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LM3914: turning on/off multiple ICs at the same time

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retkimora

New Member
Hello,

I've built couple of voltage monitors based on LM3914. Each chip monitors one 12V battery. IC takes its input power from monitored battery. This works, but I'd like to control multiple chips with a single switch. Otherwise all the chips stay on and leds are lit.

Can I use another battery as a voltage source for these ICs? I was thinking that pins 3 would go together, and so do pins 2 for each chip, and separate 12V battery powers all the chips then.

If that doesn't sound good is it possible to use optocouplers for cutting plus-wire input? I have no experience in those components but I figure those are simple enough for me to use. I don't know much about those, yet.

Thanks for all the time you spend replying me.
 

crutschow

Well-Known Member
Most Helpful Member
Do all the batteries have a common ground?

Otherwise you could use relays (mechanical or solid-state DC) or a multiple-pole (DPST) switch to control the power to each IC.
 

crutschow

Well-Known Member
Most Helpful Member
All batteries are connected in series and two wires connect IC to battery.
In that case you would need a separate battery for each unit if you wanted the indicatiors to operate off different batteries.

Otherwise just use a relay(s) or a multi-pole switch to control them as I previously mentioned.
 

CafeLogic

New Member
There are many ways you could do that. You could monitor all batteries using just one chip by referencing the voltage difference across the battery terminals to ground. See "differential amplifier" here: Operational amplifier applications - Wikipedia, the free encyclopedia
Another way you could accomplish this is interrupting each chips positive rail with a P mosfet. Connect the source to the + on the battery, connect
. You could accomplish the same thing with opto-couplers as you mentioned, but that might require calibration.
the drain to the positive on your chip. Pull the gate up to the source with a weak resistor, 470k or so.

Next grab an n mosfet. Connect the source to common ground, connect the drain to the gate of the psmofet through a resistor divider. Set up the divider so that you don't exceed the maximum source to gate voltage of the pmostfet. For example: top battery is at 48v. maxium Vgs is 20v. Connect 20k resistor from battery + to gate then 60k resistor from gate to N Moset drain. When nmosfet is on, pmosfet Vgs will be about -12 + bias from the 470k pullup, plenty to turn it on.

Now when you pull the N-Mosfet gate high, the device will be on. When low, device will be off.
 

CafeLogic

New Member
This picture is worth at least as many words as I typed. Ignore the 470k pull-up thing. Wasn't thinking, didn't realize the resistor divider would provide that. Relay's would certainly do the trick but they are not really the best choice for battery powered things.
 

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retkimora

New Member
Good. How about using optocoupler as a switch instead of fets? Wouldn't it insulate two different potentials even better? I've included picture of my thinking, which should be much like CafeLogics idea.
 

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CafeLogic

New Member
I don't really understand how that would help you. An opto-coupler is an LED paired with a photosensitive diode or transistor. You light the LED like any other LED. That provides current to the base of the photo transistor at the opposite side, which allows current to flow collector to emitter. You would then need to find a way to switch your device on using the current flow. You certainly can do that but you don't get anything out of it. It costs more, opto-couplers are $1+ each. It uses a more power, you need to put at least 10 ma into each LED, so you'd be wasting at least 40ma. With just the fets, all switches on would use 1/2ma with those resistors. It would also add complication to your circuit. When you need to communicate between two devices that need to be galvonically isolated, that becomes necessary in certain situations. Even if you required isolation, since you just need very simple switching action, it would make more sense to just use relays as suggested.
 

CafeLogic

New Member
BTW, when I said ignore the 470k thing, I was talking about another post that didn't show up because it has not been approved. The 470k shown in the schematic should be there.
 

retkimora

New Member
Should there be two different ground connections in your schematic? Or maybe I misunderstood something. All the grounds go together in your schematic, right? I don't like the idea of connecting 12V system ground together with 48V system ground. Or did you mean using first BAT for switches?

There is separate 12V system and 48V system. Those should be kept separated.
 

CafeLogic

New Member
Yes, they all have the same ground. That is because you connected the batteries in series. If you are talking about the individual monitoring uCs, the "ground" is the positive terminal of the battery beneath it.
 

CafeLogic

New Member
Yes, remember voltage is always relative and "ground" is just some label people make up to make schematics easier to interpret. You can take your digital camera and connect 10,000 volts relative to earth ground to the - battery terminal, then connect 10,003 volts to the + terminal and it would act just like u had AAs in there. Although, I wouldn't try to use it while standing on earth ground, lol.
 
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