Help with 12v relay and transistor

[StealthRT]

Hey all, i have been up and down trying to figure out why this is happening to me! This is the stats:
- Small Signal Transistors NPN 30V 100mA HFE/8
- 1k resistor
- 10k resistor
- 12v SPDT relay
- 1N4004 400v 1a standard rectifier

I am modeling it after this schema:
**broken link removed**

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Here is the picture of my board...
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I supply the 12v to it & the 5v. I get the these values:

Emitter: 0v
Base: 0.75v
Collector: 1.39v

I'm using this transistor (Small Signal Transistors NPN 30V 100mA HFE/8):
SS8050CTA

Here the relay i am using (Automotive SPDT 12VDC AUTO RELAY):
http://pewa.panasonic.com/pcsd/product/pwr/pdf/mech_eng_js.pdf

What its doing is when I supply the 12v to one side of the relay it opens it up. I want it to open up once i add the 5v to the other side. I'm using an ATMEGA168 chip with the digital pin out for the 5v (but right now I'm just hooking it straight up to 5v). But for some reason its just opening without even the 5v being applied. And I'm getting those low volts from the base and collector and i have no idea why..

Any help would be greatly appreciated!

David

 

[duffy]

Actually, that's a HIGH voltage on the base if you want it to be "off". It should be near 0V for what you are describing. Find out where that base voltage is coming from.

You seem to have it hooked up correctly. Another possibility is a bad transistor.

 

[StealthRT]

But i want it to come ON once i supply the 5v. But nothing happens when i do that. Right now i corrected the problem with the 12v triggering it open. But still can not open it with the 5v.

I tested the relay to make sure it was working and it does. I put 12v on one side and 5v on the other and it opened. Took the 5v away and it closed.

But with the resistors/transistor, i cant do the same thing for some reason??

David

 

[duffy]

Oh. Reduce that 1k resistor to 220 ohms, you aren't saturating the transistor if it has a 1.39V C-E drop.

 

[StealthRT]

And leave the 10k on it as well?

David

 

[duffy]

Yes, that's fine. It isn't really necessary with a port pin drive... but it's helpfull for this debugging stage since it will make sure the transistor is "off" when you don't apply power to the base.

 

[StealthRT]

Ok, do u happen to have the color code for the 220k? I have a lot of resistors here so it would be nice to know the color rings

David

 

[duffy]

I didn't say 220k, I said 220 ohms. Red red brown.

 

[StealthRT]

red/red/brown/gold

David

 

[duffy]

Gold or silver or brown or red or others - there's lots of tolerance bands.

By the way, do you have an older brother?

 

[StealthRT]

Still no click.

Base: 4.63v
collector: 12.10v

????

David

 

[StealthRT]

And no, no older brother. Only an older sister by one year.

David

 

[StealthRT]

With the 1k in and 220 out:

Base: 4.15v
Collector: 12.10v

David

 

[Nigel Goodwin]

With the 1k in and 220 out:

Base: 4.15v
Collector: 12.10v

You've either connected it wrongly, or the transistor is blown.

You can't have 4.15V on the base, it should be about 0.7V, no more.

 

[StealthRT]

You've either connected it wrongly, or the transistor is blown.

You can't have 4.15V on the base, it should be about 0.7V, no more.

How can i check the NPN to see if its blown or not? I have a multimeter that has a NPN/PNP tester and it came up with 058 if that means anything?

David

 

[duffy]

Do a diode check from base to emitter, then from base to collector, both ways. It should read about .7V one way, open the other on both junctions.

 

[StealthRT]

I did the test like so: (replace 9v with 5v)

B (led lead)/E (ground) = LED lights up (0.71v at base)
B (led lead)/C (ground) = LED lights up (0.69v at base)

**broken link removed**

David

 

[StealthRT]

On another NPN i tested i got:

B (led lead)/E (ground) = LED lights up (1.36v at base)
B (led lead)/C (ground) = LED lights up (0.70v at base)

So which one is correct???

David

 

[duffy]

The first one, unless the second is a Darlington transistor.

 

[StealthRT]

Ok, replaced it with a brand new one and this is the values i get

B (led lead)/E (ground) = LED lights up (0.74v at base)
B (led lead)/C (ground) = LED lights up (0.74v at base)

David