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 be the answer?
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.
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?
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 forget about having to worry about base current at all.
The other problem might be that you're wiring your lamps like this:
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.
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.
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.