Running a DC motor from BS2px

Here is a document that might interest you:

My suggestion is to start with a very simple circuit using a 2N2222 or

2N3904 and a 5K base resistor. For maximum learning potential be prepared to cook a transistor or two, they're cheap. If you have access to an oscilloscope, that would be great since you could see the actual voltages showing up on your motor terminals. You will want to experiment with clamping diodes as well. Motors are great for learning the trials of connecting your "fragile" PIC to a real world inductive load.

MOSFETs are great too. 2n7000 is a nice logic level input low-power MOSFET. For higher power, use one of the IRLxxx types as these have so-called "logic level" inputs. This means that 5V is usually enough to turn the MOSFET on hard.

A little more costly, but easier approach would be an Internationar Rectifier PVN012 Solid State Relay. Just put a 200ish ohm resistor in series with the LED, and you can switch AC or DC. Digikey has them for $5.85 in single qty. Considering the BS2PX runs about $79 each, I'd certainly prefer the electrical isolation of an SSR.

Chris

The suggestion to start simple is darn good, but needs a little more said. Those transistors are great for experimenting, cheap, and so what if you fry them.

The transistors mentioned are not the best choices for connection to the stamp or PIC. So the experiemnts should initially be done without it. When he goes to the stamp/PIC/uC the 2N3904 & 2N7000 are way too small in terms of continuous collector/drain current, with absolute maximum ratings of 200 mA. He might squeek by with the 2N2222 for a short time before it overheats and dies, if the motor is loaded. If the transistor is connected to the stamp and shorts, the stamp potentially dies. A TIP33 would work well. It has sufficient gain, continuous Ic and power dissipation ratings to switch the motor current on/off.

Ed

You're right, I should have checked first. I thought the 2N3904 would handle 500mA or so, oops.

As for the 2N7000, I didn't intend for that to be interpreted as a recommendation for the motor. I only said it was a handy part for low power switching. ;-)

On that I must disagree. Even if the transistor shorts to ground, the base resistor will limit the current protecting the STAMP.

My personal favorite is a TIP120 for heavier loads, probably because radio shack used to carry them. They will handle the OP's current requirements easily and the off leakage is very small. Being darlingtons, they don't need much drive either (Hfe is above 1000).

Yabut - it's really a very minor oops - it could be a valuable learning experience, which you stated at the outset. Frying a few transistors - which you warned him of - can really make the lesson sink in.

Don't get me started. I am drooling for the day when they make jellybean Mosfets that work well as switches with 1 volt on the gate - or (grudgingly) 3 v on the gate - and can carry heavy current with low Rds on. Give me some 10 amp, 6 cent, low Vg,

Yup, you're right. I forgot the base resistor.

Good point. I like them, too, but you pay a Vcesat penalty with darlingtons. It (the penalty) might work real well for him in this case. He could use a 4.5 or 5 v supply, and the Vcesat would drop it near nominal.

Ed

It's "only" about .6V at 1A. Logic-level MOSFETs are much nicer though IMO.