voltage divider not working when load is applied

voltage divider not working when load is applied
Posted by abdullahseba on 05 May, 2018 22:44
Hi
I have a wemos D1 mini pro that I want to power with a 3.7v LiPo battery and a TP4056 module.

I have a 91k and a 750k resistor acting as voltage dividers on the output of the module to reduce the voltage from 3.7v to 3.3v.
Measuring the output of the divider directly does give 3.35v as expected. However if I connect the D1 mini to it, all I get is 0.6v and the wemos does not power on.
Nothing gets hot and the charging module shows the battery as fully charged. The D1 mini works fine with USB power.
Here is a schematic of the D1 mini:
https://wiki.wemos.cc/_media/products:d1:sch_d1_mini_pro_v1.1.0.pdf
I have the voltage divider output connected to the 3.3v and GND pin. There is no diode, so it should work.
I have the TP4056 module's input connected to the 5v pin of the D1 mini. This lets me charge the battery when the D1 is plugged in and works fine. I have tried disconnecting it though to make sure that was not the issue but I still get 0.6v.
Anyone have any ideas?
Thanks in advance.

#1 Reply
Posted by Karlo_Moharic on 05 May, 2018 22:52
You are putting to much load on your voltage divider (it uses too much current). You need voltage regulator.
Something like this

https://www.ebay.com/itm/5PCS-5V-3-3V-DC-DC-Step-Down-Power-Supply-Buck-Module-AMS1117-LDO-800MA/201662468680?hash=item2ef4051248:g:Ga0AAOSwMgdXzhOz

#3 Reply
Posted by IanB on 05 May, 2018 23:37
Also a lithium ion battery does not produce 3.7 V; the voltage varies with the state of charge and the load between 4.2 V and 3.3 V or so.

#4 Reply
Posted by PA4TIM on 05 May, 2018 23:37
Are you trying to feed the D1 via the voltage divider ? Without a load there is 5uA going to the divider.
If you measure 0,6V there is a 3,1 voltage drop over the 91k there is 34 uA running to the resistor. 800nA for the 750k resistor and 33uA for the D1. That is not enough.

Use a low drop 3V3 voltage regulator

#5 Reply
Posted by Brumby on 06 May, 2018 00:20
The very first question is - how much current are you going to need? This is the question you did not ask, nor did you include in your voltage divider.

I agree on the low dropout regulator - and if your current needs do not exceed 100mA, then I suggest checking out this page: http://www.ti.com/product/LP2950-N

#7 Reply
Posted by Jeff1946 on 07 May, 2018 21:03
Look at specs on a Schotty diode. If you put it in series with the power supply, it should work

https://www.mouser.com/datasheet/2/308/MBR150-D-106444.pdf

A quick look at an MBR150 shows it starts conducting at 0.3 V and at 0.1A is dropping about 0.4 volts.