Troubleshooting Transistor Switching with Arduino

[nothign]

Transistor Switching with the Arduino

I know nothing about electrical engineering..

I have a TIP3055 NPN Transistor rated at 60V and 10Amps .. I have 12v 2amp current coming off of a "power supply" that's plugged into a regular american grounded wall outlet. The 12v is plugged into the transistor's collector and the ground is plugged into the emitter. But when I do an analogWrite off 255 from the 3rd pin on my arduino to the Base it doesn't respond (except for oddly messing up an LCD display I have hooked up to it).

But when I connect 5v from the arduino to the collector and a resistor followed by an LED to ground to the emitter then it lights up...

SOO how can I trouble shoot this?

I'm basically trying to switch that wall current on and off to heat up some nicrome because 5v obviously won't do it but I don't want it continuously on.

 

[skeptic2]

The 12v is plugged into the transistor's collector and the ground is plugged into the emitter.

Where was the nichrome wire connected?

 

[nothign]

The nicrome is connected after the emitter and goes to ground..

so 12v (from powersupply) -> Transistor Collector -> Transistor Emitter -> Nicrome -> Ground (from power supply)

Then arduino pin 3 -> Transistor Base

 

[jim hardy]

how much current can your arduino pump into the base?
note 3055 doesn't have a lot of current gain.

 

[ColinW]

Putting a load between the emitter and ground doesn't work on an npn transistor. When current flows through the load, the emitter voltage rises and Vbe falls partially switching off the transistor. Put the load between the 12V supply and the collector.

 

[nothign]

Moving the load between the 12v and the collector didnt seem to change anything.. the LCD screen gets crazy and the nichrome remains unaffected.

Im still frustrated by this but now I think I am going to switch to using a relay to see if it works any better.

 

[vk6kro]

You might be safer using a relay to do the actual switching.

These have a coil which attracts a piece of soft iron and this pulls some contacts together. You can use these contacts to do the actual switching.

The Arduino may be able to supply enough current for the coil or you may need to use your transistor to amplify the current. This may be safer for the Arduino.

You would use a circuit like this:
http://dl.dropbox.com/u/4222062/relay%20driver2.JPG

The contacts above the rectangle marked "relay" would be used to switch the power to the nichrome wire.

The resistor marked "R" will depend on the resistance of the coil relay, but something like 1000 ohms might be OK. Don't omit this resistor.

 

[ColinW]

I'm not familiar with the Arduino. Are you trying to use an analogue output or a pulse width modulated (PWM) output? If it's an analogue output I think you will struggle because most interface cards have very limited output current capability. If it's a PWM output then you need to assume it acts like a switch to ground and not a voltage source. Logic 1 is when the "switch" is closed and the output is 0V. Logic 0 is when the switch is open and the output just floats. A small npn transistor can be turned on by adding a pullup resistor but not something big like a 3055. You could use a pnp, either a big one to drive the load directly or better, a small one that is then used to drive the 3055.
Big power transistors have low gain so they need a lot of base current.