Author Topic: Function generator output floating 300V!  (Read 6129 times)

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Offline venoftTopic starter

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Function generator output floating 300V!
« on: December 19, 2015, 03:06:32 pm »
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. :-BROKE

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.
« Last Edit: December 19, 2015, 03:22:58 pm by venoft »
 

Online Simon

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Re: Function generator output floating 300V!
« Reply #1 on: December 19, 2015, 03:09:11 pm »
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.
 

Online Mechatrommer

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Re: Function generator output floating 300V!
« Reply #2 on: December 19, 2015, 03:23:34 pm »
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...
Could I replace the 2-prong with a 3-pronged plug and then attach earth ground to the common to force it down?
mod 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...

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. :-BROKE
floating 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...

However my oscilloscope's BNC mantles are also earthed and that doesn't seem to make a difference.
the fg output bnc gnd is probably NC, so you are still seeing floated FG signal output..
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline venoftTopic starter

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Re: Function generator output floating 300V!
« Reply #3 on: December 19, 2015, 03:31:34 pm »
@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.
« Last Edit: December 19, 2015, 03:39:51 pm by venoft »
 

Online Simon

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Re: Function generator output floating 300V!
« Reply #4 on: December 19, 2015, 03:33:41 pm »
sounds like you are outputting sweet "F" all and the scope is picking up static. Try loading the output and see what happens
 

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Re: Function generator output floating 300V!
« Reply #5 on: December 19, 2015, 03:46:42 pm »
Can you directly attach a common to earth?
if 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...
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline venoftTopic starter

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Re: Function generator output floating 300V!
« Reply #6 on: December 19, 2015, 03:49:19 pm »
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.
« Last Edit: December 19, 2015, 03:52:20 pm by venoft »
 

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Re: Function generator output floating 300V!
« Reply #7 on: December 19, 2015, 03:59:33 pm »
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.
 

Offline venoftTopic starter

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Re: Function generator output floating 300V!
« Reply #8 on: December 19, 2015, 04:30:39 pm »
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.
 

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Re: Function generator output floating 300V!
« Reply #9 on: December 19, 2015, 04:34:58 pm »
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.
 

Offline alsetalokin4017

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Re: Function generator output floating 300V!
« Reply #10 on: December 19, 2015, 05:27:34 pm »
Yes, the original problem symptoms didn't make sense, it sounded to me more like a probe issue than a problem with the FG.
The easiest person to fool is yourself. -- Richard Feynman
 

Online Mechatrommer

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Re: Function generator output floating 300V!
« Reply #11 on: December 20, 2015, 05:09:35 am »
now it works beautifully, zero noise
well, thats really something.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline LukeW

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Re: Function generator output floating 300V!
« Reply #12 on: December 20, 2015, 10:42:25 am »
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?
 

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Re: Function generator output floating 300V!
« Reply #13 on: December 20, 2015, 10:47:08 am »
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?

yes 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.
 


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