Active voltage divider? - Page 1
Forum section: electronics / projects

author: hlavac
author_rank: Frequent Contributor
timestamp: December 18, 2018, 03:54:55 pm
content: I have a battery powered circuit that requires several reference voltage levels.Normally one would use a simple voltage divider, but I don't want to piss away any current once the reference level is reached.One way I came up with is make the voltage divider really high resistance, and then add BJT emitter follower to charge the referene cap to voltage Vbe smaller than the divider.Any better ideas?

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 18, 2018, 03:57:54 pm
content: You would definitively require a follower, but instead of using a BJT with its inaccurate Vbe, I would use a micropower opamp as a buffer.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 04:05:57 pm
content: Hi hlavacWe would need a lot more information on what you are doing. For example what are the various voltage references and what are they used for. How much current are the the voltage references required to supply.There are a host of micro power techniques and chips but, we will not be able to advise you about them unless we have a complete description, with schematic, of your objectives.

author: StillTrying
author_rank: Super Contributor
timestamp: December 18, 2018, 04:06:45 pm
content: Switch the divider off when it's not needed ?

author: hlavac
author_rank: Frequent Contributor
timestamp: December 18, 2018, 04:23:10 pm
content: I'm thinking about making a battery powered version of myold logic probeI want to get rid of the big voltage divider (R6-R9).Want to have low enough power consumption to not need any power switch.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 04:40:58 pm
content: Quote from: hlavac on December 18, 2018, 04:23:10 pmThank you- an interesting project. How do you feel about a completely different, simpler circuit?

author: hlavac
author_rank: Frequent Contributor
timestamp: December 18, 2018, 04:59:35 pm
content: As long as it can do what mine can (~1M input impedance, 15ns pulse detection)

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 05:24:50 pm
content: Quote from: hlavac on December 18, 2018, 04:59:35 pmOK, I will investigate and advise.What battery are you planning on and what battery duration.I would suggest a single cell LiIon battery with a high efficiency switch mode PSU.

author: hlavac
author_rank: Frequent Contributor
timestamp: December 18, 2018, 05:34:17 pm
content: I was thinking more like two LR44 cells to make it smallSwitching supply would consume power and would need to be turned on/off

author: hlavac
author_rank: Frequent Contributor
timestamp: December 18, 2018, 05:40:55 pm
content: I mean, I don't need it to run on 5V. Was looking into reference chips like LT1004 to provide reference 2.5V, so I can run on 3V from two LR44...It should only consume significant power when a LED is lit

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2018, 05:45:37 pm
content: You already have transistors buffering 2 of the 3 reference nodes but the tail current of those differential pairs will have to be lowered also.The reference nodes could be buffered with micropower low input bias current operational amplifiers configured as voltage followers. That would get into the 10s of microamps range of supply current. Two of them replace Q1, R10, Q2, and R11 to drive the emitters of Q3 and Q4 directly. Problem solved.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 09:07:08 pm
content: Quote from: hlavac on December 18, 2018, 05:34:17 pmQuote from: hlavac on December 18, 2018, 05:40:55 pmUnderstand.My approach would use two high speed comparators with CMOS inputs and would consume around 5mA including LEDS. I was thinking of a CR3 LiIon rechargeable cell for the PSU. But this approach is not in line with your thinking.Getting back to your original question of the low power reference voltage. The solution is quite simple. All you need is a dual, micro power opamp with hi Z inputs configured as unity gain buffers. Then you can increase the divider chain resistance and use the buffers to provide a low z source for the bases of the transistor comparators in your original design.These opamps may be of interest, but there are lower power types:TLV2241, 42, 44LM0462, 64TSV711, 12, 14TSU111, 12MCP606, 08, 07, 09MAX9910, 11, 12: