Author Topic: Power consumption of circuit  (Read 1617 times)

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Offline raff5184Topic starter

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Power consumption of circuit
« on: March 27, 2020, 08:40:56 pm »
Hi all,
I am trying to realize a low power voltage monitor with hysteresis.
I have a battery or capacitor, and I want to monitor its charge using a comparator (TLC3702). I set 2 thresholds for the comparator so that I have a hysterical type of comparator. The output of the comparator drives a solid state SPST relay (CPC1114N).

The tricky thing is that from the battery I derive and keep independently 3 things:
1) the supply of the comparator Vs
2) the reference voltage of the comparator Vref, that creates the 2 thresholds (voltage divider R4-R5)
3) the voltage across the battery that I want to monitor Vin, I scale it with the voltage divider R6-R7 so that I can compare it with Vref as I need.

To generate a Vsupply (Vs)  and the Vref both at 2.5V, but keeping them independently, I use two LM285LPE 3-2-5 (http://www.ti.com/lit/ds/symlink/lm285-2.5.pdf).
Functionally this works and it does what it is supposed to do.

Now, my problem is the power consumption of the whole circuit, I mean the current drawn from the battery. The comparator TLC3702 is supposed to draw current in the order of micro-amps. But my whole circuit nabsorbs 3-4 mA. I don't know which component is absorbing that much.
I thought it was the switch but very little changes if I remove it
« Last Edit: March 27, 2020, 09:03:12 pm by raff5184 »
 

Offline Vovk_Z

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Re: Power consumption of circuit
« Reply #1 on: March 27, 2020, 08:58:22 pm »
Are you talking about power consumption from B1? Then you should increase nominals of voltage dividers and ballast resistors (by an order). R1, R2, R6 etc.
Second (not used) comparator have to be set to some stable steady state - fully on or fully off, otherwise it can oscillate.
« Last Edit: March 27, 2020, 09:01:51 pm by Vovk_Z »
 

Offline Chalcogenide

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Re: Power consumption of circuit
« Reply #2 on: March 27, 2020, 09:00:33 pm »
R1 and R2 will each draw a current of (V(B1_pos) - 2.5 v) / 3.3k; then you will have the direct path to ground through R6 and R7.
Finally, have you left the second comparator in the TLC3702 with its inputs floating? That may also be a problem as it might be oscillating.
 

Offline Prehistoricman

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Re: Power consumption of circuit
« Reply #3 on: March 27, 2020, 09:06:16 pm »
Your schematic isn't nice to read. Ground is usually expected at the bottom and power at the top, with use of the ground symbols to make everything neat.
 
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Offline raff5184Topic starter

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Re: Power consumption of circuit
« Reply #4 on: March 27, 2020, 09:09:37 pm »
Are you talking about power consumption from B1?
Yes, exactly

Second (not used) comparator have to be set to some stable steady state - fully on or fully off, otherwise it can oscillate.
I want it fully off, how do I do it?

then you will have the direct path to ground through R6 and R7.
Is this an issue?

Your schematic isn't nice to read. Ground is usually expected at the bottom and power at the top, with use of the ground symbols to make everything neat.
I'll clean it and reupload

Anyways, I'll try different resistors and see what happens
 

Offline raff5184Topic starter

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Re: Power consumption of circuit
« Reply #5 on: March 27, 2020, 09:36:06 pm »
I hope it's better
 
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Offline Chalcogenide

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Re: Power consumption of circuit
« Reply #6 on: March 27, 2020, 09:43:55 pm »
I want it fully off, how do I do it?

Connect its inputs + and - to gnd and vdd, respectively. This will force its output to 0V. The other way around is also fine.
I also noticed that you are powering the TLC3702 at 2.5 V, while the datasheet suggest a minimum value of 3 V, so that might also be a problem (but unlikely the reason of high power consumption).

Is this an issue?

Well, that's power you are just wasting. Increase their values by a couple orders of magnitude, at least.
You should size R1 and R2 not to waste too much current but still correctly bias the comparator (include in your calculation the output current through R3).
 

Offline daqq

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Re: Power consumption of circuit
« Reply #7 on: March 27, 2020, 09:48:11 pm »
The main loads will be the references (shunt references with resistors will have a current consumption equal to (input voltage - reference voltage)/shunt ) and the R6/R7 divider.

Also, why are you trying to power the comparator by a voltage reference and a shunt resistor (R1)? Can't you just connect it directly to the battery?

You could greatly trim the power consumption by using much higher resistance values (check on how high you can go with your reference), removing the "voltage regulator" for the comparators. Or use a dedicated voltage monitoring IC.
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Offline raff5184Topic starter

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Re: Power consumption of circuit
« Reply #8 on: March 27, 2020, 09:49:51 pm »
Great thank you everybody. For one of the two LM285 I was using a smaller R, I changed that and now I've current in the order of 300 uA which makes sense. I'll replace the other resistors to reduce it even more
 

Offline StillTrying

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Re: Power consumption of circuit
« Reply #9 on: March 27, 2020, 10:03:14 pm »
The CPC1114N needs a resistor in series with it, to supply it with about 2 to 5 mA.
.  That took much longer than I thought it would.
 

Offline Prehistoricman

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Re: Power consumption of circuit
« Reply #10 on: March 27, 2020, 10:30:01 pm »
I want it fully off, how do I do it?

Connect its inputs + and - to gnd and vdd, respectively. This will force its output to 0V. The other way around is also fine.
Quite possibly one of these states will have less consumption than the other. Worth checking out briefly.

Offline Vovk_Z

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Re: Power consumption of circuit
« Reply #11 on: March 28, 2020, 02:01:29 pm »
« Last Edit: March 28, 2020, 02:10:50 pm by Vovk_Z »
 
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Offline raff5184Topic starter

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Re: Power consumption of circuit
« Reply #12 on: March 28, 2020, 02:47:30 pm »
The CPC1114N needs a resistor in series with it, to supply it with about 2 to 5 mA.
It works without the series resistor and it doesn't absorb as much current, but apparently it is not the proper way of doing it. So, do you know of any Form B relays that can be activated with current in the order of microamps? Even a digital switch that can work with a supply as low as 1.5-2V would work.
All the relays I found need >1mA, which is at least 3x more current than the remaining part of the circuit absorbs
 

Offline raff5184Topic starter

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Re: Power consumption of circuit
« Reply #13 on: March 28, 2020, 08:54:04 pm »
Also, why are you trying to power the comparator by a voltage reference and a shunt resistor (R1)? Can't you just connect it directly to the battery?
That's because like I said I was trying to keep the Vs, Vin and Vref independent from each other. But since I already have a reference (LM285) for Vref I don't need it any more on Vs. However I still use a 33k Ohm between the Vbatt+ and the supply of the comparator to limit the supply current, otherwise it is too high
 


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