Low-Voltage detection circuit
- Thread starter FrogBoy
- Start date
MrAl
Flashlight Enthusiast
Hello there,
Did you want to build the circuit yourself?
There are undervoltage detect chips that can drive a MOSFET
to turn off the light once voltage gets below the
set level. If you're interested i'll look up some
part numbers.
Take care,
Al
Did you want to build the circuit yourself?
There are undervoltage detect chips that can drive a MOSFET
to turn off the light once voltage gets below the
set level. If you're interested i'll look up some
part numbers.
Take care,
Al
MrAl,
I don't know about FB, but I'd like to build one. I got started at one point with the three terminal sensors used to detect power (from Digikey) but got sidetracked. If you've some advice, I'd love to hear it....
TIA
Doug Owen
I don't know about FB, but I'd like to build one. I got started at one point with the three terminal sensors used to detect power (from Digikey) but got sidetracked. If you've some advice, I'd love to hear it....
TIA
Doug Owen
MrAl
Flashlight Enthusiast
Hello there again Doug and Frogboy,
I was just going to suggest a 3 terminal undervoltage sense chip,
one for EACH (presumably series) cell that drives a negative
logic AND gate, with the output of the AND gate driving a
N Channel MOSFET.
MORE DETAILS
The MOSFET would be a N channel, low gate voltage, low 'on'
resistance type.
The 3 terminal undervoltage sense chip(s) would have to trip at
something like 1.1 volts to protect each cell. I dont know if
it's possible to find this yet or not. If not, or if it has
to be custom ordered, perhaps one of National's low voltage
comparator chips would work. The top sense chip would have to
sense a negative voltage referenced to the positive supply
because most likely the chip wont run at only 1.1 volts.
All the sense chip power supplies connect to max plus voltage
and ground (most negative battery terminal). With three series
cells this would provide 4.5v during batt high and 3.3v during
batt low to each sense chip.
The negative logic AND gate would simply be schottky diodes,
one for each sensor chip. The common of all the diodes
connect through a 100k resistor to max battery plus terminal.
This junction also connects to the gate of the MOSFET. When
either of the sensor chips outputs goes low it turns off
the MOSFET. This also provides a convenient node for a low
current on/off switch. Total supply drain during 'off' mode
is 45ua plus a small additional current that the sensors
draw (very small also).
I suspect that because the trip voltage is so low (1.1 volts)
the low voltage comparators would have to be used. They can
all use the same reference voltage if also using a resistive
voltage divider to provide references of 1.1v, 2.2v, with the
top comparator using a lower reference (also from the
divider) and a separate two resistor voltage divider.
Perhaps someone makes a chip with several sense inputs already?
Would be nice :-)
Any ideas comments?
Take care,
Al
I was just going to suggest a 3 terminal undervoltage sense chip,
one for EACH (presumably series) cell that drives a negative
logic AND gate, with the output of the AND gate driving a
N Channel MOSFET.
MORE DETAILS
The MOSFET would be a N channel, low gate voltage, low 'on'
resistance type.
The 3 terminal undervoltage sense chip(s) would have to trip at
something like 1.1 volts to protect each cell. I dont know if
it's possible to find this yet or not. If not, or if it has
to be custom ordered, perhaps one of National's low voltage
comparator chips would work. The top sense chip would have to
sense a negative voltage referenced to the positive supply
because most likely the chip wont run at only 1.1 volts.
All the sense chip power supplies connect to max plus voltage
and ground (most negative battery terminal). With three series
cells this would provide 4.5v during batt high and 3.3v during
batt low to each sense chip.
The negative logic AND gate would simply be schottky diodes,
one for each sensor chip. The common of all the diodes
connect through a 100k resistor to max battery plus terminal.
This junction also connects to the gate of the MOSFET. When
either of the sensor chips outputs goes low it turns off
the MOSFET. This also provides a convenient node for a low
current on/off switch. Total supply drain during 'off' mode
is 45ua plus a small additional current that the sensors
draw (very small also).
I suspect that because the trip voltage is so low (1.1 volts)
the low voltage comparators would have to be used. They can
all use the same reference voltage if also using a resistive
voltage divider to provide references of 1.1v, 2.2v, with the
top comparator using a lower reference (also from the
divider) and a separate two resistor voltage divider.
Perhaps someone makes a chip with several sense inputs already?
Would be nice :-)
Any ideas comments?
Take care,
Al
Thanks, MrAl.
I'm interested in a protection circuit as well, that I could possibly drop into any future bike light mods I do, since they tend to require quite a few (4-6) NiMH cells.
I hacked the low-battery indicator on my bike light to trigger at a higher Vbattery (ended up at 4.17V for 4AA). Basically, it compares two voltages dependent on Vbattery, a resistor and diode and a resistor and resistor. Vrr starts high, but falls faster, and when Vrr < Vrd, the indicator comes on. I'll write up a diagram from my notes sometime next week if anyone is interested.
What are your thoughts on measuring the total voltage versus the individual cells? Total voltage is easier and probably requires fewer components, but wildly-mismatched cells could have problems.
I'm interested in a protection circuit as well, that I could possibly drop into any future bike light mods I do, since they tend to require quite a few (4-6) NiMH cells.
I hacked the low-battery indicator on my bike light to trigger at a higher Vbattery (ended up at 4.17V for 4AA). Basically, it compares two voltages dependent on Vbattery, a resistor and diode and a resistor and resistor. Vrr starts high, but falls faster, and when Vrr < Vrd, the indicator comes on. I'll write up a diagram from my notes sometime next week if anyone is interested.
What are your thoughts on measuring the total voltage versus the individual cells? Total voltage is easier and probably requires fewer components, but wildly-mismatched cells could have problems.
MrAl
Flashlight Enthusiast
Hi there Flatscan,
Thanks for bringing that up :-)
Interestingly, I came here just now to comment on that
very same issue.
As i thought about the details a little more, i realized
that to measure every cell individually would be the
ultimate, but would require differential voltage measurements
instead of the simpler 'from the ground up' type.
This would complicate the circuit quite a bit, but
i guess that's life. For the most part it would be
mostly resistors which could be tiny chip types too.
The other complication is that EVERY tap in the series
connection of cells has to have a wire comming off of it
to the circuit so that each node can be measured. This
can be quite a hassle too :-) but i guess if you want the
best, you have to do it.
This is the same problem i ran into when i wanted to
charge a cordless drill batt pack by doing each cell
individually. Lots of connection wires even for a
modest eight cell pack.
The simpler single voltage measurement might be
good enough for some apps so perhaps a circuit
for single measurements and another circuit for
multiple measurements is in order.
Yes wildly mismatched cells would throw off the simple
single measurement method.
Im going offline again to take a closer look, draw up
preliminary circuits, etc.
Take care,
Al
Thanks for bringing that up :-)
Interestingly, I came here just now to comment on that
very same issue.
As i thought about the details a little more, i realized
that to measure every cell individually would be the
ultimate, but would require differential voltage measurements
instead of the simpler 'from the ground up' type.
This would complicate the circuit quite a bit, but
i guess that's life. For the most part it would be
mostly resistors which could be tiny chip types too.
The other complication is that EVERY tap in the series
connection of cells has to have a wire comming off of it
to the circuit so that each node can be measured. This
can be quite a hassle too :-) but i guess if you want the
best, you have to do it.
This is the same problem i ran into when i wanted to
charge a cordless drill batt pack by doing each cell
individually. Lots of connection wires even for a
modest eight cell pack.
The simpler single voltage measurement might be
good enough for some apps so perhaps a circuit
for single measurements and another circuit for
multiple measurements is in order.
Yes wildly mismatched cells would throw off the simple
single measurement method.
Im going offline again to take a closer look, draw up
preliminary circuits, etc.
Take care,
Al
MrAl
Flashlight Enthusiast
[This is in addition to the previous post]
Hello again,
I've taken a little more time to look at this and
i've been able to find at least three different
approaches to the problem of keeping an eye on
several cells at once.
Approach #1 is the most straight forward and is going to
work the first time it's turned on.
#2 is a little more tricky.
#3 is the most advanced and therefore may require a little
R&D time on the bench, but uses the least parts for the
most cells.
#4 (which i didnt show) would require PIC programming,
so i dont know if many people would go for that.
My personal favorite is probably the approach that stems
from sampled data theory (#3), but i like the simple #1
too because there's no chance it wont work. I've even
built and tested something very similar to #1 already
for one Li-ion cell, and it worked the first time it
was turned on too.
---------------------------------------------------------------
The following is an Overview of three approaches to
undervoltage protection of NiMH cells in series
---------------------------------------------------------------
Not including the MOSFET which is optional...(see below)
[1]
Straight forward differential amps and open collector comparators...
Parts per cell: 1 chip + 4 resistors
Other parts: one voltage ref diode + 3 resistors
[2]
Voltage difference reference diode...
Parts per cell: Ref diode chip + 3 set resistors + one-half op amp chip
[3]
Sampled data theory approach...
Approximate parts for up to eight cells:
a. 5 resistors + op amp or voltage ref diode
b. two CMOS analog switch chips (16 pins each)
c. CMOS binary counter chip
d. oscillator (single chip or chip + 4 resistors + cap)
e. small cap
Approximate parts for up to sixteen cells:
a. almost double of that for eight cells.
For all approaches:
Note that the MOSFET is optional because there is always the
possibility that instead of turning off the circuit there
could be a single red LED turned on, which would indicate
to the user that the light should be turned off as soon as
possible.
Another possibility is to have a red LED for each cell...
where the ones that light show which cells are low.
This would be pretty neat :-) and very functional.
Comments/ideas/suggestions or just what sounds best to you?
Take care,
Al
Hello again,
I've taken a little more time to look at this and
i've been able to find at least three different
approaches to the problem of keeping an eye on
several cells at once.
Approach #1 is the most straight forward and is going to
work the first time it's turned on.
#2 is a little more tricky.
#3 is the most advanced and therefore may require a little
R&D time on the bench, but uses the least parts for the
most cells.
#4 (which i didnt show) would require PIC programming,
so i dont know if many people would go for that.
My personal favorite is probably the approach that stems
from sampled data theory (#3), but i like the simple #1
too because there's no chance it wont work. I've even
built and tested something very similar to #1 already
for one Li-ion cell, and it worked the first time it
was turned on too.
---------------------------------------------------------------
The following is an Overview of three approaches to
undervoltage protection of NiMH cells in series
---------------------------------------------------------------
Not including the MOSFET which is optional...(see below)
[1]
Straight forward differential amps and open collector comparators...
Parts per cell: 1 chip + 4 resistors
Other parts: one voltage ref diode + 3 resistors
[2]
Voltage difference reference diode...
Parts per cell: Ref diode chip + 3 set resistors + one-half op amp chip
[3]
Sampled data theory approach...
Approximate parts for up to eight cells:
a. 5 resistors + op amp or voltage ref diode
b. two CMOS analog switch chips (16 pins each)
c. CMOS binary counter chip
d. oscillator (single chip or chip + 4 resistors + cap)
e. small cap
Approximate parts for up to sixteen cells:
a. almost double of that for eight cells.
For all approaches:
Note that the MOSFET is optional because there is always the
possibility that instead of turning off the circuit there
could be a single red LED turned on, which would indicate
to the user that the light should be turned off as soon as
possible.
Another possibility is to have a red LED for each cell...
where the ones that light show which cells are low.
This would be pretty neat :-) and very functional.
Comments/ideas/suggestions or just what sounds best to you?
Take care,
Al
MrAl,
Yup, that's where I went, but gave up. I didn't try the analog switch version (neat idea), but got hung up with the need for taps in the battery pack and the 'negative voltage' issue at the top of the stack. That got me back to reading overall battery voltage (probably OK for me, as I was looking at 3 cell packs).
A solution for larger batteries might be breaking them into three (or so) cell blocks and summing the outputs.
I agree that the output can be handled in a number of ways, including having the output start to flicker (that is go from full bright to dimmer in a regualr pattern) rather than have it die flat out.
Doug Owen
Yup, that's where I went, but gave up. I didn't try the analog switch version (neat idea), but got hung up with the need for taps in the battery pack and the 'negative voltage' issue at the top of the stack. That got me back to reading overall battery voltage (probably OK for me, as I was looking at 3 cell packs).
A solution for larger batteries might be breaking them into three (or so) cell blocks and summing the outputs.
I agree that the output can be handled in a number of ways, including having the output start to flicker (that is go from full bright to dimmer in a regualr pattern) rather than have it die flat out.
Doug Owen
MrAl
Flashlight Enthusiast
Hello there FrogBoy,
I dont think i'd use the 311 because it consumes a bit
too much power for battery operation. There are many
low cost chips out there that use way less then 1/10th
the power of the 311. Even the common LM339.
What i need to know now is a few things about your
preferences and the number of cells, etc., ...
1. What kinds of packages do you prefer?
2. What size is the flashlight (or other device)?
3. Can you find a source for the parts (such as LMC7221)?
4. How many cells does your light (or device) take?
There are some packages that are very small, such as
SOT23-5 so im wondering if you've worked with these
or just the usual dip packages.
What you tell me here will make a big difference on the
parts selected for the circuit, although the circuit
itself probably wont change that much so i'll start
drawing it up.
Take care,
Al
I dont think i'd use the 311 because it consumes a bit
too much power for battery operation. There are many
low cost chips out there that use way less then 1/10th
the power of the 311. Even the common LM339.
What i need to know now is a few things about your
preferences and the number of cells, etc., ...
1. What kinds of packages do you prefer?
2. What size is the flashlight (or other device)?
3. Can you find a source for the parts (such as LMC7221)?
4. How many cells does your light (or device) take?
There are some packages that are very small, such as
SOT23-5 so im wondering if you've worked with these
or just the usual dip packages.
What you tell me here will make a big difference on the
parts selected for the circuit, although the circuit
itself probably wont change that much so i'll start
drawing it up.
Take care,
Al
MrAl,
Size isn't too much of an issue with me in the project I am currently working on so I am not too concerned with package size (in fact given my soldering skills, larger is probably better)
In terms of parts, I could probably get a hold of more exotic parts, but it is going to be much, much easier to get a hold of your more standard ones.
As to cells, I haven't entirely decided. it would be either 2, 3, or 4 AA NiMh.
Thankyou so much for all of the assistance you have been providing.
Size isn't too much of an issue with me in the project I am currently working on so I am not too concerned with package size (in fact given my soldering skills, larger is probably better)
In terms of parts, I could probably get a hold of more exotic parts, but it is going to be much, much easier to get a hold of your more standard ones.
As to cells, I haven't entirely decided. it would be either 2, 3, or 4 AA NiMh.
Thankyou so much for all of the assistance you have been providing.
MrAl
Flashlight Enthusiast
Hello again FrogBoy,
Looks like the two cell version requires a special design
because of the very low voltage of only two cells, but
the basic design looks like it would work up to several
cells too.
Im almost done with the schematic so i should be able to
post it by tomorrow morning.
Take care,
Al
Looks like the two cell version requires a special design
because of the very low voltage of only two cells, but
the basic design looks like it would work up to several
cells too.
Im almost done with the schematic so i should be able to
post it by tomorrow morning.
Take care,
Al
MrAl
Flashlight Enthusiast
Hey there FrogBoy,
Here's the circuit. Let me know if it makes sense to
you. Note the IRF MOSFET device is typical...the main
requirement is low gate turn-on voltage.
Any questions just yell :-)
http://hometown.aol.com/xaxo/page8.html
Take care,
Al
Here's the circuit. Let me know if it makes sense to
you. Note the IRF MOSFET device is typical...the main
requirement is low gate turn-on voltage.
Any questions just yell :-)
http://hometown.aol.com/xaxo/page8.html
Take care,
Al
NewBie
*Retired*
The NCS2200S comparator will work down to 0.85V, available as small as a 2mm sq. package:
http://www.onsemi.com/site/products/summary/0,4450,NCS2200,00.html?tax=583
The MC33201D op-amp will work down to 1.8V:
http://www.onsemi.com/site/products/summary/0,4450,MC33201,00.html?tax=373
http://www.onsemi.com/site/products/summary/0,4450,NCS2200,00.html?tax=583
The MC33201D op-amp will work down to 1.8V:
http://www.onsemi.com/site/products/summary/0,4450,MC33201,00.html?tax=373
Al,
First rate.
I wonder how it will work as a system? That is when the power switch is off. I see issues if either the V+ or ground leads are broken but the rest of the battery stack is still hooked up.
Neat idea. If you bump up the needed drive to shut down (lower R12) you could add a 'battery's dead' LED in series, or maybe even individual LEDs to tell which cell had died (probably need to be red?)?
Anyway, any advice on my original idea (three terminal 'supply OK' chips driving the FET)?
TIA
Doug Owen
First rate.
I wonder how it will work as a system? That is when the power switch is off. I see issues if either the V+ or ground leads are broken but the rest of the battery stack is still hooked up.
Neat idea. If you bump up the needed drive to shut down (lower R12) you could add a 'battery's dead' LED in series, or maybe even individual LEDs to tell which cell had died (probably need to be red?)?
Anyway, any advice on my original idea (three terminal 'supply OK' chips driving the FET)?
TIA
Doug Owen
MrAl
Flashlight Enthusiast
Hello there,
NewBie:
Thanks for the chip lookup. I like that comparator quite a bit.
Max voltage is 6 volts so it's only good for maybe three cells,
but there are a lot of apps using three cells.
The SOT23-5 package isnt that hard to work with either.
I'd say the 2202 chip would work in this app quite well.
Doug:
Thanks for bringing that power on issue up...i forgot to
show the power switch :-) Basically, any low current
switch (10ma is good enough) wired to short out D1 to
turn the light (or other device) off.
Multiple LEDs: yeah i like that :-) One red led per batt :-)
I thought i said something about the p/s monitor chips...
I built one already with a 2.63v supply monitor chip and
it worked great, but i couldnt find any chips that would
sense 1.1 volts (or even 1.0 volts) needed for NiMH or
similar cells. Mine was for an Li-ion recharge cell.
I guess if you could find a 2.2v power supply monitor
chip you could use it for two cells in series, if you
dont mind not doing EVERY individual cell like we are
doing here. Perhaps there is a chip out there we
havent found yet.
In any case, the circuit is really simple... output of
p/s monitor chip goes to MOSFET gate. MOSFET source goes
to ground, drain in series with load. You can kill power
with a low current switch by killing the power to
the chip alone (ok, with added pulldown),
which makes it possible to control high
currents with a tiny switch (a side benefit of using these
watchdog circuits).
I hope this is what you meant...
FrogBoy:
The power switch is not shown on the schematic (yet).
Wire a low current switch across D1 for the power on/off
switch.
ADDITIONAL INFO:
The actual value of all the 10k resistors depends on
the recommendations of the manufacturer who makes
the cells you intend to use this with. Check to see
what the lowest voltage allowable is and use this
as a guide:
10k for 1.10 volts
15k for 1.05 volts
20k for 1.00 volts
All 90k resistors are really 91k (2%) or 90.9k (1%).
Use either all 1% or all 2% types.
All resistors should be 2% or 1% tolerance.
Take care,
Al
NewBie:
Thanks for the chip lookup. I like that comparator quite a bit.
Max voltage is 6 volts so it's only good for maybe three cells,
but there are a lot of apps using three cells.
The SOT23-5 package isnt that hard to work with either.
I'd say the 2202 chip would work in this app quite well.
Doug:
Thanks for bringing that power on issue up...i forgot to
show the power switch :-) Basically, any low current
switch (10ma is good enough) wired to short out D1 to
turn the light (or other device) off.
Multiple LEDs: yeah i like that :-) One red led per batt :-)
I thought i said something about the p/s monitor chips...
I built one already with a 2.63v supply monitor chip and
it worked great, but i couldnt find any chips that would
sense 1.1 volts (or even 1.0 volts) needed for NiMH or
similar cells. Mine was for an Li-ion recharge cell.
I guess if you could find a 2.2v power supply monitor
chip you could use it for two cells in series, if you
dont mind not doing EVERY individual cell like we are
doing here. Perhaps there is a chip out there we
havent found yet.
In any case, the circuit is really simple... output of
p/s monitor chip goes to MOSFET gate. MOSFET source goes
to ground, drain in series with load. You can kill power
with a low current switch by killing the power to
the chip alone (ok, with added pulldown),
which makes it possible to control high
currents with a tiny switch (a side benefit of using these
watchdog circuits).
I hope this is what you meant...
FrogBoy:
The power switch is not shown on the schematic (yet).
Wire a low current switch across D1 for the power on/off
switch.
ADDITIONAL INFO:
The actual value of all the 10k resistors depends on
the recommendations of the manufacturer who makes
the cells you intend to use this with. Check to see
what the lowest voltage allowable is and use this
as a guide:
10k for 1.10 volts
15k for 1.05 volts
20k for 1.00 volts
All 90k resistors are really 91k (2%) or 90.9k (1%).
Use either all 1% or all 2% types.
All resistors should be 2% or 1% tolerance.
Take care,
Al
Al,
I don't follow. For sure a switch grounding the gate will shut *the load* off but leaves the 339s and the rest still drawing current.
FWIW, I was working with 3 NiMH cells and thinking 2.6 to 2.8 volts for total battery was probably 'close enough for jazz'.
Fun stuff, this.
Doug Owen
I don't follow. For sure a switch grounding the gate will shut *the load* off but leaves the 339s and the rest still drawing current.
FWIW, I was working with 3 NiMH cells and thinking 2.6 to 2.8 volts for total battery was probably 'close enough for jazz'.
Fun stuff, this.
Doug Owen
