# Choosing best mosfet for low voltage application

S

#### smpowell

Jan 1, 1970
0
I'm building a circuit that sends power to a photographic flash
unit at certain times. Basically the output of a 7555 timer is
connected to the gate of a mosfet, which acts like a switch to
send 6 volts to the flash unit.

I'm not familiar with using mosfets; how does one pick the
best one(s)? Something cheap, easy to find, and a low
voltage drop in this type of circuit.

I'm thinking of a logic level mosfet like a IRL 2702-ND
(Digi-Key page 641, $0.70 US). The schematic and description are at: http://octopus.freeyellow.com/slaveflash.html Stephen Powell W #### Winfield Hill Jan 1, 1970 0 Stephen Powell (smpowell) wrote... I'm building a circuit that sends power to a photographic flash unit at certain times. Basically the output of a 7555 timer is connected to the gate of a mosfet, which acts like a switch to send 6 volts to the flash unit. I'm not familiar with using mosfets; how does one pick the best one(s)? Something cheap, easy to find, and a low voltage drop in this type of circuit. I'm thinking of a logic level mosfet like a IRL 2702-ND (Digi-Key page 641,$0.70 US).

The schematic and description are at:
http://octopus.freeyellow.com/slaveflash.html

I see nobody has answered, so I'll chime in. Ding!

Did you mean the IRL2703, which is a 30V 60-milliohm (with 4.5V
gate drive) TO-220 part? This is pretty respectable for 56 cents,
qty 10. Your 7555 circuit is OK, I suppose, but I suggest you add
a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.

Normally we'd ask you to characterize the load, but with a nice
60-milliohm switch, I'm not sure if that's necessary! But I am
Four AA cells? Usually it's important to analyze your circuit's
and logic-level FET should work down to say 4 volts, although
the Ron rises to 80 milliohms or so...

Hmm, "Trail cameras are cameras placed out in the woods to take
pictures of wild life, tripped with an IR sensor." Tell us more.

A

#### Archilochus

Jan 1, 1970
0
smpowell said:
I'm building a circuit that sends power to a photographic flash
unit at certain times. Basically the output of a 7555 timer is
connected to the gate of a mosfet, which acts like a switch to
send 6 volts to the flash unit.

I'm not familiar with using mosfets; how does one pick the
best one(s)? Something cheap, easy to find, and a low
voltage drop in this type of circuit.

I'm thinking of a logic level mosfet like a IRL 2702-ND
(Digi-Key page 641, \$0.70 US).

The schematic and description are at:
http://octopus.freeyellow.com/slaveflash.html

Stephen Powell

Hi Stephen,
Have you checked out any of the forums that discuss trail cams?
Lots of knowledgeable people over at http://www.hagshouse.com

I'm wondering what will happen with the circuit in this situation...
Animal walks by, a picture gets taken (with flash), and the "delay"
between pictures is shorter than the 555's 'flash recharge' timer.
Seems like the next picture would then have no flash, since the 555
didn't allow another recharge cycle.

Maybe you could incorporate some sort of reset logic to force the flash
to recharge immediately after the flash is discharged?

On the MOSFET - couldn't find that part # listed, but look for a one
that can handle the Maximum current the flash will require (maybe 2
Amps or so?).

Arch

S

#### smpowell

Jan 1, 1970
0
Have you checked out any of the forums that discuss trail cams?
Lots of knowledgeable people over at http://www.hagshouse.com

Yes.
Animal walks by, a picture gets taken (with flash), and the "delay"
between pictures is shorter than the 555's 'flash recharge' timer.
Seems like the next picture would then have no flash,

The camera controller controls the minimum time between pictures.
Maybe you could incorporate some sort of reset logic to force the
flash
to recharge immediately after the flash is discharged?

While there is a small possibility that a second photo might only get
the flash that is built into the camera, unless there is a very
simple way to force the recharge, I prefer the K.I.S.S.
principle.

Stephen Powell
Electronic Hobby Information
http://octopus.freeyellow.com/

S

#### smpowell

Jan 1, 1970
0
I see nobody has answered, so I'll chime in. Ding!

Thanks!
Did you mean the IRL2703, which is a 30V 60-milliohm (with 4.5V
gate drive) TO-220 part?

Yes.
Your 7555 circuit is OK, I suppose, but I suggest you add
a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.

Is there a reason for wanting to slow the FET's rise/fall time?

It's a common Vivitar 2000 Flash. I measured a load of
80 ma after it was charged. I'm guessing 2-3 amps
from a cold start up. The flash uses 4 AA's; we might
use an external pack of 4 D cells instead.

I expect the circuit to reduce power consumption by a
factor of 10.

The IRL2703 was my best guess after learning what I could
about these parts and looking through the Digi-Key catalog.
Mostly I figured something cheap with a very low voltage
loss when the flash recycles.

I'm hoping that some experts will second guess me, in case
it turns out that I really don't understand as much as I think
I do .
For example, is there any reason to add an external component(s)
to protect the MOSFET, or perhaps there is another part that would
be more tolerant of any transients that might occur.
Hmm, "Trail cameras are cameras placed out in the woods to take
pictures of wild life, tripped with an IR sensor." Tell us more.

My brother wants to build a couple of these things. Apparently there
is a following for these devices. A couple of web sites on these:

http://www.jesseshunting.com/forums/index.php?s=cc79c1a0b6a5350b8082cfc5dd025be6&showforum=50

http://www.hagshouse.com

Stephen Powell
Electronic Hobby Information
http://octopus.freeyellow.com/

A

#### Archilochus

Jan 1, 1970
0
smpowell said:
SNIP
While there is a small possibility that a second photo might only get
the flash that is built into the camera, unless there is a very
simple way to force the recharge, I prefer the K.I.S.S.
principle.

Stephen Powell
Electronic Hobby Information
http://octopus.freeyellow.com/

Here's an idea that should cover both the "K.I.S.S. principle" and the
issue of the missed flashes...
Use a micropower dual comparator in an 8 pin package (one with a
built-in reference).
One comparator goes to the CDS cell, the other to a pair of high value
resistors forming a voltage divider that monitors the voltage on the
flash capacitor. The 2 comparator outputs are connected in such a way
that the CDS comparators output holds the gate of the MOSFET low in
daytime, at night the flash monitors output drives the MOSFET On or Off
as needed to keep the flash topped up (use some positive feedback for
hysteresis).

Cost and complexity should be about the same as the 555 based circuit -
and you'll probably see even higher power savings than the timed
recharge scheme.

Now it's off to the new year's festivities!

Arch

W

#### Winfield Hill

Jan 1, 1970
0
smpowell wrote...
Is there a reason for wanting to slow the FET's rise/fall time?

Yes, reduces damaging V = L di/dt inductive spikes, reduces the
dV/dt in the switched circuit, reduces RFI. Recommended.
I'm hoping that some experts will second guess me, in case
it turns out that I really don't understand as much as I think
I do .
For example, is there any reason to add an external component(s)
to protect the MOSFET, or perhaps there is another part that
would be more tolerant of any transients that might occur.

Nah, I think you're set to go. The MOSFET is avalanche rated
in case of a short spike. Go for it (adding in the relevant
timing refinements suggested by others).

S

#### smpowell

Jan 1, 1970
0
From STTOS:

"&*&% Jim, I'm a doctor, not an engineer!"
From Stephen Powell:

"&*&* guys, I'm a metallurgist, not a EE!"
Use a micropower dual comparator in an 8 pin package (one with a
built-in reference).

Hmmm, I could use a part #, preferably a common part that's available
as a DIP and likely to be around for decades to come.
One comparator goes to the CDS cell, the other to a pair of high value
resistors forming a voltage divider that monitors the voltage on the
flash capacitor.

Any tricks to getting this to work with the -300VDC on the cap?

Stephen Powell
Electronic Hobby Information
http://octopus.freeyellow.com/

A

#### Archilochus

Jan 1, 1970
0
From Stephen Powell:
Hmmm, I could use a part #, preferably a common part that's available
as a DIP and likely to be around for decades to come.

There are hundreds that would suit the application - I like the
cheapies from Mouser (about 50 - 70 cents) - http://www.mouser.com
# 511-TS3V393IN for example - an inexpensive dual in a DIP package with
a 'standard' pinout - not the finest comparator by a long shot - but
suitable for the job.
As far as availability goes - that would be up to the manufacturer -
but if you use a 'standard' pinout, you should be OK for quite a while.
I'd be more worried about the 555 supply drying up, the way things are
Any tricks to getting this to work with the -300VDC on the cap?

That's what the high value voltage divider is for - scales down the
300+ volts to a manageable level, with only minimal current bled off
the cap.

Arch

W

#### Winfield Hill

Jan 1, 1970
0
Archilochus wrote...
There are hundreds that would suit the application - I like the
cheapies from Mouser (about 50 - 70 cents) - http://www.mouser.com
# 511-TS3V393IN for example - an inexpensive dual in a DIP package
with a 'standard' pinout - not the finest comparator by a long shot
- but suitable for the job.

The TS3V393 was a nice comparator, operating on 20uA or less, down
to 2.7V. Some parts may be in the distributor pipeline, but sadly
it was discontinued by ST last year. ST's TS393 and TI's TLC393 are
similar parts in steady production; either would be a better choice.

S

#### smpowell

Jan 1, 1970
0
Use a micropower dual comparator in an 8 pin package (one with a
# 511-TS3V393IN
discontinued by ST last year. ST's TS393 and TI's TLC393 are similar
parts

I'm not following the part about "one with a built-in reference".
Didn't see
this
would eliminate the need for an external voltage reference.

Is there a reason for not using a TLC3702 with a push-pull output to
eliminate
the two pullup resistors needed with the other parts?

I've put a second schematic using a comparator at:
http://octopus.freeyellow.com/slaveflash.html

I've included a LM4040 2.5V voltage referance in the circuit, this
choice
was pretty much guesswork. Is there another choice with less power
draw?
That's what the high value voltage divider is for - scales down the
300+ volts

By trick, I was thinking of avoiding putting minus 300 volts on the
comparator when the batteries are removed. I've included a
Schottky diode on the new drawing that would probably do the job.

Stephen Powell
Electronic Hobby Information
http://octopus.freeyellow.com/

A

#### Archilochus

Jan 1, 1970
0
smpowell said:
I'm not following the part about "one with a built-in reference".
Didn't see
this
would eliminate the need for an external voltage reference.

Ohhh - I dropped that and went with a more general purpose comparator
when you mentioned wanting a part that would be around for a long time
('course the example I picked is already obsolete!)
Many comparators have a built-in reference hooked up internally to an
input on one or both comparators. Maxim makes some really fine parts -
but I hear that availability can be a real problem with them (not to
mention all those tiny surface mount packages Maxim is so fond of).
I'll poke around in some data sheets and see if I can find something
that looks good.
Is there a reason for not using a TLC3702 with a push-pull output to
eliminate
the two pullup resistors needed with the other parts?

Not that I can think of - didn't realize the '393 was not a push/pull
type.
I've put a second schematic using a comparator at:
http://octopus.freeyellow.com/slaveflash.html

I've included a LM4040 2.5V voltage referance in the circuit, this
choice
was pretty much guesswork. Is there another choice with less power
draw?

Most of the stuff I toss together is just hobby level. I usually just
use a pair of 1% resistors as a divider for the reference (on circuits
with a voltage regulator).
By trick, I was thinking of avoiding putting minus 300 volts on the
comparator when the batteries are removed. I've included a
Schottky diode on the new drawing that would probably do the job.

That's a great point! Glad you mentioned it, as I'm working on building
a similar rig for my own cameras.

Arch

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