Will this transistor work ?

Status
Not open for further replies.
J

Jessem16

New Member
Hi all,
I am wondering will these transistors work happily on 12V (In a night sensing circuit for a car)
Sorry but I have no clue what all that means.

BC108 NPN Silicon Planar Transistor
Description:
VCE max: 25V
VCB max: 30V
VEB max: 5V
IC max: 200mA
PTOT: 600mW
hFE min/max: 110/800@Ic=2mA
fT: 150MHz
Case: TO-18 ( Metal Can )
Manufacturer - CDIL

Thanks in advance.
 
Here's the circuit diagram, It does state in this its for a 9v use but I have run this on 12V without any problem but i was using a transistor that was laying around.

View attachment 64321
 
Last edited:
You'll need a diode across the relay coil, or you'll damage the transistor when it turns off.
Something like a 1N4001.
 
thanks HouseOFwax I'll get a diode for that,
Last thing I want is to kill a new transistor and Here in Australia you can't find it, so I had to order some from the UK.
 
You'll need a diode across the relay coil, or you'll damage the transistor when it turns off.
Click to expand...

Not really. The transistor turns off so slowly that no back EMF is generated.
That's the reason why you need to learn the basics of electronics.
 
Last edited:
I would also put a current limiting resistor in series with VR1 as you could fry the base-emitter junction of the tranny when you wind VR1 to zero ohms.
 
I would also put a current limiting resistor in series with VR1 as you could fry the base-emitter junction of the tranny when you wind VR1 to zero ohms.
Click to expand...
In this connection (emitter follower) the limiting base current resistor is not needed.
The relay is self limiting the current.
 
Last edited:
Yes it is. Depends on the series resistance of the relay coil. Chances are it will fook the tranny. A BC108 is NOT a power tranny.
Sorry but I have no clue what all that means.
Click to expand...
Not my words!
 
Last edited:
colin55 said:
Not really. The transistor turns off so slowly that no back EMF is generated.
That's the reason why you need to learn the basics of electronics.
Click to expand...

What rubbish. I think you need to understand the bigger picture and not be so arrogant in the company you are in on this forum.

You cannot expect to become a designer of circuits on your assumptions of how it will performs in your simple environment.

Anything could happen to the LDR fast enough to cause fast change that could fry the tranny. Again depending of all sorts of relay coil parameters you clearly don't understand.

Bad solder joints that ping off could also cause the same effect.

To be a good designer, you need to look after the don't care states that can occur and deal with them in a safe manner.

You need to listen and learn.
 
In this particular circuit, the emitter follower with base connect to the ground via fairly low resistor, the relay buck EMF does not exceed - 0.7V.
 
You do not want to FOOK the tranny!
But the transistor will cook if is turned on with an unlimited amount of base current and its emitter load is shorted.
 
Last edited:
Jessem16 said:
thanks HouseOFwax I'll get a diode for that,
Last thing I want is to kill a new transistor and Here in Australia you can't find it, so I had to order some from the UK.
Click to expand...

I wouldn't try importing from the UK for this transistor. Even if you can't find a BC108 there are hundreds of varietiesthat will do the job. Try a BC548 26c from Jaycar or 21c from Altronics
 
A BC108 is from THE DARK AGES.
I used them 48 years ago. Today their very old metal case is very obsolete.

They were just an ordinary little silicon transistor but with a wide range of current gain. Some had high gain and others had low gain. Different versions (BC108A, BC108B or BC108C) were measured for reduced ranges of current gain.
 
Oh darn I already ordered them But its no biggie They where about $6 Au including postage.
So in my circuit I can use these BC548 26c Without any troubles, If I need to ? I wan't to have a good adjustment range for this as i'm using it for automatic headlights.
When you say I don't need to have a diode in place to stop the transistor from frying because of the relay, What if I say for instance I drive into a tunnel or I start the car during the night Will this cause it to fry ?
Sorry for the noob Question I'm only just starting to learn electronics so I don't Know much
 
Time to take a step back. I have been making my head hurt by doing some research and some maths. We are talking about automotive relays here. Typical coil resistance is between 50 to 120 ohms. I=V/R so 13.8/120 equals 115mA. Well above what a BC108 or BC548 will handle. So in answer to Jessem's first question, the BC108 will NOT do the job. I would suggest using a BD649 darlington. Rated way above what is needed but will do the job reliably and tolerant of newbie mistakes.

A clamping diode is definately called for, if only for best practice, as some of my reading suggests 200V back emf on one of these relays when switched. I am not sure of the response times of an LDR but you always design for worst case. I would suggest a 1N4004 for the reasons given above.

If you are buying components in Australia, I suggest Jaycar as they have branches everywhere. Altronics is good for mail order. In Brisbane I also use Prime Electronics but they are not cheap unless you buy large quantities.

While researching I came across a good introduction to automotive relays https://www.electro-tech-online.com/custompdfs/2012/05/hweb2.pdf

I hope this helps
 
WTF Pepper, Its ok I'll just keep the ones coming and plus there already on there way so I can't get my money back.

hdc090360, Thanks and On this I'm using a circuit board mounted relay thats rated ok for this use (So I'v been told by jaycar)
The the output of that relay powers a second relay being a automotive relay. So there's no big strain.
And yea Jaycars great thats the only place I get my stuff from and there very helpful to.
 
Last edited:
Status
Not open for further replies.

Similar threads

G
Will this circuit work?
2 3
Replies
44
Views
7K
audioguru
  • Question
Simple Network – Will it Work?
Replies
15
Views
2K
AnalogKid
T
  • Solved
please help, i need this circuit to work :(
Replies
13
Views
2K
shokjok
S
A
  • Question
Latch LS374 works for sometimes and doesn't work at all next clocks
2
Replies
26
Views
1K
Tony Stewart
7
  • Question
problems getting M74HC259B1R to work
Replies
18
Views
2K
777xxx
7
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…