Super diode (active rectification) question

Discussion in 'General Electronics Chat' started by molecool, Oct 24, 2011.

  1. molecool

    Thread Starter Member

    Oct 24, 2011
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    Hi guys - my first post on this forum and my electronics knowledge is a bit rusty (mostly from my childhood). So please be gentle if I'm talking nonsense ;)

    I currently have a tiny pager motor which I'm driving directly via a 1.5V AAA battery - so far so good. Now what I'm trying to do is to limit the polarity going to that motor (for certain mysterious reasons - LOL) - meaning I only want it to turn if input A is negative and input B is positive, not the inverse.

    I can accomplish this by simply putting a regular junction diode in series - done and done. But as expected it'll eat up almost half my current as standard silicon diodes have a forward voltage drop of about 0.6V! So, I considered using a Schottky diode and that one will work but still eat up ~0.3V - thus affecting the rpm of my motor.

    [​IMG]

    Now I found this circuit above browsing around and was wondering if it may work just for my purpose by replacing the 'load' with my tiny pager motor?

    If it does - the IRLM6401 appears to be tough to get here in the U.S. - can someone suggest a more commonly available replacement?

    Any input would be greatly welcome - many thanks in advance.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    A very quick search on digikey brought up several suitable transistors. And I narrowed the search to almost the same main parameters of the IRLM.

    Btw, the same can be done with an n-MOSFET in the ground return path. I don't even think that the npn transistor is really necessary in your circuit...
     
  3. molecool

    Thread Starter Member

    Oct 24, 2011
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    Hey thanks for that. So since you didn't say otherwise this circuit would work for running my little pager motor?

    That last part however is completely over my head. Remember, my electronics skills are very basic. Could I bother you to draw an example on a napkin? ;-)

    Many thanks in advance :)
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    For this to work, the MOSFET would need to be well turned on by the voltage of a nearly-discharged battery. That is going to be less than 1V for a single AA cell. This is hard to achieve, not sure if you could find one at all easily. (I may be out of date with that though, so it may be worth a trawl.)

    I can't find a datasheet for IRLM6401, but did find IRLML6401: http://www.irf.com/product-info/datasheets/data/irlml6401.pdf

    This may or may not be the same thing: from its characteristics it will typically just about be struggling to come on on by 1V, but that can't be guaranteed. Good luck finding a better FET for the job: you are looking for an extremely low threshold device.

    Since you have only 1.5V to block*, why not just use a PNP transistor? You will need to set the base resistor so that the base gets about 1/10 as much current as the motor, which is a bit wasteful, but at least the device is not made of "unobtainium". You would lose maybe 0.3V across it though, maybe not so much better than a Schottky.

    *By which I mean, a typical base-emitter junction can just about hack blocking a couple of volts. Don't try this with bigger voltages though, as big reverse VBE usually wrecks transistors.

    Edit: It seems that I am indeed out of date with this, as with many things these days. Ah well...
     
    Last edited: Oct 24, 2011
  5. molecool

    Thread Starter Member

    Oct 24, 2011
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    BTW, here is the description for the circuit above which (as I already mentioned) dug up online:

    Much of that is chinese to me - I do understand what's being done but as mentioned already I don't have the skills to reconfigure it. IF someone could help out with replacing the IRLM6401 and possibly the PNP I would be grateful.
     
  6. molecool

    Thread Starter Member

    Oct 24, 2011
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    You lost me there - are you sure? The description that came with that diagram indicated that this would work at about 1.6V (very near to my 1.5V of my AAA battery).

    I'm all about avoiding unobtainium ;-) The reason why I'm not 'just using a PNP transistor' is because I don't know better. My electronics skills are basic at best. I can follow instructions, put a circuit together on breadboard, solder, etc. but I'm not capable of producing my own circuit. I'm a quick learner however!

    Since you mentioned the Schottky - here's a naive question: How about putting two or three of these suckers in parallel and be done with it? Shouldn't that reduce the overall resistance even further?

    Thanks!
     
  7. praondevou

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    Jul 9, 2011
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    Please go to digikey.com and type in MOSFET in the search field. Then click on FETs - Single (25,368 items). You'll have to choose the parameters to narrow your search.
    Choose p-channel, VDSS 12V, VGS(TH) max select everything from 400mV to 1V.
    Then check "in stock" and click on apply filters.

    Several mosfets will be shown. Start with the MOSFETs that have the lowest VGS(th) voltage. You'll have to open each and every datasheet to see what threshold voltage they have at your load current. (btw, how much current does the motor draw?) . If the gate voltage parameter says 400mV, that doesn't mean you can draw nominal drain current at this voltage... at about 1.2 V however a few 100mA should be possible... e.g. http://search.digikey.com/us/en/products/SI1417EDH-T1-E3/SI1417EDH-T1-E3CT-ND/1656804
     
  8. praondevou

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    If you need higher drain current than you can get at the lowest battery voltage available (down to 1V according to Adjuster), there might be a possiblity using a charge pump to increase the voltage available for the gate, even though I don't know if this works with voltages that low.

    Everything depends on how much current your load draws and how low your battery voltage goes.
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You would need to discharge the battery down to a lot lower than 1V to achieve some battery manufacturer's inflated mAh claims, but anyone who can get a circuit to go much below 0.6V deserves a medal. Germanium transistors maybe? (That's a joke, at least I think it is!)

    Note however that praondevou has found some lower threshold devices. 400mV! What is the world coming to these days, why in my day it took 2V just to heat the filaments...

    Actually, I've wondered about trying to make a step-up supply using MOSFETS from a low voltage input: the circuit could get gate bias from its own output, but only once it was working. Perhaps a bipolar "starting handle" that switches off once the FET circuit starts.

    All of which is getting seriously off-topic. Time to call it a night.
     
    Last edited: Oct 24, 2011
  10. molecool

    Thread Starter Member

    Oct 24, 2011
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    What's a 'charge pump'? Does it drain the battery quicker toward the end to keep a stable voltage? That would be extremely useful if it can be done without too much complexity (i.e. component cost).

    Maybe I should just hire one of you guys to design this thing for me. I rather pay someone and not waste hours/days of my own time, seriously. If anyone is interested please contact me directly.
     
  11. molecool

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    Oct 24, 2011
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    Say WHAT? You seriously lost me there - what's a step-up supply?
     
  12. praondevou

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    Similar to a charge pump :D You "step-up" an input voltage to an higher output voltage.

    Even your IRLML6401 is available at Digikey.

    Unfortunately the graph showing the relationship of VGS to IDS gets unclear near 1V VGS. According to the graph you should be able to draw 1A at 25 Celsius at 1V gate voltage...

    Question: how much current does the motor draw? How low do you want your battery to work?
     
  13. Ron H

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    I think you will be hard pressed to find a MOSFET in a leaded package that will be guaranteed work with Vgs=1.5V and less. I found several at Fairchild, but they require a reflow oven in order to be soldered.
     
  14. molecool

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    Oct 24, 2011
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    I really appreciate the effort but look guys - this does not help me very much. As I said in my intro - I am not savvy enough in electronics to put this together on my own. Most likely I would order some parts and the circuits wouldn't work because I wired them up wrong.

    What I am looking for is for someone to *draw me a circuit* that will do exactly what I'm looking for doing (see above in my intro). Everything else suggested is well meaning but not useful to me and thus simply academic from my perspective.

    I am being candid as I don't want to waste anyone's time.
     
  15. molecool

    Thread Starter Member

    Oct 24, 2011
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    Okay, now we are getting somwhere :)

    I would like the battery to be cut off around 1V or so - if there's a way to somehow 'pump' the voltage as you suggested that would be great. Again, I would need someone to design a complete circuit - and don't mind paying for that.

    Here's the URL of the motor's data sheet:

    http://www.seeedstudio.com/depot/datasheet/vibmot.pdf

    Many thanks for all the help!
     
  16. Ron H

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    I'll repeat praondevou's question: How much current does your motor draw? The answer is critical to the design. Given that it is a pager motor, I would guess that it draws less than 100mA, but I hate to guess.
     
  17. praondevou

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  18. molecool

    Thread Starter Member

    Oct 24, 2011
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    I posted the datasheet above - here it the URL for your convenience.

    http://www.seeedstudio.com/depot/datasheet/vibmot.pdf

    Sorry, probably should have provided that earlier as it's a key consideration. BTW, I did experiment around with that thing and it still runs when supplied about 0.7V - it's extremely responsive. But of course it's turning half as fast then. Thus some kind of cut off or 'voltage pump' as previously suggested would definitely sweaten the deal.
     
  19. praondevou

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    I wonder why it even works... starting voltage 2VDC?? 120mA current should be doable with a MOSFET, though.

    Sorry, I have to call it a day. ;)

    Cheers

    edit: we are all cross-posting, LOL
     
  20. molecool

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    Oct 24, 2011
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