Supplying voltage to two circuits from the same source - Page 1
Forum section: electronics / beginners

author: kjiwa
author_rank: Newbie
timestamp: August 24, 2015, 08:00:31 am
content: Hi, I have two circuits that I would like to power from a single power source (e.g. a battery). The first circuit requires a supply voltage of 1.8V and the second circuit requires 1.9V. I'm having trouble understanding how I can use a single power source to provide an accurate and consistent voltage to each circuit. Should they be connected in series? In parallel? What if I use a 9V battery -- how can I go from 9V to 1.8V and 1.9V? Thanks!

author: tron9000
author_rank: Frequent Contributor
timestamp: August 24, 2015, 08:23:59 am
content: HiFor those 2 voltages you require I would connect them in parallel or in other words: connect the input to both from the power source.Have you come across voltage regulators? If not look into them, there's loads.Careful of the pitfalls of using a battery source and linear voltage regulators, especially using a 9V battery, it'll be flat before you know it!

author: homebrew
author_rank: Frequent Contributor
timestamp: August 24, 2015, 08:34:16 am
content: Quote from: kjiwa on August 24, 2015, 08:00:31 amPlease give us some more info on the types of loads. What exactly are you powering?

author: kjiwa
author_rank: Newbie
timestamp: August 24, 2015, 08:41:46 am
content: Thanks for your replies. I'm trying to power an ATTiny45 and an nRF24L01+. I'd like to take advantage of each of their low power modes, so using a voltage regulator concerns me.As for voltage regulators, I have some 5V ones, but I'm not clear about how I can use those to supply 1.8V and 1.9V.Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: August 24, 2015, 08:47:41 am
content: If you're bothered about efficiency, you need to use a switching regulator.Using a linear regulator to get 1.8V from 9V will give a crappy efficiency of just 20%.Another option is to use a 1.5V battery and a boost converter.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 24, 2015, 09:01:50 am
content: I would use abuck converterand they can be bought as amoduleif you don't want to build one.Don't bother thinking in terms of 1.8and1.9 volts, they will both run fine off of 2 volts.Attached is the buck.xlsbuck converter calculator from Texas Instruments

author: homebrew
author_rank: Frequent Contributor
timestamp: August 24, 2015, 09:26:30 am
content: Quote from: kjiwa on August 24, 2015, 08:41:46 amThat's perfect... because we are apparently designing quite similar same things :-) I work on a wireless sensor node, too and I have to make the same decisions right now. At the moment I'm thinking about using an LDO regulator as well as a switch mode one. In standby the system would be powered by the LDO one, in operational mode the SM regulator would kick in. With these LDOs you get an incredibly low quiescent down to 500nA. Hence the system would draw something around 2.5uA when in power down mode. I would call that acceptable. However, I do need the full 3.3V for all the sensors I have on my board. So when the node wakes up every couple of minutes or so I would enable the switch mode Boost-Regulator to drive the 3.3V rail. I'll use Schotky Diodes (i.e. STPS1L30) to OR the supplies together.The whole device will be powered from two AAA Batteries (or NiMH cells) and hence provide a voltage of max. 3V input voltage. Given the 0.3V Schottky forward voltage drop and the 0.1V LDO drop, the assembly should work down to approx. 0.92V per cell which is perfectly fine for me.I'll post the schematic once I'm done with that. Probably this evening (CET timezone).Pete

author: homebrew
author_rank: Frequent Contributor
timestamp: August 24, 2015, 09:24:24 pm
content: As promised, here is the design.As said, absolutely untested! Maybe it contains severe errors.Would be happy to hear some feedback ...

author: lgbeno
author_rank: Frequent Contributor
timestamp: August 25, 2015, 02:55:33 am
content: You do know that both of these devices can be run direct from the two AAA cells. If you save power with this circuit, great but I would think that simply putting then in deep sleep modes at battery voltage would be lower power.Sent from my iPhone using Tapatalk

author: homebrew
author_rank: Frequent Contributor
timestamp: August 25, 2015, 07:12:43 am
content: Quote from: lgbeno on August 25, 2015, 02:55:33 amYes of course! No regulator is saving even more power in this case ...However, other components on MY board (i.e. attached sensors) require 3.3V. Hence I cannot skip the boost regulator. I assumed the same for the TO's project ...Nevermind, if it happily runs without the regulators, go for it!

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 25, 2015, 07:20:28 am
content: What's great about running it off of batteries is that someone can make another Batterizer joke soon