Function generator output floating 300V! - Page 1
Forum section: electronics / beginners

author: venoft
author_rank: Newbie
timestamp: December 19, 2015, 03:06:32 pm
content: I have bought an function generator off ebay (FY3224S). But when I measure the output voltage at the BNC connector, my oscilloscope says the output is a (slightly distorted) 50Hz sine of almost 300v!? Almost gave a a heart attack when I thought of my poor scope.I should mention that it's powered by a 2-prong mains cable, and it has it's own internal power supply. I measured the internal commons, and they are all interconnected, at least the DC part is.The internal power supply circuit has a little clip attached to the common that looks like it's supposed to be connected to earth ground, but since it's powered by 2 prongs there is no ground. Google tells me that function generators are usually floated, but in this case 300V seems a little excessive. I'm afraid I'll fry every circuit I'll attach it to.Could I replace the 2-prong with a 3-pronged plug and then attach earth ground to the common to force it down? However my oscilloscope's BNC mantles are also earthed and that doesn't seem to make a difference.I hope you guys can tell me what to do.

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 03:09:11 pm
content: As you are connecting it to an earthed oscilloscope it is earthed. Just a thought, check your probe setting and you scope setting match, it's not like you have the scope set for a x10 probe and the probe set at x1, this will make the scope read everything x10 so it would be 30V not 300V.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 19, 2015, 03:23:34 pm
content: Quote from: venoft on December 19, 2015, 03:06:32 pmmod the cable to use the 3 prongs, connect earth to the clip, if its functioning, then its ok, if elcb is tripped then there is design flaw in the fg psu...Quote from: venoft on December 19, 2015, 03:06:32 pmfloating voltage is close to nothing maybe due to capacitive coupling in the transformer or somewhere else, depending on how much energy in the floating system, probably you'll just get a little trickle by touching it before everything level out, same thing with floated or grounded DUT...Quote from: venoft on December 19, 2015, 03:06:32 pmthe fg output bnc gnd is probably NC, so you are still seeing floated FG signal output..

author: venoft
author_rank: Newbie
timestamp: December 19, 2015, 03:31:34 pm
content: @simon:No that was the first thing I checked. And besides, my scope doesn't go higher than 10v at 1x.I forgot to say the sine was 50Hz (I'm in Europe).To be sure I measured the output with my (600V max) multimeter and it gave a AC voltage of around 13v, and it didn't change when I changed to output voltage to 0v.Edit: so this probably confirms Simons suspicion in the next comment about loading the function gen. So that's the next thing I'll try.The function gen also has an input port with freq counter. When I directly connect the output to input it gives the right freq back.@Mechatrommer:Can you directly attach a common to earth? I my mind this causes the voltage potential to be shorted which causes a current drain right? But I'll try it and hope my lights stay on.

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 03:33:41 pm
content: sounds like you are outputting sweet "F" all and the scope is picking up static. Try loading the output and see what happens

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 19, 2015, 03:46:42 pm
content: Quote from: venoft on December 19, 2015, 03:31:34 pmif the common is isolated from the mains neutral... yes. btw, as suspected, your device is floated and maybe just pick up normal mains potential (inteference) through the air, thats why your are seeing 50Hz wave...

author: venoft
author_rank: Newbie
timestamp: December 19, 2015, 03:49:19 pm
content: Ok so I tried loading the output of the fg. First with 4k7, that got it down to 120v, then I added 1k (so, about 800ohm total) and that got it down to 36V. Now when I change the frequency on the fg I actually see some change (with a 36V 50Hz mixed in). So that's part of the issue fixed. Thanks!I get that a floating fg is handy because you can easily add a DC offset, but how do high end function generators tackle this problem? Or do you always have to load them (with 50ohm is my guess)?Edit: I just measured it and yes, common is isolated from mains neutral.

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 03:59:33 pm
content: sounds like a duff one to me, 120V on 4K7 is 3W, what power was the resistor and how long was it on for ?, if you used a regular 0.25W resistor it would have gone smoky pretty quick.

author: venoft
author_rank: Newbie
timestamp: December 19, 2015, 04:30:39 pm
content: I added a 3-pronged connector and now it works beautifully, zero noise (well, nothing more than I expected). Thank you all so much.@simon: the resistors didn't get hot at all. I admit not having thought about that potential disaster when I attached them, but I didn't even notice them getting lukewarm. Is it possible that the coupled mains hum couldn't deliver that much current? Although in that case I'm baffled why it stayed at 120v.I just repeated the experiment (without earth ground) and now the I only got about 10v with 4k7. So I don't know what changed, maybe I did have a wrong probe setting and it was actually 12v. But it's all fixed now, thank you all for your help.

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 04:34:58 pm
content: If you still had that voltage with that resistor then 3W should have been smoking it's way out. I think you had something wrong with your probe setting, it's easily knocked. I've had to double check often. The mains hum was the more worrying bit, a function generator should only generate the function asked of it.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: December 19, 2015, 05:27:34 pm
content: Yes, the original problem symptoms didn't make sense, it sounded to me more like a probe issue than a problem with the FG.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 20, 2015, 05:09:35 am
content: Quote from: venoft on December 19, 2015, 04:30:39 pmwell, thats really something.

author: LukeW
author_rank: Frequent Contributor
timestamp: December 20, 2015, 10:42:25 am
content: Keep in mind that it's easy to pick up some 50Hz hum just about anywhere. Try simply touching your oscilloscope probe with your finger and see what you get. How does that compare with what you're measuring?

author: Simon
author_rank: Unknown
timestamp: December 20, 2015, 10:47:08 am
content: Quote from: LukeW on December 20, 2015, 10:42:25 amyes but as soon as you load it that will not happen. Most function generators have an output impedence that will "load away" any pickup. the fact that he has 300V sine waves at 50Hz suggests he was not actually connected to the functiona generator.