How to proportionally lower Volts (-20V always) from 100v down to 70v - Page 1
Forum section: electronics / beginners

author: marce002
author_rank: Regular Contributor
timestamp: June 16, 2024, 05:29:23 pm
content: HiI need someone to help me with a simple circuit that takes a volt input like 100v and return 80v current max 800ma... it will be used to lower range for an ebike speedmeter already setup for 75v and im using 100v ... so stock max battery gauge needs my range... i have at hand lm317 resistors etc thks a lot!

author: mariush
author_rank: Super Contributor
timestamp: June 16, 2024, 05:35:52 pm
content: Maybe a whole bunch of diodes in series? Sounds the simplest method. Each 1n400x diode would drop around 0.7v or more (varies with current), and they don't take up a lot of space.I don't think there's gonna be a lot of current going into the speedometer, so no idea why the 800mA value is mentioned

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 16, 2024, 06:17:26 pm
content: 800mA is a lot for a simple gauge! Likely you simple need a resistor divider somewhere.

author: marce002
author_rank: Regular Contributor
timestamp: June 16, 2024, 06:26:04 pm
content: I have made a perfect volt divider but my output when loaded with the speedmeter went down to 22v and barelly iluminated rhe backlight.... the output were perfect 75v when no load .... need more current... yes 800mah was invented.... but might be close? ... i do not have so many... 25diodes and sound unproffesional but ....

author: langwadt
author_rank: Super Contributor
timestamp: June 16, 2024, 06:52:11 pm
content: Quote from: marce002 on June 16, 2024, 06:26:04 pm800mA * 75V = 60W, that would one toasty backlight

author: marce002
author_rank: Regular Contributor
timestamp: June 16, 2024, 06:55:34 pm
content: ok forget 800ma it should be far less... was just a guess... anyway im unable to find a simple solution for this because dont need regulated output just a" shift range"...i mwasured takes 0.013A even though the volt divider did not work.. used this (attached pic)

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 16, 2024, 08:05:47 pm
content: A 20V zener in series should do it for only 13mA (1N4747A).Switch to any other zener for the desired drop.If getting hot, you can distribute the power between lower voltage zeners in series, for example usign two 10V zeners instead.Though 13mA * 20V is just 260mW.For more power, you can add a transistor and a small heatsink.

author: marce002
author_rank: Regular Contributor
timestamp: June 16, 2024, 09:34:17 pm
content: gracias David now i will try with 3 diodes i ve found a z10c and 2 diodes z6.8A

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2024, 01:06:19 am
content: Some variation of a high current shunt regulator will work when wired in series. This could be with a zener diode or with a TL431 as shown below:https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/114728/question-about-tl431-application-high-current-shunt-regulator

author: tooki
author_rank: Super Contributor
timestamp: June 17, 2024, 02:01:36 am
content: The title says “-20V always”, but that’s right after saying “proportionally”. Those two statements are contradictory. If it’s proportional, then you’re reducing by a percentage. If it’s always -20V, then you want a voltage shift.And how does “100V down to 70V” work, since that is -30V, not -20V?(Also, V is always capitalized.)

author: marce002
author_rank: Regular Contributor
timestamp: June 17, 2024, 02:19:45 am
content: David understood very well my need... the need was explained in the first post...the zeners solution worked just perfect to suit my needs... sorry my english ... the remaining battery gauge now it is "in range" and not 100% all time.the problem now with this speedmeter is "the speed indication!!!) it works up to 50kmh and then locked at 50 even if you go faster... the way it work is with a vol 0-30v yranslates to 0-50kmh .... if you manually add eg 35v it will remain at 50kmh because as i said it is locked ...anyone knows if there would be a trick for unlock this (attached images)....