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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 18114
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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| Posted: Thu Mar 26, 2009 8:55 pm Post subject: Electronics experts: What size fuse to protect this thing? |
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Could have sworn I was being careful, but I managed to smoke this 0-700V AC panel mount meter and similar one beside it: !@(!@#&*!&!!!!!!
Manual: http://www.sure-electronics.net/measure,tools/ME-SP100.pdf
Really simple hookup:
Pin 1: AC Voltage ~
Pin 2: No Connection
Pin 3: AC Voltage ~
Pin 4: Power GND (DC)
Pin 5: Power + 5V (DC)
I tested it separately before installing but SOMEHOW I think I managed to put the AC Hot (120V) on Pin 4 (DC GND) which smoked the thing and a similar one that shared the same DC GND. Both worked fine when I originalled tested them. Popped the EEPROM on the things. Fried them completely. At least the 5V power supply is still ok.
So..... obviously not wanting to do that again (and mistakes/shorts do happen) I figured I should put some sort of fuse in line with AC HOT line going into to Pin 1 or 3.
What size fuse? I'm assuming that the resistance across pins 1 and 3 would be *very* high since this meter isn't meant to handle any current at all. If I measure 1 MegOhm (I'm guessing) can I use Ohm's law here where I = V/R = 120 / 1,000,000 = 0.12 mA? Do they make glass fuses that small? Would a 0.5A or 1.0A glass fuse probably be fine? They seem more common.
Kal
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macgyver655
Joined: 22 Aug 2007
Posts: 8508
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| Posted: Thu Mar 26, 2009 9:01 pm Post subject: |
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So your blowing up sh*t today too.......I'm missing out on all the fun.......
Anyways, fuses are usually just above the maximum draw of the circuit. Of course if its a direct short then almost any rating will blow. If you dont think its drawing more then 0.5 then try it. If it blows for no reason then up it a little. If you dont know the amp or wattage draw then you cant calculate it properly.
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r.bauer
Joined: 08 Apr 2006
Posts: 280
Location: The Netherlands
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| Posted: Thu Mar 26, 2009 9:22 pm Post subject: |
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What is the power consumption of this meter?
Divide the power consumption by 110V and you have the value of the fuse in Ampere.
So if it consumes 5W, the fuse coulde be 5/110=0,045A=45mA. The smalles fuse I have here is 100mA, so that would be just fine.
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Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
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| Posted: Thu Mar 26, 2009 10:16 pm Post subject: |
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You need the small penny nails, not the big railroad tie things.
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Xeus
Joined: 07 Mar 2006
Posts: 111
Location: N.E. Wisconsin
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| Posted: Thu Mar 26, 2009 11:31 pm Post subject: |
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A fuse is intended to prevent fires, I doubt it would blow fast enough to protect the meter against that type of error.
Matt
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macgyver655
Joined: 22 Aug 2007
Posts: 8508
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| Posted: Thu Mar 26, 2009 11:37 pm Post subject: |
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| Xeus wrote: | A fuse is intended to prevent fires, I doubt it would blow fast enough to protect the meter against that type of error.
Matt |
Not necessarily true. Many complex circuits have fuses to protect expensive parts against damage. An ICP is an ic protector and is a fuse. Pico fuses are usually just for circuit damage protection. They dont use enough current to cause heat which is what causes fires. Usually only the main line fuse is for fire protection in which the short can follow the power cord to the wall socket. 
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lostmandan
Joined: 09 Jul 2008
Posts: 146
Location: Kitchener/Waterloo, Ontario, Canada
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| Posted: Fri Mar 27, 2009 1:13 am Post subject: |
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I doubt that fusing will make much difference on the AC inputs. I assume that the AC input is applied across a high resistance (as you mentioned) and there is virtually no current flow.
It probably cooked because one side of the AC line was connected to ground. Depending on the configuration of the 5V supply this could cause all sort of weird things to happen including pulling the ground 120VAC (+5VDC) below the Vcc rail. This would be enough to make any chip pop!
The trick is to make sure it is hooked up properly 
Speaking of hooking things up properly...
I remember a guy in high school electronics class who had a big surprise. We were building linear power supplies and he decided to test the secondary circuitry (rectification and filtering) by connecting it directly to the 120VAC line). BOOOM!! And the smell of electrolytic capacitors filled the air!
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Xeus
Joined: 07 Mar 2006
Posts: 111
Location: N.E. Wisconsin
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| Posted: Fri Mar 27, 2009 1:50 am Post subject: |
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| macgyver655 wrote: | | Xeus wrote: | A fuse is intended to prevent fires, I doubt it would blow fast enough to protect the meter against that type of error.
Matt |
Not necessarily true. Many complex circuits have fuses to protect expensive parts against damage. An ICP is an ic protector and is a fuse. Pico fuses are usually just for circuit damage protection. They dont use enough current to cause heat which is what causes fires. Usually only the main line fuse is for fire protection in which the short can follow the power cord to the wall socket. |
I agree, but I was referring to the connection mistake that Kal made on the meters. In that case I doubt a fuse would have protected the meter.
Matt
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jask
Joined: 17 Mar 2006
Posts: 10187
Location: kamloops BC
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| Posted: Fri Mar 27, 2009 1:59 am Post subject: |
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Even on the DC side a fuse might be too little too late when you feed it line AC.... that really sucks Kal.
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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 18114
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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| Posted: Fri Mar 27, 2009 3:55 am Post subject: |
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| r.bauer wrote: | | What is the power consumption of this meter? |
I don't know... that's the whole problem. It's not rated. If it said what power the 5V DC supply needed to supply that wouldn't help either. What I need to know is what the measurement (AC) side draws. It's an AC voltmeter that's powered by a 5VDC supply.
| lostmandan wrote: | | I doubt that fusing will make much difference on the AC inputs. I assume that the AC input is applied across a high resistance (as you mentioned) and there is virtually no current flow. |
Correct. My understanding as well given the really tiny ribbon cable they give you to connect up to 700V AC!
| lostmandan wrote: | | It probably cooked because one side of the AC line was connected to ground. |
Yup. That's what I had mentioned in my original post. I likely (even though I thought I was careful) somehow shorted the AC hot line to the DC GND line. Which also managed to fry the second meter which shared the same DC GND.
| Quote: | | The trick is to make sure it is hooked up properly |
Very helpful. Thank you.
Actually, after it popped I looked at it very closely and measured again and again with a continuity meter and couldn't see anything I did wrong with my wiring. That's the part that bugs me. But when I took it out of circuit and measured the AC hot pin to the DC GND pin they were shorted together. I'm starting to think there was a lose strand or something that fell in there or something. Hence the reason I thought that maybe I should throw a 100mA or so fuse in line with the AC hot line or something... just in case.
Just not sure if that would actually help anything. Seems the opinions are conflicting.
Kal
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RVonse
Joined: 09 Mar 2006
Posts: 3152
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| Posted: Sun Mar 29, 2009 2:51 am Post subject: |
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The fuse definitely needs to be on the dc ground side of the device and as another poster has mentioned its going to be a very small value. As a starting value you need to know what the current being used in normal operation is. You can determine that value by placing an amp probe in series while it is in operation. Or you may find a current rating published on the data plate of the 5 volt supply that it uses. You want your fuse to just barely exceed that amount of current and you also want it to be a fast blow type.
As already mentioned, even with a fuse you can not always protect everything from anything with a fuse. Which is why well designed televisions and electronic gear with proper fusing and circuit breakers can still get fried during an electrical storm.
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beun
Joined: 28 Jun 2006
Posts: 676
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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 18114
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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| Posted: Wed Apr 08, 2009 5:06 pm Post subject: |
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Got my replacement 0-700V AC panel meter today and I'm looking it over carefully.
Now here's the weird part: pins 3 and 4 are actually shorted together right on the circuit board! That's one AC line to DC GND. Huh??? The pinouts again:
| kal wrote: | Pin 1: AC Voltage ~ (0-700V)
Pin 2: No Connection
Pin 3: AC Voltage ~ (0-700V)
Pin 4: Power GND (DC)
Pin 5: Power + 5V (DC) |
I decided to test it carefully anyway:
I gave it 5VDC to pins 4 and 5 to power it up from my bench power supply. The meter lights up with "000". Ok, so far so good. I'm also sure I'm reading the direction of pins 1->5 correctly as feeding DC power to the two pins at the other end (pins 1 and 2) does nothing. I definitely have the orientation right.
So then I take an old AC stepdown transformer and I feed 20V AC into pins 1 and 3 (the two AC lines) and it lights up with "020". Ok, so it seems to work great.
I then wire it into my panel, feed it from the 5VDC power supply in there and hook up the two AC input lines to the AC HOT (120V) and AC NEUTRAL with 2 low amp fuses in series (just in case). It lights up with "117". I reverse the two AC lines and it comes up with "117" again as it should. So it works just fine. But I could have sworn I did the same thing last time too but I fried it ...
Question:
How could it be normal that pin 3 (DC GND) is internally connected to pin 3 (AC voltage)? That doesn't seem right to me. This thing's supposed to measure any AC voltage from 0 to 700V so my plan is to use it to measure the 240VAC across two AC "hots". (I've only tested with a 120V AC hot and a neutral as mentioned above).
What's going to happen when I go with two 120V AC hots? How can it be normal to feed in 120V AC into something that's directly connected to the DC GND of my power supply?
It's an isolated DC GND, not hooked up to the chassis (AC) GND. I'm assuming if the two GNDs weren't isolated that I'd fry some wires/pop the circuit breaker immediately as that 120V AC would be direct path to GND: AC hot (120V) -> DC GND -> AC GND.
The manual (see link above) however states:
!!! The power supply and the signal being measured can have the same GND or the power converter can be a separate one.
You can tie the GNDs together? How's the possible? It was $10 off Ebay, shipping included - from China. Go figure.
Is this thing safe??
Kal
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WTS
Joined: 08 Mar 2006
Posts: 1276
Location: Calgary
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| Posted: Wed Apr 08, 2009 5:49 pm Post subject: |
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I would separate the 2 gnds and leave the AC inputs floating like they should be. Having a 120VAC or a 240VAC will make no difference to this setup.
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Walter
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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 18114
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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| Posted: Wed Apr 08, 2009 6:35 pm Post subject: |
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| WTS wrote: | | I would separate the 2 gnds and leave the AC inputs floating like they should be. Having a 120VAC or a 240VAC will make no difference to this setup. |
Thanks Walter - so you'd modify the panel meter to separate (isolate) the AC and DC GND? This would leave the AC inputs floating in the panel meter (no longer connected to DC GND) which is probably a good thing given that I use the DC supply for other things in the panel.
But why would they hardwire one of the AC inputs to DC GND in the first place? If I look at their circuit board they've clearly put a copper trace between the two inputs on purpose. I don't understand why they did this. Is there any reason why this would be done that you can think of?
Kal
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WTS
Joined: 08 Mar 2006
Posts: 1276
Location: Calgary
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| Posted: Wed Apr 08, 2009 11:07 pm Post subject: |
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Hi Kal,
I have no idea why they would do that, but I would leave it isolated.
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Walter
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beun
Joined: 28 Jun 2006
Posts: 676
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| Posted: Thu Apr 09, 2009 4:29 am Post subject: |
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My guess is they measure the AC with respect to the DC ground, you may try to separate them but it may not work. When the DC supply is transformer coupled you just lift the ground and if the system is double isolated it is perfectly OK. When your system ground however is connected to the case and the case goes to safety ground you have a 50% change to blow it (when neutral is not hooked up to DC ground (pin3).
Kim
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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 18114
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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| Posted: Mon Apr 13, 2009 1:31 pm Post subject: |
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Thanks guys. After what you wrote Kim I decided to email the manufacturer last week to see what they say:
| Quote: | Pins 3 and 4 are shorted together on your printed circuit board with a copper trace. This sends one of the AC inputs to DC GND. The DC power supply I use to power this panel meter is also used by other devices such as your 3½ Digital Blue LED AC 0~50A Amp Panel Meter + Shunt, so this short causes up to 700V AC to be sent to the other devices. I would prefer not to do that.
Question:
Can I cut the copper trace between pin 3 and pin 4? Will this cause the AC inputs to "float" so that they are completely isolated from the DC GNC and +5V DC power inputs? |
They replied today (key part in bold):
| Quote: | Dear sir,
Thanks for your reply.
Cut connection between pin3 and pin4 to can’t make AC inputs isolated from DC GND.
And it is not suggested to test the AC current and voltage at the same time with the meters powered by the same power supply.
It may lead to burn of your meters.
Our company will develop a new type of panel meter, in which tested signal is isolated from power supply.
Best wishes
Echo |
Ok, so would have been nice for them to mention somewhere in the documentation that whatever power supply you use for their volt meter should *not* be used to power anything else. Sheesh. At least they sent me two new meters for free. I ordered a second power supply (all of $7, S/H included) as I'm hoping that'll work.
So I'll be using two power supplies. One for each meter. It's this one:
(LM317 based, adjustable power supply, 1A max output)
Input: DC or AC 4V-30V
Output: DC 1.5V-27V 1A
Question: I'm using a 10:1 step down AC isolation transformer right now for the one power supply to feed it 12V AC. Should I use a second completely separate isolation transformer for the second power supply? Or can I use the same transformer? On the output side it has a center tap and two 12V taps.
Kal
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beun
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kal
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