Basic MOSFET Q., Also, "ground" vs "positive", & voltage mat

K

Kris Krieger

Guest
Hi, All!,

I have three questions.
First (and prob. "dumbest" ;) ) Q. -
I was looking at this page:
http://www.reuk.co.uk/DC-Voltage-Multiplier-Circuit-Plans.htm
and, in the section titled "3.3V Output from MAXC756", it says:
"By changing the input to pin 2 of the MAX756 from ground to positive, the
output is switched to 3.3 Volts."

The thing is, I thought that "positive" was the same thing as
"ground"...especially since the two are described in my various books, as
far as I can tell, as esentially the same.

Any clarification would be appreciated!

-------

Second Q. - I was thinking of setting up something like this, using a
"depletion"/?n-channel? MOSFET, to turn off the flow if the supercapacitor
exceeds its voltage rating:

D1
|----Solar cell---->|---SuperCapacitor----|
| |
| |
| R1 |
|----O--------| |---|-------^V^V^V------|
| d | V | s |
| |_______| |
| | |
| | g |
| |_______________________|
|
|
O__(to rest of circuit - phototransistor, DC-DC voltage booster,
Maxim1848 LED driver, LEDs, and return)


What confuses me is that there are numerous different MOSFET parameters on
the various data sheets, and I can't figure out which term(s) mean
"maximum or minimum volts that s, g, and d can handle and how much greater
must g be than s before it shuts off the power flow through d". The 350F
SuperCap I'm looking at puts out 2.5V, so what I'm figuring is that I can
use R1 to set the s voltage to (2.5V/2)(IOW, regardless of whetehr the V
exceeds 2.5), and keep the g connection unfettered so that, if V exceeds
(2.5V/2), it will shut down the circuit.

Am I on track, or are MOSFETS not sensitive enough for that, or do I need
some special kind of MOSFET .... or is this entire idea goofy...?

------

Third and last Q.:
I figure it'd be best if I could match the V from the solar cell to the V
rating of the SuperCap, but don't know if that is absolutely necessary - I
saw a nice inexpensive "mini-solar-panel" for a good price, but it's rated
at 3V output and I'm not sure whether that's acceptable, or whether it'd
damage teh SuperCap - or whether throwing a resistor in-between teh two
would suffice.

--------------

Many Thanks in Advance!!

Kris K.
 
On Sun, 24 May 2009 14:26:56 -0500, Kris Krieger <me@dowmuff.in>
wrote:

Hi, All!,

I have three questions.
First (and prob. "dumbest" ;) ) Q. -
I was looking at this page:
http://www.reuk.co.uk/DC-Voltage-Multiplier-Circuit-Plans.htm
and, in the section titled "3.3V Output from MAXC756", it says:
"By changing the input to pin 2 of the MAX756 from ground to positive, the
output is switched to 3.3 Volts."

The thing is, I thought that "positive" was the same thing as
"ground"...especially since the two are described in my various books, as
far as I can tell, as esentially the same.
Click to expand...
"Ground" is a much misused term in electricity and electronics.
Occasionally it refers to a connection to the earth (usually in AC
power systems, and some radio antenna systems). However, in most
electronic circuits "Ground" is simply the point in the circuit that
the designer decided to call "zero volts", and to use as a reference
when measuring voltages elsewhere in the circuit. In most digital
circuits, "ground" is the most negative terminal of the power supply,
and anything else will be positive, relative to that.

In many analog circuits, (using op-amps, for example) ground will be
the mid-point of the power supply, and you will have both positive and
negative power sources in the circuit.

Any clarification would be appreciated!

-------

Second Q. - I was thinking of setting up something like this, using a
"depletion"/?n-channel? MOSFET, to turn off the flow if the supercapacitor
exceeds its voltage rating:

D1
|----Solar cell---->|---SuperCapacitor----|
| |
| |
| R1 |
|----O--------| |---|-------^V^V^V------|
| d | V | s |
| |_______| |
| | |
| | g |
| |_______________________|
|
|
O__(to rest of circuit - phototransistor, DC-DC voltage booster,
Maxim1848 LED driver, LEDs, and return)


What confuses me is that there are numerous different MOSFET parameters on
the various data sheets, and I can't figure out which term(s) mean
"maximum or minimum volts that s, g, and d can handle and how much greater
must g be than s before it shuts off the power flow through d". The 350F
SuperCap I'm looking at puts out 2.5V, so what I'm figuring is that I can
Click to expand...
No. The SuperCap does not "put out" 2.5 volts. Its rated working
voltage is 2.5 volts. If you apply much more than 2.5 volts to it, it
is likely to be damaged. Any capacitor will only charge up to the
applied voltage, regardless of its voltage rating.

use R1 to set the s voltage to (2.5V/2)(IOW, regardless of whetehr the V
exceeds 2.5), and keep the g connection unfettered so that, if V exceeds
(2.5V/2), it will shut down the circuit.

Am I on track, or are MOSFETS not sensitive enough for that, or do I need
some special kind of MOSFET .... or is this entire idea goofy...?

------

Third and last Q.:
I figure it'd be best if I could match the V from the solar cell to the V
rating of the SuperCap, but don't know if that is absolutely necessary - I
saw a nice inexpensive "mini-solar-panel" for a good price, but it's rated
at 3V output and I'm not sure whether that's acceptable, or whether it'd
damage teh SuperCap - or whether throwing a resistor in-between teh two
would suffice.

--------------

Many Thanks in Advance!!

Kris K.
Click to expand...
--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
Peter Bennett <peterbb@somewhere.invalid> wrote in
news:4ntj15llvg1p1gbdn9fagnp9mjic5ei2eq@news.supernews.com:

I know top-posting is "evil" but I just wanted to (belatedly) thak you,
Peter, for your reply to my questions. Your info simplifies my project
(and clarified the difference between charging a battery and charging a
supercapacitor), and I'm now concentrating on matching the solar cells to
the supercapacitor and connecting them directly.

Thanks again for the assistance!

- Kris K.


On Sun, 24 May 2009 14:26:56 -0500, Kris Krieger <me@dowmuff.in
wrote:

Hi, All!,

I have three questions.
First (and prob. "dumbest" ;) ) Q. -
I was looking at this page:
http://www.reuk.co.uk/DC-Voltage-Multiplier-Circuit-Plans.htm
and, in the section titled "3.3V Output from MAXC756", it says:
"By changing the input to pin 2 of the MAX756 from ground to positive,
the output is switched to 3.3 Volts."

The thing is, I thought that "positive" was the same thing as
"ground"...especially since the two are described in my various books,
as far as I can tell, as esentially the same.

"Ground" is a much misused term in electricity and electronics.
Occasionally it refers to a connection to the earth (usually in AC
power systems, and some radio antenna systems). However, in most
electronic circuits "Ground" is simply the point in the circuit that
the designer decided to call "zero volts", and to use as a reference
when measuring voltages elsewhere in the circuit. In most digital
circuits, "ground" is the most negative terminal of the power supply,
and anything else will be positive, relative to that.

In many analog circuits, (using op-amps, for example) ground will be
the mid-point of the power supply, and you will have both positive and
negative power sources in the circuit.


Any clarification would be appreciated!

-------

Second Q. - I was thinking of setting up something like this, using a
"depletion"/?n-channel? MOSFET, to turn off the flow if the
supercapacitor exceeds its voltage rating:

D1
|----Solar cell---->|---SuperCapacitor----|
| |
| |
| R1 |
|----O--------| |---|-------^V^V^V------|
| d | V | s |
| |_______| |
| | |
| | g |
| |_______________________|
|
|
O__(to rest of circuit - phototransistor, DC-DC voltage booster,
Maxim1848 LED driver, LEDs, and return)


What confuses me is that there are numerous different MOSFET parameters
on the various data sheets, and I can't figure out which term(s) mean
"maximum or minimum volts that s, g, and d can handle and how much
greater must g be than s before it shuts off the power flow through d".
The 350F SuperCap I'm looking at puts out 2.5V, so what I'm figuring is
that I can

No. The SuperCap does not "put out" 2.5 volts. Its rated working
voltage is 2.5 volts. If you apply much more than 2.5 volts to it, it
is likely to be damaged. Any capacitor will only charge up to the
applied voltage, regardless of its voltage rating.

use R1 to set the s voltage to (2.5V/2)(IOW, regardless of whetehr the V
exceeds 2.5), and keep the g connection unfettered so that, if V exceeds
(2.5V/2), it will shut down the circuit.

Am I on track, or are MOSFETS not sensitive enough for that, or do I
need some special kind of MOSFET .... or is this entire idea goofy...?

------

Third and last Q.:
I figure it'd be best if I could match the V from the solar cell to the
V rating of the SuperCap, but don't know if that is absolutely necessary
- I saw a nice inexpensive "mini-solar-panel" for a good price, but it's
rated at 3V output and I'm not sure whether that's acceptable, or
whether it'd damage teh SuperCap - or whether throwing a resistor
in-between teh two would suffice.

--------------

Many Thanks in Advance!!

Kris K.
Click to expand...
 
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