# how to get more amps to my lamps?

Discussion in 'Electronic Basics' started by Alex Bryant, Aug 31, 2004.

1. ### Alex BryantGuest

I have a series of mini-bulbs (#44 lamps, like the kind used in
pinball machines) underneath a platform in my project. My
microcontroller hits various transistors in order to get the 6 volts
to the appropriate lamp. Everything works, but the lamps that are
farthest away from the power source get their current from small wires
about 6 feet long, and hardly glow at all. I need all these lamps to
glow their brightest, to be seen through a series of clear plastics.
Because of the way I've built this whole thing, I can't get the power
source (4 AA batteries) much closer to the bulbs. Would thicker wire

Thanks
--Alex

2. ### Michael A. CovingtonGuest

How thin is the wire? I think you have something else wrong.

The resistance of a #44 lamp, when illuminated, is about 25 ohms. Your wire
would have to be microscopically thin to have a resistance comparable to
that in a 6-foot run. Check voltages at both ends.

3. ### Alex BryantGuest

Well, it's not microscopically thin, but it's close...standard red
Radio Shack project wire. I'm on the road at the moment and can't
check the voltage, but maybe I'm implementing the transistors
wrong...Would the rating of the resistor going to the base of my
transistor, affect how much current goes out the emitter to the lamps?

thanks
--Alex

4. ### John FieldsGuest

---
BINGO!

Depending on how much current comes out of your micro's IOs when
they're feeding the base of the transistor, figure the drop across the
base resistor plus about 1.2V and subtract that from the supply
voltage. What's left over will be the voltage into the lamp(s).

What you should do is use the transistor for a low side switch, like
this:
+V
|
[LAMP]
|
C
IO>---[R]---B
E
|
GND

The value of R will depend on the gain of the transistor and how much
current your micro can source, but for a #44, which takes 250 mA at
6.3V and a transistor with a beta of at least 100 with 250mA going
through it, your IO will need to supply 2.5mA, so that's about 2000
ohms. Regardless, you should make sure your transistor goes into
saturation so the power it dissipates (which will be 'stolen' from the
lamp) won't cause excessive dimming. Or, use a logic level MOSFET and

The other problem might be that you're wiring your lamps like this:

+6V-----+- - - - - -+
| |
[LAMP 1] [LAMP n
| |
GND-----+- - - - - -+

When, if you're using thin wire, you should be wiring them like this:

+6v-------------------------+
|
+6v------------+ |
| |
+6V--+ | |
| | |
[LAMP 1] [LAMP 2] [LAMP n
| | |
D | |
IO>-G | |
S | |
| | |
GND>-+ | |
D |
IO>-----------G |
S |
| |
GND>-----------+ |
D
IO>------------------------G
S
|
GND>------------------------+
That way, the voltage drop in the wires caused by the current draw of
each lamp will be restricted to its own loop and won't affect the
other lamps.

5. ### Michael A. CovingtonGuest

Quite possibly. Can you post the circuit diagram on your web site and refer
us to it?

Also, your wire is nowhere near thin enough for its resistance to have an
appreciable effect.

6. ### Alex BryantGuest

I have posted a schematic of this setup at:

https://home.comcast.net/~alex1138/

Please let me know if there is a better way to set this up...as I'm a
novice, any feedback would be appreciated!

--alex

7. ### John FieldsGuest

---
1. Make sure the transistors can take 300mA of collector current
without coming out of saturation.

2. Connect one side of each the lamps to +6V.

3. Connect the other side of each lamp to the COLLECTOR of its
transistor.

4. Connect all of the transistor EMITTERS to -6V.

5. Replace the 10K resistors with 330 ohm resistors.

That should do it

8. ### Michael A. CovingtonGuest

Please let me know if there is a better way to set this up...as I'm a
Put the lamps in the collector lead rather than the emitter lead of the
transistors.

As it is, you are using the outputs as emitter follower voltage regulators.
The lamps do not get full voltage. They get 0.6V less than the output
voltage of the PIC.

9. ### Rich GriseGuest

And invert the logic of your outputs.
Cheers!
Rich

10. ### Rich GriseGuest

Wrong - never mind.

11. ### Alex BryantGuest

I now run the lamps in the collector lead, BEFORE the transistors in
the circuit path, and have replaced the random resistors that were in
the breadboard with new 220 Ohm resistors. Also, I noticed that some
of my transistors simply didn't work...I bought a brand new pack from
Rad Shack and replaced the transistors for the lamps that weren't
lighting, and they all started working like a charm. My 6V lamps are
now getting over 5.5V, which is plenty for me.
This is just a proof-of-concept version (translation: very
sloppy, no points for neatness, just make it work), so my next step
will be to clean up all that wiring, eliminate some of it, hopefully
replace a good bit with a 40-pin ribbon cable.
I still have a few weeks till the 'fun' phase of the project
begins. I don't know whether anyone's interested, but I'll post some
pictures, once I've brought together the functionality from the
different design phases.

Thanks to everyone for the help and advice....

--Alex