using mosfets as rectifiers?

[Albert]

I have an extremely low power application (recovering a small amount
of dc power from the audio output of a PC soundcard). I am trying to
key a transmitter when the soundcard puts out audio. It's actually a
self powered VOX, deriving all it's needed power from the audio output
of the soundcard. Right now, I'm almost succeeding, but need another
half volt or so to make the keying reliable.

I am using my laptop computer, which has only speaker out jacks. I am
feeding the speaker output into the low impedance side of an audio
transformer and taking the stepped up voltage from the secondary (8
ohm to 1K ohm impedance transformer). I am feeding the secondary into
a small signal schottky diode and filtering the rectified output to
get my dc power.

As it is now, it's barely usable. I have to really crank up the laptop
audio in order to get reliable keying.

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

Thanks,

Al

 

[Sam Goldwasser]

What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

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Important: The email address in this message header may no longer work. To
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[Albert]

What is your circuit driving? As described, I would think the
output of the audio transformer should be more than adequate.

-

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Al

 

[Bob Urz]

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Al

Why don't you just steal 5 volts off of a serial or USB port and
screw the audio rectification?

Bob

 

[Ban]

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. The mosfet is hooked to ground
and the positive keying terminal on the microphone jack. It (the
keying line) is already pulled up by the electronics inside the radio,
so it needs to be pulled down to ground in order to key the
transmitter. The mosfet pulls the keying terminal to ground when it
conducts.

There is a small (large r/small c) parallel circuit on the gate to
keep the mosfet conducting all the time the audio is applied,
otherwise it would unkey during the lower amplitudes of the applied
audio. But, this circuit doesn't waste much power.

I measure 6 ma (in the keying line) when I key the transmitter with a
switch, but I am told that I need to allow for higher currents to
accomodate different radios.

Right now, I have to crank the audio on the laptop fairly high to make
it key the transmitter, so I just ned a little more voltage on the
gate of the mosfet...hence my question about using mosfets as low
voltage drop rectifiers.

Albert, take a high beta BJT which needs only 750mV and maybe 25uA to switch
lets say 15mA of collector current. You can also double the voltage of the
transformer (if it has isolated windings).
___
o----. ,---+->|--------+----|___|--+
)|( | | + 39k |
)|( | ### | +--------o
o----' '-+ | --- | |
| | |47u | |/
+-)-----------+ +-|
| | + |>
| ### |
| --- |
| |47u |
+--|<-------+---------------+--------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
Maybe you have to use another resistor to discharge the caps, if the
transistor doesn't turn off.

 

[Kevin McMurtrie]

Why don't you just steal 5 volts off of a serial or USB port and
screw the audio rectification?

Bob

Better yet, get one of those electronics hobbyist adaptors. They
connect to a port of your computer (SCSI, printer, serial, USB) and
provide you with analog and digital I/O on screw terminals. You can
write programs to control the I/O. They're commonly used for simple
machine control and data logging. Rat Shack even sold them for a
while...something like Hobo.

 

[Sam Goldwasser]

Why not use a bipolar transistor instead of a MOSFET? Then you only need
0.7 V or so to turn it on. Of course, you would need a high volue resistor
in series with the base to get a decent time constant. If you need the
current sensitivity, this could be followed by another transistor as an
inverter and then your MOSFET. Or, instead of the transistors, a voltage
comparator or op-amp?

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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

 

[Albert]

Thanks for all the suggestions.

I chose the mosfet because it has high input impedance. I'd rather not
go to a bjt because it will draw base current and will need a series
base current limiting resistor-hence the design becomes more
complicated because the time constant of the parallel rc network has
to be maintained.

Yes, I can use an op amp, provided I can get power for it from
somewhere. Yes, I can steal power from this or that computer port.
And, some radios even have low power dc voltages available on the
microphone connectors that could be used. The power needed is very
small, yes, I could power the circuit from batteries too.

But, not all radios have dc voltages on the microphone and some
computers don't have usb or serial ports to steal power from. I want
this thing to be universal in nature, meaning I can use it on any
radio/computer anywhere/anytime. This is my motivation for NOT
stealing computer/radio power or importing it from other sources.

Since it almost works now, my hope was to try and reduce the voltage
drop in the rectifiers by using mosfets as diodes.

Can anyone offer advice on the original question, which is…

Can I bias the base on all the time and feed and use the mosfet as a
near zero forward voltage drop rectifier?

Thanks,

Al

 

[Jan Panteltje]

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

How about a germanium diode?

Would it be ok to bias the gate positive all the time (with a small
button cell 3 volt battery) and just treat the mosfet like it was a
diode??

If you are goin to use a small battery, why not use a small opamp!
CMOS opamp at 3V should take little power.
perhaps you could take power from the laptop par port?
JP

 

[Ban]

Can anyone offer advice on the original question, which is…

Can I bias the base on all the time and feed and use the mosfet as a
near zero forward voltage drop rectifier?

Sorry, a FET behaves like a controlled resistor, there is no diode
behaviour. A synchoneous rectifier is above your possibilities. I showed you
a great way of doubling your transformer voltage and you do not even see it!
Anyway there are also fets with lower threshold voltages, just last week
Winfield Hill answered a similar question, if you want to stick to a Mosfet.
What time constant do you need to switch that PTT after the voice has
finished? 1s.
Look you need anyway a bleed resistor so the 39k base resistor does that.
And the BJT costs only 5cent.

 

[Asimov]

"Albert" bravely wrote to "All" (06 Jun 04 22:28:43)
--- on the heady topic of "using mosfets as rectifiers?"

One method is to use a low power cmos opamp configured as an ideal AC
full wave rectifier and powered by a square 9V batt. If you don't want
to use a battery then you can easily get a little power from the pc by
using either the game port (+5V at pins 1,8,9,15), printer connector
(+5V 4.7Kohm pin 35, or data), and rs-232 (enable any handshake line).

Don't forget you need a time delay circuit in order to hold the key
down otherwise it will be switching on the peaks only, as you are
experiencing now BTW.

A*s*i*m*o*v


Al> From: Albert <[email protected]>
Al> I have an extremely low power application (recovering a small amount
Al> of dc power from the audio output of a PC soundcard). I am trying to
Al> key a transmitter when the soundcard puts out audio. It's actually a
Al> self powered VOX, deriving all it's needed power from the audio output
Al> of the soundcard. Right now, I'm almost succeeding, but need another
Al> half volt or so to make the keying reliable.

Al> I am using my laptop computer, which has only speaker out jacks. I am
Al> feeding the speaker output into the low impedance side of an audio
Al> transformer and taking the stepped up voltage from the secondary (8
Al> ohm to 1K ohm impedance transformer). I am feeding the secondary into
Al> a small signal schottky diode and filtering the rectified output to
Al> get my dc power.

Al> As it is now, it's barely usable. I have to really crank up the laptop
Al> audio in order to get reliable keying.

Al> Can I use small signal mosfets to rectify the signal instead of the
Al> schottky diode? This would gain my half a volt because the mosfet
Al> would have very low voltage drop once it turns on.

Al> Would it be ok to bias the gate positive all the time (with a small
Al> button cell 3 volt battery) and just treat the mosfet like it was a
Al> diode??

Al> Thanks,

Al> Al

.... Back when I was a boy, we sucked the air out of our own vacuum tubes!

 

[René]

How about a germanium diode?


If you are goin to use a small battery, why not use a small opamp!
CMOS opamp at 3V should take little power.
perhaps you could take power from the laptop par port?
JP

A voltage doubler with 2 schottkys driving a BC847c or such would
certainly work at average lsp output level. Used to key CW from a PC
that way.

....but somebody will undoubtedly advise to use a PIC or a 555 in due
time...

 

[Tam/WB2TT]

Hi Sam,

It's powering the gate of a small signal mosfet. The mosfet needs
about 3 volts to turn on completely. .....................

You should be able to find a MOSFET that requires less voltage to turn on

Tam

 

[starfire]

If you use a MOSFET in a circuit to function similar to a diode in the
conducting direction, remember most MOSFETs have the built-in body diode
which will conduct in a direction opposite to what you want to rectify, so
this may not be a desired result.

If you say you're only about 0.5V away from something useable, why don't you
just change out your silicon diode for a germanium and regain about 0.4V?

Dave

 

[Winfield Hill]

starfire wrote...

If you use a MOSFET in a circuit to function similar to a diode
in the conducting direction, remember most MOSFETs have the
built-in body diode which will conduct in a direction opposite
to what you want to rectify, so this may not be a desired result.

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Ken Smith]

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

I know they do conduct in both directions, but how can we say they are
happy about it. They may be disgusted.

Make sure you use a fairly modern MOSFET that has a spec for the recovery
of the diode. If you build the recifier with a VN10, there will be a
large recovery current when the unit is first powered up.

When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

 

[Jim Adney]

Can I use small signal mosfets to rectify the signal instead of the
schottky diode? This would gain my half a volt because the mosfet
would have very low voltage drop once it turns on.

No, because the MOSFET has an intrinsic reverse diode built into it.
You could certainly make it give you a very low forward voltage drop,
but it would still conduct in the reverse direction, too, but with the
standard silicon PN junction 0.7V drop.

Besides, where would you get the 10 volts you'd need to turn the
MOSFET fully on?

-

 

[Asimov]

"Albert" bravely wrote to "All" (07 Jun 04 09:39:42)
--- on the heady topic of "Re: using mosfets as rectifiers?"

Al> From: Albert <[email protected]>
Al> Can anyone offer advice on the original question, which is

Al> Can I bias the base on all the time and feed and use the mosfet as a
Al> near zero forward voltage drop rectifier?

Fets tend to conduct in both directions (the source interchanges with
the drain to some extent) so this makes them ill suited to behave like
a rectifier.

A*s*i*m*o*v

.... I transmit, therefore I ham!

 

[Winfield Hill]

Ken Smith wrote...

Winfield Hill wrote:
[.. mosfet rectifier ..]

This isn't an issue because a "turned-on" MOSFET happily conducts
in both directions, one can arrange to simply turn the FET around.

I know they do conduct in both directions, but how can we say they are
happy about it. They may be disgusted.

Not at all, operating as switches they are _pefectly_ happy, and remain
so up to some maximum current, which is vey high, but low enough so the
voltage drop across the FET is under say 200mV. We're talking amps here.

Make sure you use a fairly modern MOSFET that has a spec for the recovery
of the diode. If you build the recifier with a VN10, there will be a
large recovery current when the unit is first powered up.

When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

This is a somewhat theoretical discussion, because this isn't the
best way to solve the O.P.'s query, but were one to use MOSFETs as
signal rectifiers, the issue of stored charge wouldn't be an actual
problem either. That's because long before the diode would become
turned on, the circuit's comparator would have driven the gate and
turned on the FET, keeping the reverse-voltage drop across the FET
very low, which after all is exactly the job of an active rectifier.
This technique is commonly used in high-efficiency low-voltage SMPS.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Ken Smith]

When the load side is at zero volts, the body diode of the MOSFET does the
rectifying. On MOSFETs with no spec for the diode recovery, conduction in
the body diode stores lots of charge in the part. When the voltage
reverses, this charge tends to bias the parasitic NPN.

This is a somewhat theoretical discussion, because this isn't the
best way to solve the O.P.'s query, but were one to use MOSFETs as
signal rectifiers, the issue of stored charge wouldn't be an actual
problem either. That's because long before the diode would become
turned on, the circuit's comparator would have driven the gate and
turned on the FET, keeping the reverse-voltage drop across the FET
very low, which after all is exactly the job of an active rectifier.
This technique is commonly used in high-efficiency low-voltage SMPS.[/QUOTE]

Your understanding of the OP's situation must be different than mine. I
though that the only way his circuit got power was via the rectifier.
When power is first applied to the system, there is no voltage to run the
comparitor from and hence it can't be controlling the FET and hence the
body diode must do the rectifying until enough voltage is produced to run
the comparitor.