Super diode (active rectification) question

Thread Starter

molecool

Joined Oct 24, 2011
32
To the Electrician - I shared the simple circuit you proposed here and got the following input:

"The circuit that you linked uses a PNP transistor--the best PNP is rated at 5A and has significantly higher Vce sat."

Not sure I understand but I take it that perhaps the use of a NPN transistor may be more advantageous? Your comment was pretty clear on this one having a low Vce sat and as I'm a lightweight in all things electronics you can imagine I'm a bit confused by now... How does the transistor's saturation mode affect my design? Are we talking about threshold or loss of forward current?

Again, please forgive my obvious ignorance.
 

Thread Starter

molecool

Joined Oct 24, 2011
32
PNP transistors have really improved since I've last looked at low power switching! Only downside is the package. The roughly 1206 package is still big enough to solder without magnification, but you need a board to put it on, unless you use the dead bug method with wire wrap wire. A LOT of the cool stuff is coming out in SMT only, so get practicing now!
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Once more I may be able to help (time for some karma pay back!):

http://www.schmartboard.com/index.asp?page=smt_to_soic

Just ordered the 10-pack yesterday which should even give me enough to produce one functioning one for my breadboard ;-)
 
To the Electrician - I shared the simple circuit you proposed here and got the following input:

"The circuit that you linked uses a PNP transistor--the best PNP is rated at 5A and has significantly higher Vce sat."
"...has significalntly higher Vce sat.", compared to what? Ask for a link to a datasheet or part number for a NPN with significantly lower Vce sat.

ZETEX has a NPN complement with essentially the same Vce sat, the ZTX688B.

I used a PNP in my example circuit because I had some on hand so I could wire up and test an actual circuit, but there's no reason why an NPN can't be used.
 
It looks to me like we're getting closer to what you want to do. Do you want a circuit with two motors with the property that if you connect a battery in one polarity, one of the motors spins and the other doesn't, but reversing the battery reverses the motors which spin and which don't?
 

thatoneguy

Joined Feb 19, 2009
6,359
Quick question for OP:

Why not just use a battery holder? They are designed so that no electrical contact is made if the battery is inserted backward. A cheap solution that is easy to implement.
 

Ron H

Joined Apr 14, 2005
7,014
Quick question for OP:

Why not just use a battery holder? They are designed so that no electrical contact is made if the battery is inserted backward. A cheap solution that is easy to implement.
I hate you.

EDIT: I hate you. ;):p:D
 
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Ron H

Joined Apr 14, 2005
7,014
Please see the comment I posted earlier above... any takers?
If you want to spin one of two motors, depending on polarity, it would have been really cool if you had stated that in the beginning.
I don't see how this relates to the LM2623.:confused:
 

Thread Starter

molecool

Joined Oct 24, 2011
32
It looks to me like we're getting closer to what you want to do. Do you want a circuit with two motors with the property that if you connect a battery in one polarity, one of the motors spins and the other doesn't, but reversing the battery reverses the motors which spin and which don't?
EXACTLY - sorry for not mentioning this in the beginning. The reason why I didn't was because I thought I would simply use two circuits of the one I described. My bad... :rolleyes:

The other issue that has crept into this thread is boost converter of some kind as I would like to maintain the rpm of the active motor at what it is when connected to a fully charged AAA battery. I prefer a constant rpm with a dead stop over a slowly degrading extended performance curve. And that's where the LM2623 may come in - unfortunately it's very expensive and so is the TPS61200.
 

thatoneguy

Joined Feb 19, 2009
6,359
No guys - that is not going to work for my purposes. There is method to my madness.
Could you do us all a favor and explain either the method or the madness part?

There could be a sweet solution ready to go if you just let us know the project, such as how many motors, why they may be hooked up backwards, etc.

Engineers are good at solving problems like that when the problems are laid out all at once instead of a little bit in each post. I don't mean to sound harsh, but...


--ETA: If you have 2 motors and want them to spin depending on polarity, you can use a DPDT switch and not bother with switching the battery around, just hook the battery to the common, and the motors to each throw of the switch, then, instead of removing the battery and putting it back in, you just need to flip the switch.
 
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Thread Starter

molecool

Joined Oct 24, 2011
32
--ETA: If you have 2 motors and want them to spin depending on polarity, you can use a DPDT switch and not bother with switching the battery around, just hook the battery to the common, and the motors to each throw of the switch, then, instead of removing the battery and putting it back in, you just need to flip the switch.
Yeah, but that's not what I need, mate - LOL :)
 

thatoneguy

Joined Feb 19, 2009
6,359
Yeah, but that's not what I need, mate - LOL :)
What do you need? So extra stuff isn't added once a design is started? I'm looking at the original post, then the new requirements, and other things you'd like.

If you explain what you are trying to accomplish, there may already be a solution for it, is essentially what I'm saying/asking.
 
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