN doesn't want to see more than about .6-.7 volts positive applied to it relative to the emmiter. It should look like a diode and in fact it is. The 10k resistor you connected to the base should do the trick but you may also want to measure the relay current in the collector of the transistor and make sure you are appling enough current through the base resistor to keep the transistor turn on hard otherwise it may get warm if it is not operating in saturation. The hFE of the transistor is the current gain so you take the base voltage through the resistor 12volts/10k*hFE = collector emmiter current. You may also want to connect a 5k resistor between the base and emmiter to turn off the transistor when the 10k resistor connected to the Accessory power doesn't quite get to ground to turn off the transistor completely. Chances are there are devices loading the acc power down to ground pretty quick but things like a radio can have caps on the power leads that hold the ACC power high enough to prevent the transistor from turning off quickly and that is what keeps your headlights on for a few seconds. A bigger cap in the right place (base of the transistor with another diode) will give you delayed off headlights.
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Old 10-09-2008, 01:59 PM   #18
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Hmm, yeah, I did notice that the transistor is getting quite hot with the headlights on. I can't expect that the relay is drawing all that much current, but the transistor is only rated for 800ma.

For some reason my multimeter just won't measure amperage, maybe it has a fuse blown inside, I don't know, so I'm not really sure. I do know that the wires through the headlight switch are only 20 or 22 gauge and don't get hot at all. So maybe I'm not saturating the transistor?

My one friend thinks the 10k resistor might be the problem and suggested a voltage divider. So 'Acc On' -> 1k -> base -> .5k -> ground And that way it won't relay on grounding through the transistor as it's ground and should be seeing something like 5V at 10ma.

But with the headlights off and 'Acc On', so I'm putting whatever 12v/10k is through the base to ground off the emitter and that generates no heat from either the resistor or transistor so it must be drawing very little current. So I'm not totally sure the voltage divider/changing the base input will help me much.

Thoughts ?
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Old 10-10-2008, 02:00 PM   #19
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That's a great site you linked. I checked and my transistors are Hfe 200. So I need a 22k resistor, with the 10k I don't have enough current and it's not saturating which could be leading to the heat ?

Should I go get a 22k, use 2 10ks in series or can I just use a 33k?
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Old 10-11-2008, 01:28 PM   #20
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Hmm, well 33k ohm still makes it hot as all hell pretty quickly, so, hmm.
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