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Old 01-26-2009, 03:41 PM #17
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Default Re: Adjustable Micro-Driver and 803t Questions

Yes rog, this thread is done. kernelpanic, I've already looked at your driver thread, and it's absolutely under consideration. I had in fact asked rkcstr if I could rig up something using either a two way or three way switch with his driver board, but I'm pretty sure it's just up to my soldering and design ability. Thank you all for the help given.


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Old 01-27-2009, 10:28 AM #18
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Default Re: Adjustable Micro-Driver and 803t Questions

Quote:
Normal mosfets like the 2sk1112 will work. But a logic level mosfet (like an IRL-510) will work better for battery powered circuits. Also I recommend NOT using a pot to adjust power on the fly. Better to use fixed resistors and a momentary SPDT push-button to activate 'burn mode'. (Schematic attached.)
The 2SK1112 has a gate threshold voltage of 2V max and a drain current of about 3 amps with a gate voltage of 3V. I'm pretty new to this stuff but I thought those figures would make it a good candidate for this driver and in my breadboarding I was getting a dropout of about 0.2 V with a ld current of about 4.6V and driver input of 4.8V on my adjustable power supply. I adjusted some component values around, like the sense resistor I dropped to 1ohm to try to lower the voltage drop.

I realise that individual components can vary quite a bit so the results that I got may have been pure luck. I would be interested to hear your comments on this but I don't want to hijack the thread too much and I hope that this information is of interest to the OP seeing as it is related to getting a low dropout voltage and whatnot
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Old 01-27-2009, 10:57 AM #19
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Default Re: Adjustable Micro-Driver and 803t Questions

Positron,

As I said the 2sk1112 will work. There also should be no real difference in dropout either way. I use logic level mosfets so that the regulator circuit can fully open the mosfet's gate. Why? So that when the battery gets too weak to provide the set current, the regulator will open the gate on the mosfet completely and provide whatever current is available to the load with minimal waste.

If one isn't using this circuit with batteries, logic-level mosfets aren't required...

Make sense?

cheers,
kernelpanic

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Old 01-27-2009, 11:11 AM #20
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Default Re: Adjustable Micro-Driver and 803t Questions

Quote:
Originally Posted by kernelpanic
Positron,

As I said the 2sk1112 will work. There also should be no real difference in dropout either way. I use logic level mosfets so that the regulator circuit can fully open the mosfet's gate. Why? So that when the battery gets too weak to provide the set current, the regulator will open the gate on the mosfet completely and provide whatever current is available to the load with minimal waste.

If one isn't using this circuit with batteries, logic-level mosfets aren't required...

Make sense?

cheers,
kernelpanic
Thanks for the explanation. It makes perfect sense, if the battery is dropping to 2.6V or something and the mosfet is not turned on very hard it will be dropping voltage as well as the sense resistor so the ld will be left with less voltage. (at least that's how it sounds in my head )

How would you tell if a mosfet is a logic level mosfet? I have looked at the IRLXXXX datasheets and find that the gate threshold voltage and graphs that plot drain current vs gate to source voltage seem to be very similar to other mosfets that don't seem to mention that they are a logic level mosfet. Like the 2SK1112 for instance. The datasheet claims a max RDS(on) of 0.3ohms with a Vgs of 4V. To my (inexperienced) mind, that kind of seems like it would be perfect to switching decent current at a logic level of 5V. So why would it not be considered a "logic level" fet?

Sorry for all the question. Maybe I should start a new thread with questions like this. :-?
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Old 01-27-2009, 01:49 PM #21
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Default Re: Adjustable Micro-Driver and 803t Questions

Quote:
Originally Posted by positron

Thanks for the explanation. It makes perfect sense, if the battery is dropping to 2.6V or something and the mosfet is not turned on very hard it will be dropping voltage as well as the sense resistor so the ld will be left with less voltage. (at least that's how it sounds in my head )

How would you tell if a mosfet is a logic level mosfet? I have looked at the IRLXXXX datasheets and find that the gate threshold voltage and graphs that plot drain current vs gate to source voltage seem to be very similar to other mosfets that don't seem to mention that they are a logic level mosfet. Like the 2SK1112 for instance. The datasheet claims a max RDS(on) of 0.3ohms with a Vgs of 4V. To my (inexperienced) mind, that kind of seems like it would be perfect to switching decent current at a logic level of 5V. So why would it not be considered a "logic level" fet?

Sorry for all the question. Maybe I should start a new thread with questions like this. *:-?
It probably doesn't help that the only datasheet available for the 2sk1112 is in kanji. The 0.3 ohms you *see with 4v on the gate will drop to 0.15 or so with 10v on the gate. Whereas with a logic level mosfet minimum resistance occurs around 5v... Since the 'on' resistance of the 2sk1112 at 4Vgs is lower than that of the IRL-510 anyway, I doubt it makes too much of a difference.

Since the op-amp automagically keeps the sense current equal to the set current, it'll adjust to almost any N-channel mosfet. (within reason...)

cheers,
kernelpanic

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Old 01-28-2009, 10:20 AM #22
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Default Re: Adjustable Micro-Driver and 803t Questions

That makes sense.
In case anyone is interested, you can get 20 for $6 shipped from here:
http://cgi.ebay.com.au/20pcs-N-Chann...3286.m63.l1177

I have attached an English data sheet too if you want a look. It's a pdf but I had to zip it as the forum software wouldn't let me attach a pdf.
Attached Files
File Type: zip 2SK1112.zip (188.0 KB, 17 views)
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Old 01-28-2009, 01:47 PM #23
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Default Re: Adjustable Micro-Driver and 803t Questions

Quote:
Originally Posted by positron
That makes sense.
In case anyone is interested, you can get 20 for $6 shipped from here:
http://cgi.ebay.com.au/20pcs-N-Chann...3286.m63.l1177

I have attached an English data sheet too if you want a look. It's a pdf but I had to zip it as the forum software wouldn't let me attach a pdf.
O.K. Thanks for the english datasheet. It is a logic level mosfet. The phrase '4-volt gate drive' is a dead giveaway...

cheers,
kernelpanic
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Old 01-28-2009, 01:55 PM #24
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Default Re: Adjustable Micro-Driver and 803t Questions

Quote:
Originally Posted by kernelpanic

O.K. Thanks for the english datasheet. It is a logic level mosfet. The phrase '4-volt gate drive' is a dead giveaway...

cheers,
kernelpanic
haha, cheers ;D
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