Converting high voltage to 5v range - Page 1
Forum section: electronics / beginners

author: beergut
author_rank: Newbie
timestamp: July 30, 2023, 06:10:14 am
content: G'day all,I purchased a cheap portable oscilloscope to work on my car.I didnt realise but it has a maximum voltage of +5V to -5V.Its too late to return it so would like to get some more use out of it.This post also relates to some arduino projects I have in mind.Passing a know voltage range thru a series resistor would probably be good enough for what I need.How do I calculate the resistance needed without knowing current?Can I just connect the oscilloscope to a 5V source, measure current thru the probes and use that in calculations?I realise accuracy will suffer but I need to Know change in voltage for most things, accuracy of measured voltage is not a big issue, I can always double up with my multimeter.Before the obvious suggestion comes in, I cant afford another oscilloscope and have more time on my hands than money.Thanks to anyone that can help.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2023, 06:30:06 am
content: Get a set of 10x probes, recommended for the scope.Tim

author: Psi
author_rank: Super Contributor
timestamp: July 30, 2023, 06:59:30 am
content: Check the menu for the scope channels, it should have an option for 10x probes.That way the voltages will show correctly on the screen even though its only 10% of the voltage at the scope input.If there's no option for 10x probes in the menus you can still use them but the scope will show 1/10 the voltage. eg show as 1.2V on screen when you touch the tip of the probe to 12V etc..In fact, it's even possible you have 10x probes and don't even know it.See if there is anything written on the probes themselves, like 1x 10x 100x

author: Jwillis
author_rank: Super Contributor
timestamp: July 30, 2023, 02:40:27 pm
content: I'm curious as to what oscilloscope you have with such a low input voltage. Even the cheapest DSO 138 DIY Oscilloscope is 50v peak to peak.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 30, 2023, 04:19:26 pm
content: I'd suggest a fixed (no 1x switch to bump) 10x and a fixed 100x probe, the cheap ones tend to work fine. That fixed attenuation (a 9 or 99 megaohm resistor) will also provide some extra protection to the scope when poking around in a car.

author: robowaffe
author_rank: Regular Contributor
timestamp: July 30, 2023, 04:51:01 pm
content: Its easy to go down voltage, just going up voltage is impossible without an amplification.U just need resistors to do it, I can make a funny switch that involves adding a low voltage power source to it and it converts the high voltage inverts it and low voltage comes out.But the more usual way is to just use a voltage divider.

author: beergut
author_rank: Newbie
timestamp: August 30, 2023, 10:13:32 am
content: Thanks for the replies every one.Didnt know there was such a thing as 10x or 100x probes, will look into getting some for the future.In the mean time I bought a couple of Arduino voltage dividers off ebay for next to nothing. Take up to 25v and good enough for working on the car sensors.The probes will be better for control board work and such with appliances.Oscilliscope is a Digitech from Jaycar in Australia. Cheapest I could get in stock to get the car diagnosed quickly. Very basic USB unit, no screen or controls, software for laptop very limited. Wouldnt recommend it.Unfortunately finances are very tight, Im forced to be a cheap ass.

author: Infraviolet
author_rank: Super Contributor
timestamp: August 30, 2023, 06:03:13 pm
content: You don't need to buy something special for a voltage divider, just have two resistors in an appropriate ratio. Top end of the larger resistor goes to the thing being measured, bottom end of larger resistor and top end of smaller goes to your probe's input, bottom end of smaller goes to a commong ground shared by the scope and the thing being measured. V_out=V_in * (R_lower / (R_upper + R _lower) )

author: Jwillis
author_rank: Super Contributor
timestamp: August 31, 2023, 02:19:38 am
content: Quote from: beergut on August 30, 2023, 10:13:32 amDigitech QC-1929 is a Hantek 6022BE. Peak voltage protection is 35Vpk(Without external attenuation DC + peak AC < 10kHz). That being said you will still need a set of 10X probes. That should get the gain range to 200mV～50V/div@x10 probe.I found the software tends to crash a lot. There are some independent applications that are more stable.Just for reference the QC1938 is a Hantek DSO2d10