Voltage cut off circuit
Chris W
voltages(5V-16V). I mostly run it on batteries. Since in stand by mode
it draws very low current and will work with as little as 5V, it often
runs the NiCd/NIMh batteries down to too low of a voltage. Since I put
together my own battery packs, I was wondering if there was a way for me
to add a simple circuit that would just cut off the power when the
battery voltage dropped below some set point. The radio draws about 2
amps on high power transmit. Of course I don't want this circuit to
draw much additional current and preferable zero current once the shut
off point is reached. When in standby the radio only draws about 15 ma.
The set point would vary with the battery pack, some are 6 cell some
8 and some 10. The set point will probably be 1.1V/cell
--
Chris W
KE5GIX
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waz
Just off the top of my head, and I am getting very rusty.
kell
> I have a 2 way radio that will run with wide range of input
> voltages(5V-16V). I mostly run it on batteries. Since in stand by mode
> it draws very low current and will work with as little as 5V, it often
> runs the NiCd/NIMh batteries down to too low of a voltage. Since I put
> together my own battery packs, I was wondering if there was a way for me
> to add a simple circuit that would just cut off the power when the
> battery voltage dropped below some set point. The radio draws about 2
> amps on high power transmit. Of course I don't want this circuit to
> draw much additional current and preferable zero current once the shut
> off point is reached. When in standby the radio only draws about 15 ma.
> The set point would vary with the battery pack, some are 6 cell some
> 8 and some 10. The set point will probably be 1.1V/cell
>
> --
> Chris W
> KE5GIX
>
p-channel
mosfet
s d
bat------+--------- -----------+------load
pos | |___| |
| _ |
10K | |
| | |
| | ___/ zener
'---------+ /_\ diode
| |
\c |
npn |-----R-----'
/e
|
gnd
R can probably be a few K, haven't taken the time to figure ought what
it ought to be really.
When the voltage at the load drops below Vz+Vbe, the bipolar transistor
will turn off, releasing the mosfet gate, which will turn the mosfet
off (the gate-source resistor makes sure of that). Load on your
battery will then be zip. Not even a microamp, my guess.
You must also put a momentary contact switch across the mosfet. Put in
a fresh battery and push the button, turning on power to the load. It
will then stay turned on until the voltage falls, either because the
battery runs down or the load presents a short. In other words, this
circuit can also act like a circuit breaker tripped by voltage drop at
the load.
A resistor from the zener anode to ground might not be a bad idea, but
I'm not sure that's necessary.
Another option instead of the zener is a voltage divider from load to
ground, tie the transistor base to the divider midpoint and dispense
with the base resistor R.
John Fields
wrote:
---
What's going to turn on the MOSFET?
--
John Fields
Professional Circuit Designer
kell
> >>
> >try this (view in fixed font)
> >
> > p-channel
> > mosfet
> > s d
> >bat------+--------- -----------+------load
> >pos | |___| |
> > | _ |
> > 10K | |
> > | | |
> > | | ___/ zener
> > '---------+ /_\ diode
> > | |
> > \c |
> > npn |-----R-----'
> > /e
> > |
> > gnd
> >
>
> ---
> What's going to turn on the MOSFET?
>
> --
> John Fields
> Professional Circuit Designer
You have to turn the circuit on, because as you see it doesn't turn on
by itself. A momentary pushbutton parallel to the mosfet ought to
accomplish this.
John Fields
wrote:
---
Here's a better way:
IRF7404
VBAT>--+----------+-----------+-------S D---> LOAD
| | | G
| [82.5K] | |
| | +---|---[1M]--+
[412K] | | | |
| +-[1K]--+--|+\U1A |
| A | | >-------+
+---O<----------------|-/LT1017
| | |
[61.9K] | |
| B | |
+---O | +---|+\U1B
| | | | >
[37.4K] | +---|-/
| C | |
+---O | |
| K|2.5V |
[150K] [LM385] |
| | |
GND>---+----------+-----------+---------------> GND
With a Vbat of 13.2V, U1A- is connected to "C", with a Vbat of 9.6V
to "B", and with a Vbat of 7.2V to "A." That will cause the circuit
to shut down power to the load when battery voltages fall to 11V,
8.8V, and 6.6V, respectively.
Total current should be less than about 200渙, but simulating it
with:
Version 4
SHEET 1 944 680
WIRE -624 256 -624 0
WIRE -624 576 -624 336
WIRE -624 640 -624 576
WIRE -400 0 -624 0
WIRE -400 32 -400 0
WIRE -400 128 -400 112
WIRE -400 144 -400 128
WIRE -400 256 -400 224
WIRE -400 304 -400 256
WIRE -400 400 -400 384
WIRE -400 432 -400 400
WIRE -400 576 -624 576
WIRE -400 576 -400 512
WIRE -368 128 -400 128
WIRE -368 256 -400 256
WIRE -272 400 -400 400
WIRE -272 400 -272 272
WIRE -224 432 -224 304
WIRE -224 576 -400 576
WIRE -224 576 -224 512
WIRE -176 304 -224 304
WIRE -64 304 -96 304
WIRE -64 368 -64 304
WIRE -48 368 -64 368
WIRE 32 272 -272 272
WIRE 32 304 -64 304
WIRE 64 0 -400 0
WIRE 64 96 64 0
WIRE 64 256 64 176
WIRE 64 576 -224 576
WIRE 64 576 64 320
WIRE 128 288 96 288
WIRE 128 368 32 368
WIRE 128 368 128 288
WIRE 240 0 64 0
WIRE 256 288 128 288
WIRE 256 288 256 48
WIRE 256 384 256 288
WIRE 256 576 64 576
WIRE 256 576 256 464
WIRE 432 0 336 0
WIRE 432 256 432 0
WIRE 432 576 256 576
WIRE 432 576 432 336
FLAG -624 640 0
SYMBOL pmos 336 48 M270
SYMATTR InstName M2
SYMATTR Value IRF7404
SYMBOL voltage -624 240 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value PULSE(0 13.2 0 1e-3 5s 1s)
SYMATTR InstName V1
SYMBOL Comparators\\LT1017 64 288 R0
WINDOW 0 -76 -59 Left 0
WINDOW 3 -96 -91 Left 0
SYMATTR InstName U1
SYMBOL res -416 128 R0
SYMATTR InstName R2
SYMATTR Value 61.9K
SYMBOL res -416 416 R0
SYMATTR InstName R3
SYMATTR Value 150K
SYMBOL voltage -224 416 R0
WINDOW 0 -51 101 Left 0
WINDOW 3 -60 132 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 2.5
SYMBOL res 416 240 R0
SYMATTR InstName R4
SYMATTR Value 6.6
SYMBOL res -80 288 R90
WINDOW 0 61 59 VBottom 0
WINDOW 3 68 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL res 48 352 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R6
SYMATTR Value 1E6
SYMBOL res -416 16 R0
SYMATTR InstName R5
SYMATTR Value 412K
SYMBOL res -416 288 R0
SYMATTR InstName R7
SYMATTR Value 37.4K
SYMBOL res 240 368 R0
SYMATTR InstName R8
SYMATTR Value 1e6
SYMBOL res 48 80 R0
SYMATTR InstName R9
SYMATTR Value 1
TEXT -594 602 Left 0 !.tran 30
TEXT -520 400 Left 0 ;V1 = 13.2V
TEXT -512 256 Left 0 ;V1 = 9.6V
TEXT -512 128 Left 0 ;V1 = 7.2V
Shows that a single comparator is drawing roughly Vbat/1000R, when
the data sheet, at:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1004,C1013,P1227,D1268
shows it as 100渙 worst case with A 40V supply and the chip ay 125C.
Dunno what's up with that, but unless I made a mistake, there's
probably something wrong with the model. If Helmut's listening,
maybe he'd care to comment?