Power consumption of circuit - Page 1
Forum section: electronics / projects

author: raff5184
author_rank: Regular Contributor
timestamp: March 27, 2020, 08:40:56 pm
content: 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 Vs2) 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

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 27, 2020, 08:58:22 pm
content: 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.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:00:33 pm
content: 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.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:06:16 pm
content: 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.

author: raff5184
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:09:37 pm
content: Quote from: Vovk_Z on March 27, 2020, 08:58:22 pmYes, exactlyQuote from: Vovk_Z on March 27, 2020, 08:58:22 pmI want it fully off, how do I do it?Quote from: Chalcogenide on March 27, 2020, 09:00:33 pmIs this an issue?Quote from: Prehistoricman on March 27, 2020, 09:06:16 pmI'll clean it and reuploadAnyways, I'll try different resistors and see what happens

author: raff5184
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:36:06 pm
content: I hope it's better

author: Chalcogenide
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:43:55 pm
content: Quote from: raff5184 on March 27, 2020, 09:09:37 pmConnect 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).Quote from: raff5184 on March 27, 2020, 09:09:37 pmWell, 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).

author: daqq
author_rank: Super Contributor
timestamp: March 27, 2020, 09:48:11 pm
content: 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.

author: raff5184
author_rank: Regular Contributor
timestamp: March 27, 2020, 09:49:51 pm
content: 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

author: StillTrying
author_rank: Super Contributor
timestamp: March 27, 2020, 10:03:14 pm
content: The CPC1114N needs a resistor in series with it, to supply it with about 2 to 5 mA.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 27, 2020, 10:30:01 pm
content: Quote from: Chalcogenide on March 27, 2020, 09:43:55 pmQuite possibly one of these states will have less consumption than the other. Worth checking out briefly.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 28, 2020, 02:01:29 pm
content: Quote from: raff5184 on March 27, 2020, 09:09:37 pmHere is Application Design Guidelines for LM339, LM393, TL331.Look page 13 "5.1.2 Unused Comparator Input Connections".Here is opamp design notes.

author: raff5184
author_rank: Regular Contributor
timestamp: March 28, 2020, 02:47:30 pm
content: Quote from: StillTrying on March 27, 2020, 10:03:14 pmIt 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

author: raff5184
author_rank: Regular Contributor
timestamp: March 28, 2020, 08:54:04 pm
content: Quote from: daqq on March 27, 2020, 09:48:11 pmThat'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