Low voltage cut off ckt.

[Christoph]

I am trying to put together a circuit to protect 4 NIMH cells that will be used in a led lantern(of course) the lantern uses 4 d cells normally I wish to use 4 nimh 4500mah F cells.I have found this circuit I like that it only uses two parts,can I do better? TIA
Chris

 

[TakeTheActive]

I am trying to put together a circuit to protect 4 NIMH cells.. ...I have found this circuit I like that it only uses two parts...

What's the Title of the EDN article? (Do you have a LINK to the ARCHIVED article?)

I'd like to read a "Description of Circuit Operation" for the circuit on the right (with the momentary pushbutton). :thinking: IC1 and Q1 are the same - I need to GOOGLE IC2 - 74HC132.

Thanks!

 

[rmteo]

Here is the article.

A low-battery voltage-cutoff circuit prevents overdischarge of a rechargeable battery. An obvious requirement of this circuit is extremely low power consumption. Figure 1a's simple circuit has a measured current consumption of approximately 1.2 mA and uses only two components to perform the low-battery cutoff function for a four-NiCd battery.

IC1 is a 3.9V voltage detector with a maximum hysteresis of 0.3V. When the battery is charged, the 5V power supply exceeds this IC's threshold such that its output goes high to turn on Q1, an IRLZ14 MOSFET switch. The IRLZ14 is a logic-level device with an on-resistance of 0.2 Ohm. When the battery voltage drops to below IC1's threshold, the output of IC1 is zero, which turns off Q1.

If the load is heavy, the circuit may turn on and off when the battery voltage reaches the threshold. When the circuit cuts off the load, the battery voltage rises again; this higher voltage may exceed IC1's turn-on threshold. To prevent this problem, the circuit in Figure 1b uses a flip-flop to provide a clean cutoff. Pushing S1 turns on the switch. When the load has a large capacitance, R1 and C1 provide a delayed response to prevent the turn-on in-rush current from triggering the circuit. The power consumption of this circuit is in the same range as that of the circuit pictured in Figure 1a.

All the parts for this idea are available from Digi-Key (www.digi-key.com). For a lower switch resistance, you can use the IRLZ44, which has an on-resistance of 0.022 Ohm.

 

[TakeTheActive]

Here is the article.

Thanks.

I also wanted to see if there was a PDF of the article WITH the schematic all on one page (Page 4):

EDN Access: September 11, 1998 - Download PDF version

...without actually having to do the 'work' of looking for it.
​

 

[uk_caver]

I'm not sure I'd call 1.2mA extremely low power consumption.
I guess it depends how long a unit might be left post-cutoff before recharging.

 

[SemiMan]

That is uA, not mA.

Semiman

 

[Christoph]

I've seen both units used which do you think is right? I have tried reading the data sheet but I kinda get lost in some of it.I think I found it typ 1 μA max 5 μA If I'm reading the right line. The trouble is Mouser wants a min order of over 700 pieces :sick2: .I guess I keep searching this looks like exactly what I need. I found it it is 1.2 μA
C

 

[Christoph]

The part # MN1381-Q is no longer available digikey suggested this#MN13811squct in place of it . I need a little help to figure out if this will work the same way
TIA

 

[McAllan]

Haven't thought it quite through but I feel it's worth mentioning.

I had a similar need although with only 3 NiMH cells. Got the idea to use a single cell LiIon protection circuit. For 3 cells it can be used unmodified - works perfectly. You just need to be aware that after it has trigged it needs to either have some power in (to simulate charging of battery) or just be without power 10-20 seconds.

Typically such a circuit consist of a control IC and an IC (really just a mosfet in disguise??) to do the switching.
Perhaps just lowering the supply to the control IC would do the trick. One or two diode in series depending of desired cut off level.

If it'll work in practice - as said haven't tried it with 4 cells.