Voltage divider (5V -> 3.3V) to power Nokia LCD display - Page 1
Forum section: electronics / beginners

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: March 16, 2016, 01:16:42 am
content: I have a LM7805 on a project and I need to power 2 devices that use 3.3V: aCD4050(level shifter) and aNokia 5110 LCD display. The CD4050 is necessary because the LCD is 3.3V and the microcontroller will be run at 5V.Is it ok to power the 4050 and the LCD display using only 2 x 1/4W resistors in a voltage divider? I figure 390R and 200R resistors will give me 3.3V from a 5V input.Thanks in advance.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 16, 2016, 01:28:33 am
content: Why not use LD1117V33 or LM3940?

author: MK14
author_rank: Super Contributor
timestamp: March 16, 2016, 01:32:52 am
content: Quote from: AlxDroidDev on March 16, 2016, 01:16:42 amIf the load, uses any kind of significant current, the voltage would just drop and drop, because it would NOT be regulated. Best to use a LDO (Low drop out) voltage regulator, 3.3Vhttp://uk.farnell.com/microchip/mic5504-3-3ym5-tr/ldo-volt-reg-0-3a-3-3v-sot-23/dp/2510377Resistors do NOT hold the same voltage, if current is drawn.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: March 16, 2016, 01:33:22 am
content: Quote from: alsetalokin4017 on March 16, 2016, 01:28:33 amI don't have them and they are not easy to source around here. Ordering them from the internet would take too long, since our postal service is on strike.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: March 16, 2016, 01:38:40 am
content: Quote from: MK14 on March 16, 2016, 01:32:52 amI estimate less than 60mA when the backlight is on.

author: MK14
author_rank: Super Contributor
timestamp: March 16, 2016, 01:40:27 am
content: Quote from: AlxDroidDev on March 16, 2016, 01:38:40 amAn alternative would be to use a transistor or op-amp, AFTER the resistor dividers. (Configured as a simple voltage regulator alternative).The transistor circuit, needs to be about 0.7V higher, to account for the Veb drop.I DON'T know what components you have to hand, to supply circuits.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 16, 2016, 01:41:26 am
content: And that's the thing, when your backlight goes off, your voltage goes up and fries your LCD. Can you get an LM317 and run it off whatever is running the LM7805?EDIT: back when it was difficult for me to get spare parts, I used to raid junk electronics. I can't imagine it would be very hard to find a 3.3V regulator.

author: Macbeth
author_rank: Super Contributor
timestamp: March 16, 2016, 01:46:19 am
content: You can't just calculate the R1/R2 division - that will only work with no load. It's generally ok with an op-amp input, for example, as those blighters theoretically have infinite input impedance. But you are placing an actual load. It is going to be restricted in current available by R1 alone, and in parallel with R2 will totally change the ratio making your simple R1/R2 divider wrong. You need to find the parallel R2 + load.This method can actually work if you use very low resistances. Disadvantage is we are talking really high wattage ceramic wire wound resistors that glow nearly as hot as a bar fire. Back in the early 1970's colour TVs actually used this method of "dropper resistors" that would wind the electricity meter into a £££ overdrive and nearly set fire to the curtains in the living room.

author: MK14
author_rank: Super Contributor
timestamp: March 16, 2016, 01:47:47 am
content: Op-amps usually have a max output current of about 25 mA's, so 60 mA's is too much for most of them, without an extra pass transistor or particularly high current one. (Sorry, it was me who first mentioned using them in this thread).

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: March 16, 2016, 01:56:20 am
content: Thank you for the replies, fellows. I understood the explanation and the circuit.I'll keep you posted when I have it working somehow or if I have more doubts.

author: matseng
author_rank: Frequent Contributor
timestamp: March 16, 2016, 02:20:33 am
content: If I'm not completely off here the backlight is separately on the nokia lcds so you can run that from 5 volts like any other LED. Then the lcd controller chip, PCD8544, will just use a about 250 uA.With those low values in the voltage divider they probably be stiff enough especially since the PCD8544 is happy with VCCs down to 2.7 volts.

author: danadak
author_rank: Super Contributor
timestamp: March 16, 2016, 02:48:00 am
content: Use a 1.8 V zener from the regulator output to the 3.3V loads. Do the tolerance analysis tomake sure 3.3V parts stay in power supply spec.Regards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2016, 11:01:27 am
content: Quote from: danadak on March 16, 2016, 02:48:00 amA red LED may do, as long as the maximum forward current rating isn't exceeded.

author: danadak
author_rank: Super Contributor
timestamp: March 16, 2016, 12:14:00 pm
content: Problem with Red LED is its spec Vth is not tested for min max limits.Unless you can find a datasheet that calls this out. Generally speakingdatasheet min max limits are actually tested for on devices.Regards, Dana.

author: MrSlack
author_rank: Frequent Contributor
timestamp: March 16, 2016, 12:29:48 pm
content: If you have a stable source voltage you can "stiffen" a voltage divider with a transistor pretty easily if that's all you have lying around:This is usually close enough for a lot of applications. Nothing that might oscillate however.