Can a Capacitor help in this situation? - Page 1
Forum section: electronics / beginners

author: Veramacor
author_rank: Regular Contributor
timestamp: September 16, 2015, 12:01:10 pm
content: Hello,I have a low power circuit that will have a 3.3 Volt boost regulator boosting voltage from a 1.2 - 1.5 volt single cell battery.Most of the time the circuit will be drawing about 500 MICRO amps. But there is a wifi module that will need about 250-300 MILLI amps for a short 1 second burst periodically.In my search for a boost regulator, the best current rating I can find is for 100 milli amps.Will a large capacitor in parallel with the battery help? Say a 100 micro farad capacitor to handle the 250-300 milli amp burst?thanks all!

author: grumpydoc
author_rank: Super Contributor
timestamp: September 16, 2015, 12:45:29 pm
content: Energy in capacitor = 0.5CV2= 0.5x1e-4*3.3*3.3 = 0.0005445JEnergy you need that the converter can't supply = VAT = 3.3*0.2*1.0 = 0.66JIf I have the sums correct I think that might answer your question.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 16, 2015, 01:09:27 pm
content: You can calculate how much is required.The fundamental equation for a capacitor is:I = C * dV/dtThe dV/dt is a calculus thing, but if you don't know calculus, and you don't know a function for current anyway, you can assume the current is constant during some period, and change the d's to deltas, so that the voltage follows a straight diagonal line down.We know delta t, that's ~1 second.How much delta V? How much is tolerable? Say you run the module at 3.30V, and it gives up at 3.00V. That's 0.3V. More seems unlikely (if it's not made for wide operating voltages), though low voltage CMOS logic can survive down to 1.5V or so. Check the datasheet.Rearranging, C = I * dt / dV = (0.3A) * (1s) / (0.3V) = 1F.Yes, a wholefarad. You can get supercaps this size, but mind the ESR: it needs to be quite low, so you don't blow the 0.3V dropout range (or whatever it is) without even gaining any time. That's calculated similarly (< 0.3V at 0.3A is < 1 ohm ESR).Using an excessively large capacitor isn't usually the best option, because it's expensive and bulky, the regulator may not appreciate it (charging it from zero might cause undue stress, or failure), and a better battery and regulator can usually be selected instead (e.g. LiPo instead of long life lithium primary).So....yes, it can help. You're low by about four orders of magnitude. So no, it's not really the best approach either.Tim

author: Chris C
author_rank: Frequent Contributor
timestamp: September 16, 2015, 01:48:40 pm
content: From your description, I'm going to take a guess that your wifi module is a ESP8266.They will operate down to 2.5V. Which means you could charge a supercap slowly through a resistor, to avoid stressing the boost converter. And provide a separate discharge path through a Schottky diode, with a voltage drop of about 0.5V, to provide the extra current when needed. This could work so long as you don't need to transmit frequently, or soon after the battery is installed or the power switch (if present) is turned on.However, you might instead consider adding a second battery. That would increase runtime, eliminate the need for either a supercap or boost converter with unusual specs, and might even eliminate the need for a boost converter entirely.