Basic Oscilloscope Operation Question - Page 1
Forum section: electronics / beginners

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 01:20:03 am
content: Yes this is an amateur question-I have a Tek 465M. I wanted to view the output of my bridge rectifier circuit. I had it set on 50ms/Div and 20V/Div, but I get a flat line.Meanwhile, if I hook it up to my function generator with the probe, all looks good.I hooked up the ground lead to - side of the rectifier and the probe to the + side.My DMM does show that the bridge is working.Am I doing something wrong? I tried to put a 200ohm resistor on the output to act as a load. But I still get a relatively flat lineTried changing coupling, trigger level etc.Is there a reason why I get no signal on the CRT?Thanks.

author: joelby
author_rank: Frequent Contributor
timestamp: February 06, 2012, 01:27:53 am
content: Do you have ripple filtering capacitors? At 20 V/div, you probably won't see much ripple, so the output voltage will be DC, which will be a flat line. However the flat line shouldn't be at ground unless you have AC coupling set.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2012, 01:28:02 am
content: What's the voltage of the flat line?

author: BravoV
author_rank: Super Contributor
timestamp: February 06, 2012, 01:32:21 am
content: Make sure you've set the scope input coupling at AC and slowly adjust the vertical resolution.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 01:34:25 am
content: Quote from: IanB on February 06, 2012, 01:28:02 am0, Grounded

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 01:34:58 am
content: Quote from: joelby on February 06, 2012, 01:27:53 amIt's straight out of the bridge, no caps.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 01:35:40 am
content: So AC or DC coupling, though I've tried both.

author: joelby
author_rank: Frequent Contributor
timestamp: February 06, 2012, 01:36:08 am
content: Try putting the probe across a battery in DC coupling mode - you should read 1.5 V or whatever.

author: joelby
author_rank: Frequent Contributor
timestamp: February 06, 2012, 02:00:41 am
content: Ah, another thing you should check is that your probe attenuation matches that of the scope. If your scope is set to 1x and your probe is 100x, you won't see anything at 20 V/div.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2012, 02:01:45 am
content: Let's check a few basic things. Scope set to DC coupling. Scope ground set to rectifier negative output terminal. Scope test point on positive terminal. A DC voltmeter connected to the same two points should read a DC voltage. The scope should be showing a series of positive half-cycles. Make sure you have your 200 ohm load or whatever across the + and - terminals of the rectifier. Make sure no capacitors are in the circuit.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 02:02:04 am
content: So taking Joels advice, I checked a 9V battery. I took off the adapter that gives you a spring hook. Showed up as 9V. Tried the same thing with Channel 2, except I didn't take off the spring hook adapter. I didn't do this intentionally.And what do you know. There's no continuity between the spring hook and the actual probe...no wonder I was reading ground.Next question is...Why do I have this waveform? This was with no load! Perhaps that's the problem?EDIT: Ian, I guess I need the load then.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2012, 02:03:45 am
content: Quote from: FenderBender on February 06, 2012, 02:02:04 amDo you have a load resistor on the rectifier? It will do funny things like that with no current flowing. Diodes need current to switch on and off.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 02:08:10 am
content: Quote from: joelby on February 06, 2012, 02:00:41 amMy scope conveniently has two "windows" on the knob. One for 1x and one for 10x so I think I had that down.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 02:09:32 am
content: Quote from: IanB on February 06, 2012, 02:03:45 amNo there was no load. I forgot about that property.Ok good point. Damn- Either I'm stupid or whoever made made my probes should fall off a cliff. Almost gave myself a heart attack for multiple reasons there...Thank you.

author: w2aew
author_rank: Super Contributor
timestamp: February 06, 2012, 04:31:56 am
content: You may want to review my AC/DC Coupling controls video, and the 1X/10x probe video:

author: Psi
author_rank: Super Contributor
timestamp: February 06, 2012, 04:43:44 am
content: Quote from: FenderBender on February 06, 2012, 02:02:04 amThe plastic hook attachments sometimes need to be pushed on until you hear a "click" before they will make contact inside.Or, if it still doesn't work, it might be the wrong hook (maybe from a different set of probes).There is another attachment that comes with oscilloscope probes which looks like a bare metal spring with one end bent out.That's for getting a ground connection close to your test point. So if you were trying to use that to probe stuff that's why it didn't work. Its connected to ground rather than the oscilloscope input. Unless you're doing high frequency or high sensitivity work you don't need this attachment on your probe. Most people leave them in the probe bag and never use them.

author: FenderBender
author_rank: Super Contributor
timestamp: February 06, 2012, 05:36:52 am
content: Yeah I've watched your vids like 20kadrillion times. Excellent stuff. I guess once more doesn't matter.But the source of my problems was just from a faulty probe..or lack of connection between spring hook and probe.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 06, 2012, 07:13:41 am
content: This is fairly common with the smaller,more modern probes--the old "clunky" ones didn't seem to do it as much.I've found that some of these hook ends lose tension,& hence go open circuit,or as Psi said,it might be from a different set of probes.If you get funny effects like this,ditch the "hook" & probe directly with the pinpoint end of the probe as a check.After a while,checking for these effects becomes second nature,but you'll still get caught,sometimes.

author: w2aew
author_rank: Super Contributor
timestamp: February 06, 2012, 03:24:22 pm
content: Quote from: FenderBender on February 06, 2012, 05:36:52 amOK - I'm glad you found the problem. I guess I would've known that if I read all of the posts before posting my reply! Doh!I've always adopted the practice of quickly touching the probe to the Compensation signal before I use it on my circuit. Serves the purpose of ensuring the probe is working, and double-checking the compensation adjustment.