12v battery protector circuit

I'm looking for a circuit that will protect a lead acid battery against excessive discharge - i.e. cut off the supply to the main circuit when the voltage from the battery goes below about 11V. Ideally, once the circuit has switched off, the protection circuit shouldn't use any current itself, either, which is the tricky bit. The best I've come up with so far is:

Vin o--------------------o------------------------------o | |< +------| | |\ R1 .-. | Vg 100k | | o---------+ | | | | '-' .-. | | | | R2 | | | |100k | - '-' | zener, 9.5v ^ | | +--------o | | | .-. | | | R3 | | | 3.3k | '-' === | |^| Gns o-------------------o---------+|+-----------------o

I built it without the mosfet, and it does work - the circuit switches right off at Vin=~10V, and Vg goes high at Vin=~10.5V, which would switch the mosfet on. But there's still the 0.5V gap between the two, and it's not very adjustable - is there a better way of doing this?

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

P.S. R1 is just to stop the base current going too high when the transistor saturates - it doesn't have much effect on the switching (with Vg=~10V, the voltage across R1 was about 0.4 V)

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

What's wrong with 0.5V hysteresis, especially in a low voltage cutout? Or do you mean that you need more?

Thanks, Rich

--
Yes, check "Low voltage cutoff circuit" on
alt.binaries.schematics.electronic.  Jim Thompson and I have both
posted circuits which should work.

On mine you'll have to change R4 to 12.1k because of your 11V cutoff
requirement and on Jim's, you'll need to change R3 to 2.21k.

I dare to say it is extremely difficult, because once the circuit is turned off, one needs some form of energy to turn it on. This "energy" has to come from somewhere, either external to the system (external battery, manual reset, etc.) or internal. In any case, micro-power comparators or OpAmps draw ~50 microA, this would be the only device, together with the voltage sensing and reference circuit, that uses power from the battery in the off-status. Probably you could achieve confortably less than 100microA "leakage". (BTW, is cost an issue?) Gene

It's not hysteresis - the mosfet having switched on or off doesn't make any different to the switching point of the transistor circuit. A bit of hysteresis would be good, but that's not what's happening - it's that the mosfet and hence the main circuit will switch off at Vin=~10.5V, but the transistor circuit won't switch right off until Vin drops to ~10V. It would probably do the job OK (in practice, switching off the main circuit would probably be enough, because the battery would then get charged back up from the solar panel on the next day's sunshine), but I'm wondering if there's a better way, where the whole thing switches off at a single cutoff voltage. (Ideally with a bit of hysteresis to stop it dithering or browning out).

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

Could do, but I'm still looking for a way where there's no current draw at all apart from leakage once the circuit has turned off.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

Andy, your current circuit draws current because the BJT applies some gain to the small leakage current through the zener (perhaps 100X or so) and this is then allowed to flow through the collector. The zener leakage is probably something like a microamp or so, so this suggests something like 100uA of consumption for your circuit.

Frankly, I'm not even sure what your circuit is supposed to be doing. Those

100k resistors seem very big. For one thing, zeners are spec'd to operate at much higher currents than you'll get with your 100k resistor. For another, your zener will leak current to the base of the BJT and the BJT will apply gain to it to yield collector currents that are some 100X higher.

In the case where the zener hasn't yet reached its zener voltage, it will just leak at say, 1uA. Thus, your Vg will be:

Izener ~= 1uA (rough guess) V(R1) = Izener * R1 ~= 0.1V (negligible) so, Ib(BJT) ~= Izener, Ic(BJT) = beta * Ib(BJT) ~= 100uA (estimate, for now)

With an Ic of 100uA and an (R2+R3) of slightly over 100k, you'd expect to see about 10V across them. Since we already know as a rough guess that the voltage supply is below the zener voltage and thus below 10V, we can be pretty sure that V(CE) is small and that the transistor is probably saturated (reducing the beta so that the actual Ic matches up with (R2+R3) to yield about the supply voltage.

So, in effect Vg would basically just track your supply voltage before the zener voltage kicks in, placing the supply voltage directly on your MOSFET gate. Mostly because of that huge R2 value, though.

But what happens after the zener has kicked in? Well, Ib will be even higher of course. And V(CE) will just be that much less (not much less, because it was already saturated to begin with.) So Vg would be about the same then, too.

So you might as well just hook up the supply to Vg and be done with it.

This all changes some if you make R2 a smaller value. I'm not sure if you are telling us correctly about the 100k value on R2, or not. But if R2 is a lot smaller than 100k, then the case before breakdown and the case after breakdown look different, which is probably what you are after. Then Vg is held down to a low value by the much smaller R2 and R3 against the relatively smaller looking Ic=beta*(Ib=leakage) until the zener breakdown when the Ib rises (although again, I suspect your R1 should be something less than 100k as even these tiny Ib currents present a real voltage drop across something that big and your zener really needs more substantial currents after breakdown, anyway) and then more substantial Ic values can begin to work against R2+R3 and cause Vg to rise up towards the supply voltage.

But any reasonable way you work it, your Ic is going to be at least beta times the zener leakage and that's going to set the total leakage of your circuit, when off.

I also think Gene's point about having to have some kind of comparison circuit operating, even when the MOSFET is disabled, is true enough. You'll need something running. But I believe you can get this down to about a microamp or two with predictable performance, with some careful design. (I'm not a good or professional designer, but I do have an idea how this might be achieved.)

Jon

Yes, at:

snipped-for-privacy@4ax.com

and,

snipped-for-privacy@4ax.com

I just looked.

Both of these appear to require a switch to turn them back on, though, since power to the rest of the circuit is blocked by both of these once the source to gate resistor takes over and holds the MOSFET off. I'm guessing, but Andy is looking for something that does this automatically when the voltage rises *AND* where it has very low Iq when the power is disconnected from the load.

Jon

How about using something a bit more straightforward:

P-MOSFET Q3 -----------o---------o------------o-------+^+----------- VIN | | | ||| VOUT .-. | .-. === | |47k | | |220k | | |R1 | | |R4 | '-' | '-' | | ___ |< | | o-|___|-| PNP o-------' | 100k |\ Q1 | | R2 | ___ |/ | o---|___|--| NPN 10V - | 47k |> Q2 Zener ^ .-. R5 | D1 | | | | | | |220k | | '-'R3 | | | | | | | GND | | | -----------o---------o------------o----------------------

D1 = 1N4740A (10V Zener) Q1 = 2N3906 (PNP) Q2 = 2N3904 (NPN) Q3 = IRF9Z34S for example

created by Andy´s ASCII-Circuit v1.24.140803 Beta

When VCC dips below Vzener + 0.7, Q1 turns off, making Q2 turn off and Q3 turn off!

This circuit also has a fairly sharp knee, meaning it won't drag VOUT through an extended period of low voltage, it'll drop to zero fairly quickly. This is affected adversely by high impedance of the voltage source.

It also uses very little current; at VIN = 15V, it uses around 600uA. At VIN = 10, it uses almost none.

The zener should get more current to be accurate, but it doesn't really matter; it'll probably be within 5%. If you are worried about accuracy, you should instead use a voltage reference and a comparator.

Regards, Bob Monsen

I hadn't thought of that, I admit.

I was trying to keep the current down as much as possible - the main circuit only uses 0.8 mA, so I'd like this part to use maybe up to 0.2 or

0.3mA when it's on, and under 10 uA when it's off.

The idea of it is -

- when Vin is below Vzener plus 0.7 volts, there's no current except for leakage, because both devices are off (but I forgot about this being amplified, as you said).

- when Vin goes above Vz+0.7, then current will start to flow in the zener. This will create a collector current, which will start to increase the voltage across R2 and R3. R3 is meant to give a negative feedback - as the collector current increases, this will push the base voltage on the transistor closer to Vin again, tending to switch it off.

- so Vg will rise until the transistor saturates. The maths I worked out has this happening when Vin=(Vz+0.7)*(R2+R3)/R2

- then the voltage across R1 starts to rise, which stops the base current going too high after the transistor has saturated.

your

it

that

voltage.

zener

of

It didn't do that, honest. I'll try building again to be sure.

a

zener

The idea of that circuit was that you don't, because it's relying on the transistor and zener shutting off when Vin goes below their combined switch off voltage. But I hadn't thought about the transistor amplifying the leakage, as you've said.

I did build it as in the diagram, and it worked as I said with those resistor values - Vg stuck at 0V for Vin = 0 to ~10V, and swung up to near Vin as Vin went from ~10 to ~10.5V. I probably am using the devices outside the specs though, so maybe I was just lucky.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

That is what I'm looking for. I'll have a proper look at the circuits and the other comments when my head's a bit clearer, and see if I can come up with something that does what I want. The TL431 looks like a possibly-useful component apart from the 1mA minimum current - The whole circuit only uses 0.8 mA, so this is a bit high for what I want.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

Thanks - I finally found a free news server that carries that group (news.tehnicom.net). What I want is a circuit that doesn't need resetting, so I'll have a proper look at both those designs when my head's a bit clearer, and see if I can come up with something.

R3 is meant to be a pot, like you said in your reply on abse - I just tested with a static resistor because it was easier.

--
http://www.niftybits.ukfsn.org/

remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

to

*AND*

--- Try this:

PCH BAT+>----+-------+-----+-----------------S D------+ | | | G | | [845K] | | | [360K] | | | | | +----|-\ | | | | | >--------------+ | | +-------|----|+/ | | | | | U1A 4001 A--+ | | | | LMC6762 +--Y U2A | | | [22.6K] | B-----+ [LOAD] | | | | | | | U1B +--B | | +-------|----|-\ U2B Y-----+ | | | | >-----+--A | | | +----|+/ | | | [LM385-2.5] | | +--A | | | [226K] | | U2C Y-----+ | | | | +--B | | | | | | | | | | | +--A | | | | | | U2D Y-----+ | | | | +--B | | | | | BAT------+-------+-----+----------------------------+

All of the resistors are +/- 1% except for the 360k, which is +/- 5%.

The resistor string values were selected for a VBAT cutoff of =12V. You'll have to recalculate them if you want something else.

Operating currents at 11V are: for the LM385, 24µA for the resistor string, 10µA for the comparator, 25µA for the 4001, 1µA for the MOSFET, 0 ----- For a total of ....................................... 60µA

I haven't added any hysteresis because the latch makes it unnecessary, but if the circuit chatters and you find that objectionable, connect

0.1µF ceramic caps across the inputs of each comparator.

-- John Fields

--
Oops... Miswired the NORs

Should be :

                                          PCH
BAT+>----+-------+-----+-----------------S   D------+
         |       |     |                   G        |
         |     [845K]  |                   |        |
       [360K]    |     |                   |        |
         |       +----|-\                  |        |
         |       |    |  >--------------+  |        |
         +-------|----|+/               |  |        |
         |       |    U1A       4001 A--+  |        |
         |       |  LMC6762    +--Y U2A    |        |
         |    [22.6K]          |     B-----+      [LOAD]
         |       |             |           |        |
         |       |    U1B      +--A        |        |
         +-------|----|-\      | U2B Y-----+        |
         |       |    |  >--+--|--B        |        |
         |       +----|+/   |  |           |        |
    [LM385-2.5]  |     |    |  +--A        |        |
         |     [226K]  |    |  | U2C Y-----+        |
         |       |     |    +--|--B        |        |
         |       |     |    |  |           |        |
         |       |     |    |  +--A        |        |
         |       |     |    |    U2D Y-----+        |
         |       |     |    +-----B                 |
         |       |     |                            |
BAT------+-------+-----+----------------------------+

--- "That's looking promising. Thank you." would be so much nicer, don't you think?

--- Maybe, maybe not. the LMC6762 has push-pull outputs, so if you need an open-collector output, you're out of luck. But, you could use an LMC6772. ISTR that I posted a circuit using it once but you blew it off for some reason. Oh, well...

---

--- Jesus, Andy, do some of your own leg work for a change. It's easy enough to do, just point your browser around and find out who's got what in stock. Since National makes the part that would be the logical place to start, and here's what they have on it:

-- John Fields

Hi, Andy. You got very lucky. There have been an amazing number of really well thought-out answers to your original post, especially the two circuits posted by Mr. Thompson and Mr. Fields in binaries. If you start out with an inadequate problem description and keep adding to it as you go along, you're kind of wasting the time of the people who respond, as well as an opportunity to learn something. But...

If you're going for fleapower both when on and when off, and you can't deal with a reset switch, look at the available art. There are a number of battery power management ICs, including some by Maxim. One IC that comes to mind is the High-Voltage, Low-Current Voltage Monitor in SOT Package made by Maxim, specifically the MAX6460. Of course, this is a surface mount part, but it should fill the bill here. (View in fixed font or M$ Notepad):

Fleapower Undervoltage Lockout Vbat+ Vout o-----o--------------------o-----------o--+^+--------o | | ||| | | | === .-. | .-. | 2M | | | 47K| | | | | | | | | '-' | '-' | | .----o------. | | .-. | Vcc | | | | |

OK. That circuit should do the job. I guess you've got a good idea of where you're going. A few comments:

• There are few solar power problems that more panels can't help at least a little.

• If this beast is going to be outside, you need to be careful about things. In the summer, overheating the battery can cause it to go flat. In winter and in conditions of fog or high humidity (you never get that on your side of the pond, right?), the possibility of condensation is going to wreak havoc with your best-laid plans of microamp current consumption with leakage currents across the surface of the board and components, unless you either hermetically seal the circuit from the atmosphere or apply some kind of sealant on the board. You might have the best luck with a gasketed enclosure which is shielded from direct sunlight, and is somewhere that convection cooling and any breeze will help. Along these lines, also make sure any wires enter/leave the enclosure through gasketed/sealed glands. Resist the temptation to use all-temp outdoor silicone sealant here - it corrodes wires.

Good luck Chris