help with reducing consumption for very small motor / low voltage buck - Page 1
Forum section: electronics / projects

author: pommil
author_rank: Newbie
timestamp: September 25, 2024, 12:33:29 am
content: Hi,I'm trying to get a small dc motor to run for the longest possible time from the energy of a small solar panel with a liion battery (or cap). The solar panel output up to 40mA at 0.5v (close to max max power point). I have tested the motor, it starts at 0.142v/11mA (I can also do 24mA and 270mV).I'm using a bq25570 energy harvester on a board from aliexpresshttps://fr.aliexpress.com/item/1005006447191303.html?spm=a2g0o.productlist.main.7.1dc16677O5KYPj&algo_pvid=6fcbbdd6-e42e-49dd-975f-6771b66e1efa&aem_p4p_detail=202409241648145856548259165120010910300&algo_exp_id=6fcbbdd6-e42e-49dd-975f-6771b66e1efa-3&pdp_npi=4%40dis%21EUR%2114.99%2114.99%21%21%2116.26%2116.26%21%402103868817272216948587324e340f%2112000037202243228%21sea%21FR%21760932160%21X&curPageLogUid=x5iwhs9uLGKZ&utparam-url=scene%3Asearch%7Cquery_from%3A&search_p4p_id=202409241648145856548259165120010910300_1It's basically dealing with the battery management, energy harvesting and outputting 2.6v (100mA max).I want the motor to run as slowly as possible to reduce it's power consumption. So, I have 2.6v and need to reduce it to approx 0.2. I could not find a suitable buck (I'm trying to avoid smd component so can't use something like ADP5331 and anyway, it's still cannot go under 0.8v).I have tried an astable multivibrator to PWM the motor and reduce its power consumption. It's working fine to reduce the motor speed and I can get it down to 11mA and 142mv, but the problem is that the circuit consumes 11mA at 2.6v! 11 mA are going through Q3 with 142mV for the motor and the rest between Q3 and ground (about 2.56V*11mA). I have tried playing with capacitor and diode, but can't solve it. From my readings, it seems I may need to add an inductor somewhere??Or should I feed back the excess voltage inside the BQ2550 (connecting emitter of Q3 and ground to + and - of solar cell, but then I'll have to add a diode on the solar cell, possible but not sure it would work and don't really want the voltage drop of the diode + it may mess up the MPPT of the BQ25570).Thanks in advance for any suggestion, either:- on the overall idea of BQ25570 + SOLAR + BUCK / PWM + LIION BATT + DC MOTOR- on how to reduce the power consumption of the multivibrator (kind of turning it into a buck?)- on what other solutions to efficiently reduce motor consumption with through hole if poss)- on any good low voltage buck on a board or SMD if nothing elseThanks a lot in advance

author: moffy
author_rank: Super Contributor
timestamp: September 25, 2024, 05:21:24 am
content: Try the LMC555 timer, it is CMOS, so low power consumption and according to the datasheet it has a version that comes in the PDIP package:https://www.ti.com/lit/ds/symlink/lmc555.pdf?ts=1727241526018&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLMC555

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 25, 2024, 05:53:58 am
content: Quote from: pommil on September 25, 2024, 12:33:29 amAt 2.6V to 200mV you may be better to use a CMOS schmitt inverter as oscillator, skewed to give the required duty cycle and then a low gate threshold NMOS/PMOS pair to drive the motor as a sync rectifier.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 25, 2024, 07:00:14 am
content: * Your analog oscillator circuit consumes more than a small microcontroller which would be able to do proper motor control (maybe as simple as PWM with some fixed ramp speeds for inrush limiting without current sense).* Vf of the diode is a massive loss element. For freewheeling switch, you really need to use MOSFET for voltages this low.* Bipolar transistor as motor power switch has significant saturation voltage, BC547 datasheet says Vce_sat would be more than 100mV at some tens of mA, plus then you have the base current to supply.With 2.6V available there is really no reason to use bipolar transistor for switching, use a small N-channel MOSFET for low side switch and another P-channel MOSFET for synchronous freewheeling switch instead of a diode.Basically a little bit beefier than normal CMOS logic buffer would drive the motor directly pretty optimally.But the diode is the biggest problem, especially 1N4007 which is totally unsuitable type. If you want 0.2V out, even Vf=0.5V limits your efficiency to some tens of %, not even to talk about reverse recovery problem and short circuit current you get with 1N4007. A bog standard schottky would be a huge improvement already but is still limiting your efficiency to below 50%. Use a MOSFET switch.