EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: lk on April 03, 2012, 08:55:24 am
-
Greetings,
I must have taken some mental damage from the old Scifi movies i have been watching as i feel my self drawn towards analog Ammeters, i have several projects in that draw current in uA ranges, and i found many cheap ones on eBay. But as there are really no datasheets for them, i don't really know how they work, and I'm afraid that i could easily break them if fed too much current, but i don't for a fact know if this would happen. eg feeding 10mA through a 500 uA ammeter.
Then i have been pondering about how to make some sort of auto ranging for use with maybe couple a mA and uA. I have seen from Daves uSupply that the parts exist for doing this digitally, and i presume that this can be done analog as well, but the how is bit unclear.
And i don't know if my babbelings make any sense :)
-lk
-
Yes, you can damage an analog meter by 'pegging' it. I remember one of my classmates doing this to one of the instructor's meters in high school.
-
Analog uA meters are very delicate and putting 10mA through a 500uA one is going to bend the parts out of calibration and probably destroy the coil by overheating.
You can see some typical analog meter specs here:
http://www.simpsonelectric.com/uploads/File/PanelMeterCatalog_Jan09.pdf (http://www.simpsonelectric.com/uploads/File/PanelMeterCatalog_Jan09.pdf)
They can withstand 10x FS current for 1 second, or 1.5x FS continuously.
-
Then i have been pondering about how to make some sort of auto ranging for use with maybe couple a mA and uA. I have seen from Daves uSupply that the parts exist for doing this digitally, and i presume that this can be done analog as well, but the how is bit unclear.
you could use a ammeter that requires an external shunt. each range would require its own shunt to match the scale.
-sj
i have a HP 6200B with a single meter that displays 0-5V, 0-50V, 0-0.18A or 0-1.8A. *see attached schematic.
-
It's not exactly brain surgery!
If the meters are sold as microammeters,you can see what their Full Scale Deflection (FSD) is by connecting them in series with a 33k
resistor & a1.5volt AA cell.(if you are really namby pamby,you can use a 50k pot,& slowly reduce the series resistance.)
With 30k in series:
If they are 50uA FSD,they will show close to FSD
100uA FSD will be about half of FSD.
25uA FSD,you will "twang" the needle,but it shouldn't do any real harm.
500uA & 1mA FSD meters will show very low readings.
High current meters with built in shunts will show a zero reading.
Obviously,the meter coil has some resistance,but you can ignore it compared to the series resistor.
Once you know their FSD,you can then find out how to make & use shunts to increase their ranges.
-
It's not exactly brain surgery!
If the meters are sold as microammeters,you can see what their Full Scale Deflection (FSD) is by connecting them in series with a 33k
resistor & a1.5volt AA cell.(if you are really namby pamby,you can use a 50k pot,& slowly reduce the series resistance.)
With 30k in series:
If they are 50uA FSD,they will show close to FSD
100uA FSD will be about half of FSD.
25uA FSD,you will "twang" the needle,but it shouldn't do any real harm.
500uA & 1mA FSD meters will show very low readings.
High current meters with built in shunts will show a zero reading.
Obviously,the meter coil has some resistance,but you can ignore it compared to the series resistor.
Once you know their FSD,you can then find out how to make & use shunts to increase their ranges.
...and that will quite easily kill a 5uA FSD meter, the most sensitive common type available. A 300k resistor is enough to get one to FSD in series with 1.5v.
-
It's not exactly brain surgery!
If the meters are sold as microammeters,you can see what their Full Scale Deflection (FSD) is by connecting them in series with a 33k
resistor & a1.5volt AA cell.(if you are really namby pamby,you can use a 50k pot,& slowly reduce the series resistance.)
With 30k in series:
If they are 50uA FSD,they will show close to FSD
100uA FSD will be about half of FSD.
25uA FSD,you will "twang" the needle,but it shouldn't do any real harm.
500uA & 1mA FSD meters will show very low readings.
High current meters with built in shunts will show a zero reading.
Obviously,the meter coil has some resistance,but you can ignore it compared to the series resistor.
Once you know their FSD,you can then find out how to make & use shunts to increase their ranges.
...and that will quite easily kill a 5uA FSD meter, the most sensitive common type available. A 300k resistor is enough to get one to FSD in series with 1.5v.
I would dispute how common 5uA meters are--I haven't seen one for sale in years.
But--Good point!
That would be the best first step.
-
Thanks for the all the replies especially the one from you Vk6zgo,
If will order my self a couple of those and see what i cant wrest from them using your hints. And i guess if i manage to break them, I'm an experience richer.
I will resurrect this thread when i have my meters
-lk
-
Remember amyk's comment& start out with a 330k ,just in case!
-
5uA definitely isn't very common but they do show up from time to time.
Here's one that sold recently:
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=320857214190 (http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=320857214190)
-
I'd advise looking for the old Simpson 373 mA and 374 uA meters to get a bit more versatility for your buck.
These small meters are great for patching into your circuits here and there for real time monitoring purposes whilst you mess about with your designs. They have a small footprint, are rugged and accurate, and have multiple ranges, plus a "short" position, so you can remove the meter from your circuit without having to...remove it.
If you are patient you can get these for under $20 in near mint condition on Ebay. I've never paid more than $12 for any I have picked up, though...and you just have to ignore the sellers that think these are priceless collectibles. Simpson made a whole line of these small meters but I find the 373/374 models the most useful. The 374 uA model is trickier to find.