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Offline doobedoobedo

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Using a Zener for low voltage cutoff with a mosfet switch?
« on: April 28, 2014, 06:49:17 PM »
I want to create a low voltage cut-off with a mosfet/tact switch. I'm using the IRLU3114ZPBF with a 15k resistor across the gate and source. I have some 3.3V zener diodes.

I though I'd just mock something up to check so I created a basic voltage divider with a pot so I could change the voltage:



R1 and R2 are 4k7, R3 is a 100K pot. The voltage across R1 can be set by adjusting R3, the battery is at 3.8V.

Surely when the voltage across R1 is over 3.3V the voltage across R2 should be 3.3V (it doesn't appear to be)?

Am I being an idiot?

Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #1 on: April 29, 2014, 11:51:07 AM »
The problem you'll run into is you'll get into the active region of the MOSFET so it won't actually act like a switch, it will act like more like a variable resistor.

You need to provide a logic signal to the MOSFET (on or off).  You could do it with an arrangement of transistors, but with the component count, it would actually be easier to use a voltage detector.  It's a simple 3 wire chip that provides an on or off signal depending on voltage level.  It makes for a very simple circuit, simply a voltage detector and MOSFET, no other components.

Here are some that will work with a single battery;  http://www.onsemi.com/PowerSolutions/product.do?id=NCP300

I've not found ones that go up to the 6V you need for dual batts, but it should be possible to find one with some digging around.

Offline doobedoobedo

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #2 on: April 29, 2014, 12:29:27 PM »
Thanks Craig :). Single battery is what I'm after.

So something like this http://uk.farnell.com/on-semiconductor/ncp300lsn30t1g/v-detector-3vth-cmos-act-lw-5tsop/dp/2318558?Ntt=NCP300  would give me a 3V cutoff and supply 3V to the gate?

Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #3 on: April 29, 2014, 01:19:04 PM »
Looks correct assuming that MOSFET you mentioned is an N-channel.

You want the correct logic for the detector, you have the choice of active low or active high.  For an N-channel you need active low, for an P-channel, you need active high. 

It's a simple circuit, with an NCP300LSN30T1G;

Input pin on the detector to battery positive. 
Reset pin on the detector to NMOS Gate pin.
Ground pin on the detector and NMOS Source pin to battery negative.
NMOS Drain pin to the ground return on your circuit.

I would probably use a P-channel myself.  In that case with an NCP300HSN30T1G;

Detector Input pin and PMOS Source pin to battery positive.
Detector Reset pin to PMOS gate.
Detector Ground pin to battery negative.
PMOS Drain to the supply for your circuit.

I didn't look at the specs on the MOSFET you mentioned, but you need one with a pretty low VGS(th) for a single battery.  Look for one that has a RDS(on) rating for 2.5 Volts.  That's an easy way to spot them right off, otherwise you can look for a maximum gate-source threshold 1.5V and under.

Offline doobedoobedo

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #4 on: April 29, 2014, 01:34:17 PM »
Thanks again Craig.

I have a load of those N-Channels. 2.5V Vgs fully on. I've built a few mods with them now they work great. All that was missing was low V cut-off and it looks like that's going to be pretty easy now too :).

I love how by using a tact and a mosfet switch you can squeeze an unregulated mod into a tiny space.

Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #5 on: April 29, 2014, 02:31:27 PM »
Sounds good.  If it has a 2.5V rating it will work well with a single battery.  Yes, tactile and MOSFET does work well for minimizing things.  If you add a resistor, you may be able to get the MOSFET to do double duty for both the switch and cut-out.  I'd have to think about it, should be possible though.

Yeah, got it, put a pull down resistor on the NMOS gate and put the tactile inline with the reset output from the voltage detector.


« Last Edit: April 29, 2014, 02:38:35 PM by CraigHB »

Offline doobedoobedo

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #6 on: April 29, 2014, 02:51:21 PM »
This is how imagined it should be wired:


Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #7 on: April 30, 2014, 12:00:05 PM »
That should work, but I would probably put the switch inline with the reset output and NMOS gate.  It's not a problem to leave the voltage detector powered full time.  The current demand is very low for that part, only about 150nA as in billionths of an Amp.  Though the specs say 250nA, I've found that part actually draws closer to 150nA in using it myself.  That's a negligible power consumption that will have virtually zero effect on battery drain.

There's a benefit in leaving the part powered all the time in that you'll be able to take advantage of it's "hysteresis".  That's a 150mV gap between enable and disable depending on whether there's a rise or fall in voltage.  It keeps the part from cycling from one state to another on the verge of its detection.  When it has to power up every time, you won't get the hysteresis.

Also, in that configuration you would need to "debounce" the switch to filter the noise a switch creates when the contacts open and close.  It's not a big deal to do that, just a cap and a resistor, but still you would need that since it could cause the detector to operate unreliably.

If the detector is powered full time, debouncing the switch is not required, though it would still be a good idea to put a 1uF MLCC cap as close as possible to the detector's input pin and ground.  It may not be required, but it's still good practice to do that on the power supply for any chip.  It cleans up the power supply to the chip an amount to keep it operating reliably.

Offline doobedoobedo

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #8 on: April 30, 2014, 01:03:22 PM »
Thanks Craig.

As I'll be using a soldered in LiPo battery, I'd rather go for a solution that doesn't drain any power at all while it's not in use. Just in case it's left on a shelf for a while. (This happened to my re-homed vamo and it killed the pack)

I don't expect the low voltage cut-off to ever actually be required but it's peace of mind that it's there.

Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #9 on: April 30, 2014, 01:49:42 PM »
It's really not a concern, with the drain from the detector it will take a 10 years to use 10mAh.  Keep in mind, that's even much lower than the protection circuits used in the round cells which draw 500nA to 1uA. 

In any case, if you'd really rather not power the detector full time, it should still work.  If there's no problem operationally with the circuit as shown, there's no problem. 

If it seems flakey, then it's going to be because of the switch bounce.  To fix that you can increase the resistor on the NMOS to 47k and put a 100 Ohm resistor inline with the switch at the detector's input pin.  Then add a 1uF MLCC cap at the detector's input pin and ground pin.  That should clean up the input from the switch enough.


« Last Edit: April 30, 2014, 01:55:45 PM by CraigHB »

Offline doobedoobedo

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #10 on: April 30, 2014, 08:26:53 PM »
That's fair enough then if it keeps the component count down, I'll take your advice and just leave it in line and switch the output.

 ;cheers;

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #11 on: May 01, 2014, 01:11:44 PM »
OMG they are tiny!

Offline CraigHB

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #12 on: May 01, 2014, 02:13:51 PM »
Yes, they are SOT23 which is actually a big part by modern standards.  I normally would get them in an SC70 if they were available in that package and that's about half the size. 

Just make a host board for it, really easy, you can cut some traces in a small piece of copper clad FR-4 using a Dremel and then reflow solder the part in a frying pan or toaster oven or whatever.  Personally, I would make a small board for the whole lot with a surface mount MOSFET and SMD tactile.  It would take about an hour at the most to draw up the board, etch it, and assemble it.

Don't be intimidated by the size of modern electronics, it's all about the tools and some magnification when working with the parts.  It's all relative to what you get used to.  It's funny, when I first started working with SMD parts probably two decades ago, I was rather intimidated by the size and that's when they were still pretty big.  I just keep going smaller and smaller.  I look back at the stuff before and wonder why it bothered me at all.  Though I have to admit I rather avoid the caps and resistors when they get down to 1.0x.5mm which is the 0402 size.  Those you can't even see hardly without magnification.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #13 on: May 01, 2014, 04:55:09 PM »
Sadly my eyesight is not the best. I'll see if I can find something I can actually see.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #14 on: May 01, 2014, 07:38:12 PM »
Well I've managed to get some single core copper leads soldered to one and encased it in epoxy. It works exactly as advertised. I don't have a thin enough dremel blade to cut traces between the pins.

Hat off to you Craig you must have good eyesight and a steady hand.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #15 on: May 02, 2014, 12:48:11 PM »
Well, glad to hear you got it working in any case.  With the low currents there, even the finest wire is good so if that works for you, it works for you.

As far as good eyes and steady hands, I have neither.  What I do have is a 10x stereo microscope and a platform to rest my hand to keep it from shaking too much.  Though, any magnification will do as long as its strong enough.  You can get lighted magnifiers that mount on the desk like a desk lamp specifically designed for electronics work.  I have several different ones from the type you wear like glasses to my microscope, need them all.  The microscope actually goes up to 30x which I don't use for assembly, but I use it for inspection.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #16 on: September 06, 2014, 09:41:35 PM »
I'm having ZERO luck getting an NCP300 active high working with a p-chan using Craig's suggestion above:

"Detector Input pin and PMOS Source pin to battery positive.
Detector Reset pin to PMOS gate.
Detector Ground pin to battery negative.
PMOS Drain to the supply for your circuit."

For testing purposes I've got it wired up so the PMOS drain is powering an LED... but regardless of the voltage going to the NCP300, the PMOS doesn't seem to switch.  I've tried (for S&G mostly) putting a 15k across the G and S of the PMOS (as discussed in other threads), but that didn't change much either.

Am i missing something?  I'm also a bit confused as to why, when using a tact with a PMOS, the gate gets grounded (via the tact), but when using an active-high NCP300, we're... not grounding the gate? :wallbash:
http://breaktru.com/smf/index.php/topic,616.msg5417.html#msg5417

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #17 on: September 07, 2014, 04:54:43 AM »
Maybe.  For a PMOS, you ground the gate to turn it on.  There should be a resistor connected to battery positive and the MOSFET source pin.  The detector acts like a switch that either allows the connection to ground when voltage is above the threshold or opens the connection to disallow the connection to ground when battery voltage is below the threshold.  Here's a schematic, might have already posted it in this thread but here it is again anyway;

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #18 on: September 07, 2014, 04:33:43 PM »
Maybe.  For a PMOS, you ground the gate to turn it on.  There should be a resistor connected to battery positive and the MOSFET source pin.  The detector acts like a switch that either allows the connection to ground when voltage is above the threshold or opens the connection to disallow the connection to ground when battery voltage is below the threshold.  Here's a schematic, might have already posted it in this thread but here it is again anyway;

If I'm understanding you correctly:
1. when voltage is above threshold, then NCP300 allows current to flow from PMOS GATE to RESET to GND
2. when voltage is below threshold, then NCP300 pushes current from INPUT to RESET to PMOS GATE, effectively disconnecting the GND path

Then it's possible I've fried the NCP somehow and it's not allowing that 'backflow'?

Ah you know what... I don't have my input pin connected to ground via a cap, just V+ to input... maybe that's my problem...
« Last Edit: September 07, 2014, 04:37:48 PM by XombyCraft »

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #19 on: September 07, 2014, 06:15:11 PM »
The 300 series does that, yes.  Connects the reset pin to either ground or input.  When the reset pin is engaged on input, it effectively breaks the circuit to ground.

The 301 series covered in the same data sheet works a little differently.  It uses what's called an open drain which is more like switch.  Makes or breaks the circuit to ground.  You could use either series part for this, but I chose the CMOS one for whatever reason, don't recall. 

It's probably just a wiring error.  I don't think the part will fry from reverse polarity, but I'm not sure on that.  Hopefully you have more than one you can test with.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #20 on: September 07, 2014, 06:48:57 PM »
Yes I have a bunch.  I scratch-built some SMD to breadboard adapters for testing...
I'm going to test out that cap hooked to input.  Looks like a ceramic should do fine, as the symbol you used is non-polarized?

Other than that, I suppose it's possible the resistor value on my P-Mos is wrong.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #21 on: September 07, 2014, 07:07:25 PM »
The cap is not critical, but it's good practice to use a bypass capacitor for the power supply on any chip.  It protects it from spurious noise.  The circuit should work without it, but the final product should have one.  MLCC caps are best to use for decoupling because they have very low ESR.  They do the best job of filtering noise.  Also the cap should be as close as possible to the chip's power and ground pins.

The resistor value is not critical, anything from 4.7K to 470k would work.  10k is good because it provides a fast turn-off for the transistor.  The particular one I show has an unusually large gate charge and a high value resistor will cause the transistor to turn off too slow when the switch is released.  It wouldn't be anything you would notice, but too slow a transition can cause unnecessary heating for the transistor which is hard on it.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #22 on: September 07, 2014, 07:11:26 PM »
That explains the capacitor-like gradual dimming I see across the LED... with a bigger load (like a 4050c) that shut-down time would be pretty quick.

I changed to a bigger LED and, suddenly its mostly working... 
the LED (driven by DRAIN on the PMOS) is only MOSTLY off when V+ is below 3.02v, but it's still visibly lit.  compared to full bright it's nothing, but it just doesnt make any sense.

regardless of what resistor I put across G and S on the PMOS, makes no difference.

So i would assume then that while the PMOS is off it leaks current, and that current is so miniscule that it won't activate anything upstream (unless it's a super-low current device), correct?
« Last Edit: September 07, 2014, 07:26:34 PM by XombyCraft »

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #23 on: September 07, 2014, 08:00:33 PM »
A MOSFET does leak, but it should not be enough to light a diode.  There's a leakage from gate to source and a leakage from source to drain (drain to source for an NMOS).  These are typically very small currents, not even measurable with a standard ammeter.  Even my ammeter with resolution down to 10nA does not normally see them.

There's also a body diode that allows unbounded current flow from drain to source (source to drain for an NMOS).  Three wire MOSFETs only control current in one direction.  There are four wire MOSFETs that control current in both directions, but they are not common and not used for power switches.

If you're getting a lot of leakage, check your gate-source voltages by applying your meter probes to the gate and source connections.  You should see virtually zero when the switch is relaxed and battery voltage when the switch is engaged.  If that checks out, then you have a leaky MOSFET for whatever reason.  Either it's been damaged (which is easy to do by handling) or it's one that's just a leaky part and you should select a better one.

Also, LEDs make a poor load since they typically require a 2V drop before they draw much current.  It's better to use something like a 470 Ohm resistor as a load and measure voltages with your meter.


« Last Edit: September 07, 2014, 08:20:36 PM by CraigHB »

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #24 on: September 07, 2014, 08:46:26 PM »
Thanks for the input, Craig.  I'll check that out. 

There's definitely enough current from drain to ground through the 5mm whilte LED that it glows.  not enough to light it up brightly, but it's definitely not "off", even with a current limiting resistor inline.

[edit] Turned out to be a failed PMOS.  Swapped in a fresh one from the strip and registered 0.01 to 0.00 mV at the 470ohm resistor (as load, instead of an LED). Setup works perfectly.
Thanks again Craig.  I wouldn't have thought to try a fresh part if you didn't confirm that sillyness can/will happen with ESD...  You don't think it's really a possiblity when you're careful.  I suppose I may have popped this PMOS off my tweezers a time or two, and maybe overheated it while mounting to a testing board. PMOS do not take kindly to playing tiddlywinks...[/edit]
« Last Edit: September 08, 2014, 11:59:16 AM by XombyCraft »

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #25 on: September 08, 2014, 08:40:51 PM »
I get the feeling people don't believe me when I say how delicate MOSFETs are.  It's the gate pin that's the culprit.  The insulation between the gate junction and the source-drain channel is literally only molecules thick.  It's very easy to damage with any voltage above the absolute maximum even for a nanosecond. 

Just walking around, a person typically carries around 20V or 30V of static charge on them.  Not enough to make a static spark, but enough to fry a MOSFET.  A ground strap is required when working with them.  Optionally, protected ones are available that have built-in TVS diodes and are safe to handle without any ESD grounding.  However, the unprotected ones are much more common.

In terms of heat tolerance, they're pretty robust.  For test circuits, I've resoldered them several times without failure.  Of course, if you're using a soldering pencil that does not have a regulated temperature, all bets are off.  You can overheat one on the first solder job with one of those.


« Last Edit: September 08, 2014, 09:10:46 PM by CraigHB »

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #26 on: September 09, 2014, 04:05:33 PM »
I get the feeling people don't believe me when I say how delicate MOSFETs are.  It's the gate pin that's the culprit.  The insulation between the gate junction and the source-drain channel is literally only molecules thick.  It's very easy to damage with any voltage above the absolute maximum even for a nanosecond. 

Just walking around, a person typically carries around 20V or 30V of static charge on them.  Not enough to make a static spark, but enough to fry a MOSFET.  A ground strap is required when working with them.  Optionally, protected ones are available that have built-in TVS diodes and are safe to handle without any ESD grounding.  However, the unprotected ones are much more common.

In terms of heat tolerance, they're pretty robust.  For test circuits, I've resoldered them several times without failure.  Of course, if you're using a soldering pencil that does not have a regulated temperature, all bets are off.  You can overheat one on the first solder job with one of those.

Hakko-888D

But, interesting thing... turns out that chip is just FINE... I removed it from the adapter I made, and found resistance between the separated pads...  I think the flux residue was conductive... HOW WIERD IS THAT!?  Anyway, after cleaning it up, and remounting the SAME chip... well... it worked just fine.  Phew! fainting:

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #27 on: September 09, 2014, 06:32:58 PM »
That ~is~ weird.  Normally flux is not conductive at all, a little capacitive maybe.  If it is conductive, that's not the right stuff.

The main reason you need to clean the flux off is the standard activated stuff is somewhat corrosive.  Though you can get "no clean" fluxes that are non-corrosive and don't have to be cleaned off, but they don't work as well.  You can get water soluble flux that can be cleaned off with water, but nothing works as good as regular activated or mildly activiated rosin flux.

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Re: Using a Zener for low voltage cutoff with a mosfet switch?
« Reply #28 on: September 10, 2014, 10:19:48 AM »
That ~is~ weird.  Normally flux is not conductive at all, a little capacitive maybe.  If it is conductive, that's not the right stuff.

The main reason you need to clean the flux off is the standard activated stuff is somewhat corrosive.  Though you can get "no clean" fluxes that are non-corrosive and don't have to be cleaned off, but they don't work as well.  You can get water soluble flux that can be cleaned off with water, but nothing works as good as regular activated or mildly activiated rosin flux.
agreed, on all counts.  best i can figure is... well dunno really, but i had megaohm level resistance between pads until i cleaned some of it off.  i suspect there was some copper filings stuck in the grooves between pads that contributed to this effect.  note to everyone cutting traces with a dremel....

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