smd parts...and a little knowledge needed

[strokedmaro]

hello again...attached is a schematic for a preivious project I completed (with a lot of help) and Im in need of a little info and help with parts. Ok a little operation info..on the left of the diagram there is "A" "B" and "OFF.....these inputs come from a 4028 to turn the solenoids on and off. Im really trying to get this by myself so no bashing...just help if I make a mistake.

In the diagram the solenoids are 25ohm's...at 13.8vdc (typical car voltage) that would be 552mA right?

looking at the TIP41's data sheet (also attached) I read it as being able to sink up to 6A...Im having a hard time understanding the saturation part...a saturated transistor is one that conducts the best between collector and emitter?

ok back to the diagram... I understand the operation of this circuit but am not sure if I could figure out the values needed on my own...Now to the 2n2222's...in the diagram the 2n2222's "steal" the current from the Tip41's bases preventing them from "conducting" (whats the word for this) between collector and emitter. So doing the math again 13.8 volts and 220ohm resistor gives me 62mA right?

If these calculations are correct can I swap the 2n2222's for ANY npn transistor capable of sinking 62mA or more? Same with the Tip41's ...if the calculations are correct I would need a transistor 1/12th the size to do the same job (approx....I would go with at least one amp to be safe)

Any help appreciated. I'm trying to do an smd board as small as possible for my next project and it would help to figure this out

If I'm way off don't hesitate to slap/correct me and as always "Thank you"

EDIT: Whats a small smd diode that could replace the 1n4001's I use in this diagram?

 

[strokedmaro]

will this work in place of the 1n4001's? (attached)

 

[philba]

S1A looks like a direct replacement to the 1N4001.

My general purpose SMD NPN is an MMBT3904 ($.06/1 or less) which is the SM version of the 2N3904. It's a little less beefy than the 2222. MMBT2222A is the SM version of the 2N2222A if you want an exact match. This is still not quite up to the Ic of the non-A version. I don't know the SM version of the TIP41C but there are a number that would fit the need. In general, switching transistors are pretty interchangable as long as the Ic is OK. I'd check the Hfe as well but have never had an issue with that as most circuits are designed very conservatively in that regard.

 

[Valence_4]

Yep ! MMBT3904's will do the job. They will work in a 2.4volt-60mA environment and the 3904 will do the same job as the 2222.

Same thing for the S1A diodes. They will do the job. You could also install 1N400x diodes directly on the solenoid's terminals also. That configuration will prevent any back-EMF shock in the solenoids are unplugged from the circuit while energized.

Hope that will help !