Low Voltage Cutoff Circuit

[Me]

Hello,

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast, which
in turn drops the voltage and shuts off the ballast. This turns into a
vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

Thanks

 

[Adrian Jansen]

Just wire a relay with one NO contact in series with its coil. Then arrange
it that when the voltage drops too low, the relay opens, which cuts the
power permanently, until you close a pushbutton across that same relay
contact.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

 

[Winfield Hill]

Me wrote...

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast,
which in turn drops the voltage and shuts off the ballast. This turns into
a vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

You'll need a comparator for the 10V decision and a power MOSFET to effect
the power shutoff. Comparators should generally have some hysteresis, and
the easiest way to turn a comparator into the latch you need is to increase
its hysteresis to the max, with a positive-feedback diode.

| latched low-voltage cutoff
| Win Hill 24Aug04
| 12V in out
| (O)--+----+---------+--- S D ------(O)
| | | | G
| 3.3k R2 1.0k | p-channel FET
| | 3.0k | | IRF9z34, etc
| start | | __ +------'
| _|_ +-------|- \ | R2
| ,-o o--+ | | >--+ V_stop = Vref ( 1 + -- )
| | | +--|+ / | R1
| gnd | | LM358 |
| \_|_ +---|>|---' = 10V as shown
| 2.5V /_\ |
| LM336 | '-- R1 --- gnd
| gnd 1.0k

 

[Rob Paisley]

Hello,

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast, which
in turn drops the voltage and shuts off the ballast. This turns into a
vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

Thanks

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

Have a look at the first circuit at -

http://home.cogeco.ca/~rpaisley4/xLowBattery.html

Rob.

 

[Jonathan Kirwan]

You'll need a comparator for the 10V decision and a power MOSFET to effect
the power shutoff. Comparators should generally have some hysteresis, and
the easiest way to turn a comparator into the latch you need is to increase
its hysteresis to the max, with a positive-feedback diode.

| latched low-voltage cutoff
| Win Hill 24Aug04
| 12V in out
| (O)--+----+---------+--- S D ------(O)
| | | | G
| 3.3k R2 1.0k | p-channel FET
| | 3.0k | | IRF9z34, etc
| start | | __ +------'
| _|_ +-------|- \ | R2
| ,-o o--+ | | >--+ V_stop = Vref ( 1 + -- )
| | | +--|+ / | R1
| gnd | | LM358 |
| \_|_ +---|>|---' = 10V as shown
| 2.5V /_\ |
| LM336 | '-- R1 --- gnd
| gnd 1.0k

Uh..

Say again? p-channel? Don't you have the 358 latching it's output __low__ when
the voltage drops below 10V? Or am I misreading this?

Jon

 

[Winfield Hill]

Jonathan Kirwan wrote...

You'll need a comparator for the 10V decision and a power MOSFET to
effect the power shutoff. Comparators should generally have some
hysteresis, and the easiest way to turn a comparator into the latch
you need is to increase its hysteresis to the max, with a positive-
feedback diode.

latched low-voltage cutoff [ snip ]

Uh..

Say again? p-channel? Don't you have the 358 latching it's output
__low__ when the voltage drops below 10V? Or am I misreading this?

No, you're right, I botched it! Whoa! Sorry!

It's easily fixed using an n-channel FET instead, like this:
(suited only for low-side switching of course)

| latched low-voltage cutoff - LO side switch
|
| 12V in
| (O)--+----+-----------------------(O)
| | | SWITCHED
| 3.3k R2 LOAD
| | 3.0k 1/2 ,-----(O)
| start | | __ LM358 |
| _|_ +-------|- \ D
| ,-o o--+ | | >--+---- G
| | | +--|+_/ | S n-channel FET
| gnd | | | |
| \_|_ +---|>|---' gnd
| 2.5V /_\ |
| LM336 | '-- R1 --- gnd V_stop = Vref (1 + R2/R1)
| gnd 1.0k = 10V as shown

(The LM358 opamp is used as a comparator with an active pullup.)

Or we can have convenient high-side switching by using the second
LM358 section to invert the logic signal for a p-channel FET.

| latched low-voltage cutoff - HI side switch
|
| 12V in out
| (O)--+----+----------------------+--- S D ------(O)
| | | | G
| 3.3k R2 LM358 10k | p-channel FET
| | 3k __ | | IRF9z34, etc
| start | | ,------------|+ \ | |
| _|_ +-------+--|- \ | >--+------'
| ,-o o--+ | | >--+--|-_/
| | | +--+--|+_/ |
| gnd | | | |
| \_|_ R1 '---|>|---' V_stop = Vref (1 + R2/R1)
| 2.5V /_\ 1k = 10V as shown
| LM336 | |
| gnd gnd

The 10k pullup on the 9z34 FET's gate insures 0V off-state voltage,
probably overly conservative, given the FET's 2V to 4V threshold.

Or we can swap the LM358 input pins and do the job with just one
section. The circuit seems awkward, but the LM358 does work with
its + input pulled above the common-mode limit in the ON state,
and it doesn't mind its output being shorted to ground. Gagck!

| latched low-voltage cutoff - HI side switch
|
| 12V in out
| (O)--+--------+------------+--- S D ------(O)
| | | | G
| 3.3k R2 10k | p-channel FET
| | 3k 1n4148 | | IRF9z34, etc
| | | ,---|<|---+------'
| +--3.3k-----+--|+ \ |
| | | | >--+ start
| \_|_ +-----|- / | _|_
| 2.5V /_\ | 1/2 LM358 '---o o--gnd
| LM336 | R1
| gnd 1k V_stop = Vref (1 + R2/R1)
| | = 10V as shown
| gnd

 

[Jonathan Kirwan]

Ah... But now that you've gone through all that, Win, what about "zero power"
versions, eh? Remember the CD4069/4007 discussion a while back? Those 358
circuits you just added suck power from that poor battery after it is already
kind of down and out...



Jon

 

[Winfield Hill]

Jonathan Kirwan wrote...

Ah... But now that you've gone through all that, Win, what about
"zero power" versions, eh? Remember the CD4069/4007 discussion a
while back? Those 358 circuits you just added suck power from that
poor battery after it is already kind of down and out...

Hmm, Jon can you find that discussion on the Google groups database?

 

[Jonathan Kirwan]

Jonathan Kirwan wrote...

Hmm, Jon can you find that discussion on the Google groups database?

Well, I did an advanced search on google groups, using sci.electronics.design
and "zero-power toggle":

http://groups.google.com/groups?as_ugroup=sci.electronics.design&as_usubject=zero-power toggle

That seemed to hook the thread, just fine.

One of the examples of a post from you, near the end of the section on using the
CD4069/4007 was:

http://groups.google.com/groups?q=g:thl2902914686d&[email protected]

Refresh your memory?

Jon

 

[Ken Smith]

Ah... But now that you've gone through all that, Win, what about "zero power"
versions, eh? Remember the CD4069/4007 discussion a while back? Those 358
circuits you just added suck power from that poor battery after it is already
kind of down and out...

How about this:

Use a comparitor that allows the output to be pulled above Vcc. Have the
comparitor pull down on the gate of a P channel switch. Power the
comparitor, reference and divider on the load side. Put the start switch
across the output of the comparitor.

This way, it draws no power when off and latches off without any added
parts to make it do it.

 

[John Fields]

Hello,

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast, which
in turn drops the voltage and shuts off the ballast. This turns into a
vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

 

[Winfield Hill]

Ken Smith wrote...

How about this:

Use a comparitor that allows the output to be pulled above Vcc. Have the
comparitor pull down on the gate of a P channel switch. Power the
comparitor, reference and divider on the load side. Put the start switch
across the output of the comparitor. This way, it draws no power when off
and latches off without any added parts to make it do it.

Very nice. Makes a pretty compact circuit, with a left-over comparator.

| low-voltage cutoff with HI side switch, zero-power latched
|
| 12V in ,------------------, out
| (O)-------------- | ----+--- S D --+---(O)
| ,-----+------+ | G
| | | | 10K | p-channel FET
| 3.3k R2=3k | | | IRF9z34, etc
| | | __| +------'
| +---------|+ \ |
| | | | >---+ start
| \_|_ +---|-__/ | _|_
| 2.5V /_\ | | '-----o o--gnd
| LM336 | R1=1k GND
| gnd | V_stop = Vref (1 + R2/R1)
| gnd 1/2 = 10V as shown
| LM393 assuming low Rds(on) drop

This has the further attractive property that an excessive current draw
(or short circuit) will also cause the circuit to shut down. Given the
9z34's low Ron (0.1 ohm typ) many amps (20) of current could be required
to effect the shutdown, endangering the battery and FET. Alternately
the extra comparator could be used to add a real overcurrent feature.

 

[Jim Thompson]

Hello,

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast, which
in turn drops the voltage and shuts off the ballast. This turns into a
vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

Thanks

See...

Newsgroups: alt.binaries.schematics.electronic
Subject: Low voltage cutoff circuit - LowVoltageCutOff.pdf
Message-ID: <[email protected]>

This is just a variant of "PowerSwitch.pdf" that I posted about a
month and a half ago.

(Put a pushbutton switch across the power MOSFET to reset... I left it
off of the drawing

...Jim Thompson

 

[Me]

Hello,

I need a variation of a Low Voltage Cutoff circuit.

I have a ballast that runs on a 12V battery. When the battery voltage drops
below 10V, the ballast shuts off, opening the circuit. After that happens,
the battery voltage rises due to lack of load, turning on the ballast, which
in turn drops the voltage and shuts off the ballast. This turns into a
vicious cycle.

I need to create a circuit that cuts off the voltage when the ballast opens,
(or when the battery voltage drops below a preset value, whichever is easier
to create), and stays cutoff, until I hit a Reset button.

Any ideas on the simplest approach?

Thanks

Thanks for the replies.

Here's what I was thinking. An SCR connected in series with the Ballast. A
momentary switch to turn the whole thing on is connected to the Gate of the
SCR. When the battery voltage drops below 10V, the Ballast will
automatically shut off and open the circuit. That will stop the current
flowing through the SCR and turn it off (open it). When the Battery voltage
rises, the SCR will be opened, so no current will flow keeping the Ballast
off.

Here's my issue. In order to turn the circuit off, I have to put a momentary
switch in series with the SCR/Ballast. That will work fine, but I'm trying
to think of a way to use one switch to turn the Ballast on and off, and use
another switch to reset the circuit. Instead of using one switch to turn it
on, and another switch to turn it off.

I was also thinking of using just one switch, setup as a toggle using a "T"
type latch, one push for on, and one push for off, and not even have a reset
button, but I don't want to make the circuit too complicated.

BTW, I have to use the existing switch for the On/Off, which is momentary,
but can be locked on. I can add another switch of any type for the reset.

Any idea's?

Thanks

 

[Rich Grise]

Thanks for the replies.

Here's what I was thinking. An SCR connected in series with the Ballast. A
momentary switch to turn the whole thing on is connected to the Gate of
the SCR. When the battery voltage drops below 10V, the Ballast will
automatically shut off and open the circuit. That will stop the current
flowing through the SCR and turn it off (open it). When the Battery
voltage rises, the SCR will be opened, so no current will flow keeping the
Ballast off.

Here's my issue. In order to turn the circuit off, I have to put a
momentary switch in series with the SCR/Ballast. That will work fine, but
I'm trying to think of a way to use one switch to turn the Ballast on and
off, and use another switch to reset the circuit. Instead of using one
switch to turn it on, and another switch to turn it off.

I was also thinking of using just one switch, setup as a toggle using a
"T" type latch, one push for on, and one push for off, and not even have a
reset button, but I don't want to make the circuit too complicated.

BTW, I have to use the existing switch for the On/Off, which is
momentary, but can be locked on. I can add another switch of any type for
the reset.

Any idea's?

Not in the possessive, but I might have some ideas. Do not use an apostrophe
to pluralize, except in certain limited cases.

One is, if the switch is on the high side, run a cap from switch/scr anode
to scr gate, and maybe a 100K from there to ground. When you turn on the
switch, the capacitor charging through the gate turns on the SCR. Steady-
state, the cap charges to VSupply - Vgk or so. Maybe another 100K R to
discharge the cap when it's off so that it will turn on reliably next time.

Good Luck!
Rich

 

[Fred Bloggs]

Thanks for the replies.

Here's what I was thinking. An SCR connected in series with the Ballast. A
momentary switch to turn the whole thing on is connected to the Gate of the
SCR. When the battery voltage drops below 10V, the Ballast will
automatically shut off and open the circuit. That will stop the current
flowing through the SCR and turn it off (open it). When the Battery voltage
rises, the SCR will be opened, so no current will flow keeping the Ballast
off.

Here's my issue. In order to turn the circuit off, I have to put a momentary
switch in series with the SCR/Ballast. That will work fine, but I'm trying
to think of a way to use one switch to turn the Ballast on and off, and use
another switch to reset the circuit. Instead of using one switch to turn it
on, and another switch to turn it off.

I was also thinking of using just one switch, setup as a toggle using a "T"
type latch, one push for on, and one push for off, and not even have a reset
button, but I don't want to make the circuit too complicated.

BTW, I have to use the existing switch for the On/Off, which is momentary,
but can be locked on. I can add another switch of any type for the reset.

Any idea's?

Thanks

You can use a self-latching dual comparator and SCR as shown below. The
SCR turns on via gate bias of R1, COMP1 senses the output voltage rise
through R2/R3 divider, indicating SCR has turned on, and grounds SCR
gate, COMP1 then opens the R3/GND path, forcing the comparators to
remain latched in this state. When the ballast cuts off due to low
voltage, the SCR cuts off permanently. The ON/OFF switch is then used
for both ON/OFF control and the RESET. There are some details pertaining
to reliable latch operation that are omitted- but where's the fun if we
tell you everything.

View in a fixed-width font such as Courier.

SCR
(sensitive gate)
SW
+--o-->o-+-----+---------|>|----------------------+
| | | / |
| | / | |
| | 10K | |
| | R1 | |
| | \ | |
| | | 1N914 | 1N914 |
| | +---|>|---+ +--------|<|----+
| | | D1 | D2 |
| | | / |
| | | R5 \ |
| +-----|----+--/\/\-+ R2 |
|BATT | | | | \ |
----- / | | | | |
--- \ | |COMP1 | | |
| R4 | /-|-----------+ |
| \ +---< | | | |
| | | \+|--+ | | |
| | | | | / +-+-+
| +-----|---------+ | \ | B |
| | | | R3 | L |
| | | COMP2 | \ | S |
| | +---|-\ | | | T |
| | | >-----+-----+ +-+-+
| +---------|+/ |
| | |
| /-/ vref |
| ^ |
| | |
+--------+-------------+--------------------------+
|
GND

 

[John Fields]

Thanks for the replies.

Here's what I was thinking. An SCR connected in series with the Ballast.

....

 

[Fred Bloggs]

...

good question- assuming that ballast cut-off current does fall below the
SCR holding current, the simplest ckt would use a momentary ON-OFF-ON
rocker switch- these are available in surplus:

View in a fixed-width font such as Courier.




rocker sw
+--------+
| o--|-------------+
| | |
+-|-o---> | |
| | | |
| | o | |
| | | | |
| +--------+ |
| | SCR |
| | |
Vbatt>---+-------------------|>|--+-> Load
| /
| 100 |
+---/\/\----+

 

[Me]

...

Max current is 6.8Amps, 3.3 typical. I don't believe there is any current
draw when cutoff, but I'm going to check to make sure.

 

[Me]

good question- assuming that ballast cut-off current does fall below the
SCR holding current, the simplest ckt would use a momentary ON-OFF-ON
rocker switch- these are available in surplus:

View in a fixed-width font such as Courier.




rocker sw
+--------+
| o--|-------------+
| | |
+-|-o---> | |
| | | |
| | o | |
| | | | |
| +--------+ |
| | SCR |
| | |
Vbatt>---+-------------------|>|--+-> Load
| /
| 100 |
+---/\/\----+

The problem is, I have to use an existing switch, Momentary with lock on.