# I'm not biased, and neither is my transistor

Discussion in 'Electronic Basics' started by Phideaux, Jan 11, 2004.

1. ### PhideauxGuest

It's amazing how the simple things seem to elude me. I've set up this
circuit to drive a relay from a CMOS gate output.

http://im1.shutterfly.com/procserv/47b4df30b3127cce866a50d709b60000003610

When the gate goes high, it puts 10.8V on the 1K resistor to base.
The transistor just sits there. No current flow from collector to
emitter. Yes, they do use the same power supply, i.e. they share the
same ground and source. No glow from the LED. Nadda. FYI, I can't
seem to grasp the art of oscillators either. Maybe a torch is needed
to bring some warm to this transistor? What have I done wrong?

2. ### TimGuest

Have you tried driving the base of the transistor from the power supply
rather than through the CMOS gate? Does it work that way? Most CMOS gate
outputs only drive small current devices. Could that be the challenge?

3. ### John PopelishGuest

Your diagram appears to be using a transistor correctly. So I suspect
that your circuit is not actually connected the same as your diagram.
Measure the voltage on the base, emitter and collector pins with
respect to ground during operation and get back to us. A digital
photo of the circuit posted to the web site might also be helpful. I
made made silly errors on circuits and absolutely could not see them
until someone else pointed them out.

4. ### John LarkinGuest

The CMOS source may not be able to actually put 10 volts into a 2K
load, and the assumption of b=200 is way optimistic.

Put a voltmeter across the base and collector resistors and calculate
the actual currents to see what's going on.

Figure this out - quantitatively - before you tackle oscillators.

John

5. ### Ian BellGuest

Indeed, most people forget beta depends on vce and at saturation, which is
where this circuit should end up, beta is probably little more than 20.

Ian

6. ### Thomas C. SefranekGuest

So many things....

2N1104 is a ANTIQUE!
It is INCAPABLE of Hfe over 150.

You need to graduate to power FETs.
Even the simplest Power FET will do this job with room to spare.
For example, IRF210.
No gate (Base) resistor needed, and CMOS will drive it just fine.

--
*
| __O Thomas C. Sefranek
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://www.harvardrepeater.org

7. ### PhideauxGuest

Affirmative.

Does it work that way?

Negative.

Most CMOS gate outputs only drive small current devices.

True, sad to say, but true none the less.

Could that be the challenge?

It could prove to be one, but I don't think so in this case.

All good points Tim. I quess it's quite often the simple things that
we overlook. In another circuit, I miswired one lead of an IC and the
circuit that had been working quit. It took 30 minutes of going over
it for me to find it. In this case, I think there's something wrong

8. ### JoeGuest

Hi Phideaux,

Are you sure you have the part number of the transistor correct? I did a
google search and checked a couple of catalogs. No sign of a 2N1104. The
closest I found was a 2N1100 germanium pnp transistor.

Assuming it really is a npn transistor, why the 120 ohm resistor in the
collector line? Also, what type of cmos gate is driving it?

Joe

9. ### TimGuest

I could not find a pinout of the 2N1104, so I got a cross-reference from
NTE. Is this the pinout of the transistor?
http://www.nteinc.com/graphics/diag21a.gif

Are you certain you have it connected correctly? If so, have you tried using
the 12v power supply to activate the base to light the LED by itself?
(remove the 120ohm resistor/relay)

If that does not work, have you tried jumping the 2N1104 collector and
emitter with a jumper wire to insure the LED and relay work when hardwired?

After that, it will be voltage checking with an o-scope or voltmeter. I
would recommend getting the LED/transistor circuit working first. Then add
the relay and get it to work, then the CMOS gate.

One step at a time...

10. ### John LarkinGuest

It's a TO-5 silicon NPN, actually not a bad part for its age.

John

11. ### JeffMGuest

http://im1.shutterfly.com/procserv/47b4df30b3127cce866a50d709b60000003610
I'm with TCS on this one.
Microamps beat milliamps and there's fewer parts to buy/fail.
BTW, what's with that 120 ohm shunt?
Pull that sucker out first thing.

12. ### PhideauxGuest

You are correct about the 2N1104 being an antique. Looking at your
call sign, WA1RHP, you may be old enough to appreciate MARS program
refuse. Gee, I must have 1,000 or so lying about, so they are handy,
plentiful, and most importantly FREE. Perhaps you have made the
aquaintence of an old friend of mine, Paul Childress, WA5JIY? I have
him to thank for the abundance of antiquated surplus! Anyone in need
of a fan cooled 500W dummy load, in a water tight metal case? (Gives
new meaning to the phrase Old Navy!) Or silver plated variable air
gap capacitors? The list goes on and on. I still would like to
master the art of biasing transistors. Old Paul tried to teach me,
but I was young and not wanting to think much about serious matters
back then. Youth, it is indeed wasted on young people.

the job done, I just hope Radio Shack has them at the store. And
thanks, the trip down memory lane has been good.

13. ### PhideauxGuest

Joe, it is quite old, and it is an NPN silcon. The "7" digit output
from a CD4017B decade counter is driving it.

14. ### Thomas C. SefranekGuest

Yea, Free is often tempting...
Sorry, out of the 650,000 hams here, I know a lot of them, but not Paul.
You will get many requests for this, I lust after these kinds of loads.
I have some of these.
Send me your questions via E-Mail.
--
*
| __O Thomas C. Sefranek
(*)/ (*) Bicycle mobile on 145.41, 448.625 MHz

http://www.harvardrepeater.org

15. ### MichaelGuest

How come nobody has told you to put a diode accross the relay coil to
prevent back EMF blowing up the transistor?

Connect a 1amp diode, with cathode (stripe) to the + side of the coil.

problem, but is still required.