[EE] MOSFET Gate Voltage

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[EE] MOSFET Gate Voltage

Josh Koffman
Hi all,

I'm modifying an existing battery powered LED lamp. I haven't fully
completed my investigation, but I believe I have identified a MOSFET
that is responsible for controlling the LEDs. I want to hijack that
MOSFET and inject my own control signal. If my current theories are
correct, this shouldn't be too hard.

However, the circuit is running off a battery at 3.3V. The control
signal I plan on using is going to be 5V. So that means the gate of
the MOSFET will have more voltage on it than across D-S. Will this
work, or will this end badly for the MOSFET?

Thank you!

Josh
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Re: [EE] MOSFET Gate Voltage

Jason White-20
I'd advise reducing the control signal to 3.3 volts. Without more
information it's hard to say what might (or might not) happen.

On Saturday, May 23, 2020, Josh Koffman <[hidden email]> wrote:

> Hi all,
>
> I'm modifying an existing battery powered LED lamp. I haven't fully
> completed my investigation, but I believe I have identified a MOSFET
> that is responsible for controlling the LEDs. I want to hijack that
> MOSFET and inject my own control signal. If my current theories are
> correct, this shouldn't be too hard.
>
> However, the circuit is running off a battery at 3.3V. The control
> signal I plan on using is going to be 5V. So that means the gate of
> the MOSFET will have more voltage on it than across D-S. Will this
> work, or will this end badly for the MOSFET?
>
> Thank you!
>
> Josh
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
>         -Douglas Adams
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>


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Re: [EE] MOSFET Gate Voltage

Harold Hallikainen-3
I'd certainly look at the datasheet of the FET. But, in general, I see
nothing wrong with the gate voltage being higher than the drain voltage.
In fact, that is a typical condition once a FET is switched on.

Harold

> I'd advise reducing the control signal to 3.3 volts. Without more
> information it's hard to say what might (or might not) happen.
>
> On Saturday, May 23, 2020, Josh Koffman <[hidden email]> wrote:
>
>> Hi all,
>>
>> I'm modifying an existing battery powered LED lamp. I haven't fully
>> completed my investigation, but I believe I have identified a MOSFET
>> that is responsible for controlling the LEDs. I want to hijack that
>> MOSFET and inject my own control signal. If my current theories are
>> correct, this shouldn't be too hard.
>>
>> However, the circuit is running off a battery at 3.3V. The control
>> signal I plan on using is going to be 5V. So that means the gate of
>> the MOSFET will have more voltage on it than across D-S. Will this
>> work, or will this end badly for the MOSFET?
>>
>> Thank you!
>>
>> Josh
>> --
>> A common mistake that people make when trying to design something
>> completely foolproof is to underestimate the ingenuity of complete
>> fools.
>>         -Douglas Adams
>> --
>> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>
>
>
> --
> Jason White
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
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>


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Re: [EE] MOSFET Gate Voltage

Josh Koffman
On Sat, May 23, 2020 at 2:15 PM Harold Hallikainen
<[hidden email]> wrote:
> I'd certainly look at the datasheet of the FET. But, in general, I see
> nothing wrong with the gate voltage being higher than the drain voltage.
> In fact, that is a typical condition once a FET is switched on.

Thank you Harold and Jason,

Since this is an existing mass produced product I'm a bit limited in
figuring out what the transistor is. It's an SO-23 package marked AOH.
I did a bit of searching and while I did find a fascinating website on
the Ancient Order of Hibernians, I haven't found a datasheet for this
one yet. I also haven't come across a transistor marking guide.

Thanks!

Josh
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Re: [EE] MOSFET Gate Voltage

Isaac Marino Bavaresco-2
In reply to this post by Josh Koffman
Josh,

Normal gate-level MOSFETs usually support +-20 V as Vgs(max), while
logic-level MOSFETs support up to +-12 V.

It seems that there are some very special devices with very low
Vgs(max), but they are very rare.

I think it is safe to assume that your MOSFET should be a normal-level
or logic-level device.

Cheers,

Isaac


Em 23/05/2020 13:57, Josh Koffman escreveu:

> Hi all,
>
> I'm modifying an existing battery powered LED lamp. I haven't fully
> completed my investigation, but I believe I have identified a MOSFET
> that is responsible for controlling the LEDs. I want to hijack that
> MOSFET and inject my own control signal. If my current theories are
> correct, this shouldn't be too hard.
>
> However, the circuit is running off a battery at 3.3V. The control
> signal I plan on using is going to be 5V. So that means the gate of
> the MOSFET will have more voltage on it than across D-S. Will this
> work, or will this end badly for the MOSFET?
>
> Thank you!
>
> Josh

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Re: [EE] MOSFET Gate Voltage

Mike Rigby-Jones
In reply to this post by Josh Koffman
http://www.dl7avf.info/charts/smdcode/caf.html

This lists suggests AOH could be an LM4040 shunt reference in an 3 pin Sc70
package, or a Seiko linear regulator in a 5 pin SOT89.

These smd code lists are not exhaustive, but worth checking that neither of
these functions would make sense for the circuit you are looking at.

Mike

On Sat, 23 May 2020, 21:12 Josh Koffman, <[hidden email]> wrote:

> On Sat, May 23, 2020 at 2:15 PM Harold Hallikainen
> <[hidden email]> wrote:
> > I'd certainly look at the datasheet of the FET. But, in general, I see
> > nothing wrong with the gate voltage being higher than the drain voltage.
> > In fact, that is a typical condition once a FET is switched on.
>
> Thank you Harold and Jason,
>
> Since this is an existing mass produced product I'm a bit limited in
> figuring out what the transistor is. It's an SO-23 package marked AOH.
> I did a bit of searching and while I did find a fascinating website on
> the Ancient Order of Hibernians, I haven't found a datasheet for this
> one yet. I also haven't come across a transistor marking guide.
>
> Thanks!
>
> Josh
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
>         -Douglas Adams
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
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Re: [EE] MOSFET Gate Voltage

Josh Koffman
In reply to this post by Isaac Marino Bavaresco-2
On Sat, May 23, 2020 at 4:34 PM Isaac M. Bavaresco
<[hidden email]> wrote:
> Normal gate-level MOSFETs usually support +-20 V as Vgs(max), while
> logic-level MOSFETs support up to +-12 V.
>
> It seems that there are some very special devices with very low
> Vgs(max), but they are very rare.
>
> I think it is safe to assume that your MOSFET should be a normal-level
> or logic-level device.

Thank you for the encouragement. The more I look at this I believe
that feeding it 1.2V more than it's getting now should be ok.

Thank you!

Josh
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Re: [EE] MOSFET Gate Voltage

Josh Koffman
In reply to this post by Mike Rigby-Jones
On Sun, May 24, 2020 at 3:49 AM Mike Rigby-Jones
<[hidden email]> wrote:
>
> http://www.dl7avf.info/charts/smdcode/caf.html
>
> This lists suggests AOH could be an LM4040 shunt reference in an 3 pin Sc70
> package, or a Seiko linear regulator in a 5 pin SOT89.
>
> These smd code lists are not exhaustive, but worth checking that neither of
> these functions would make sense for the circuit you are looking at.

Hi Mike,

Thanks for the resource! I had a look, but neither device matches what
I'm seeing here.

Thank you!

Josh
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Re: [EE] MOSFET Gate Voltage

Bob Blick-5
Hi Josh,

Might be an Alpha and Omega part like the AO3400A, they are very popular.

Best regards, Bob

________________________________________
From: [hidden email] <[hidden email]> on behalf of Josh Koffman <[hidden email]>
Sent: Sunday, May 24, 2020 7:36 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] MOSFET Gate Voltage

On Sun, May 24, 2020 at 3:49 AM Mike Rigby-Jones
<[hidden email]> wrote:
>
> http://www.dl7avf.info/charts/smdcode/caf.html
>
> This lists suggests AOH could be an LM4040 shunt reference in an 3 pin Sc70
> package, or a Seiko linear regulator in a 5 pin SOT89.
>
> These smd code lists are not exhaustive, but worth checking that neither of
> these functions would make sense for the circuit you are looking at.

Hi Mike,

Thanks for the resource! I had a look, but neither device matches what
I'm seeing here.

Thank you!

Josh
--
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completely foolproof is to underestimate the ingenuity of complete
fools.
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Re: [EE] MOSFET Gate Voltage

Josh Koffman
On Sun, May 24, 2020 at 11:51 PM Bob Blick <[hidden email]> wrote:
> Might be an Alpha and Omega part like the AO3400A, they are very popular.

Ah, good thought, but sadly no. The marking on the device is wrong,
and the gate and source pins appear to be swapped.

Thanks for the suggestion though!

Josh
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