Inexpensive voltmeter - Page 4 - Fiberglass RV


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Old 12-13-2012, 05:14 PM   #43
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Is that enough to light the LED display?
If it's multiplexed, in a dark room, barely. More importantly if your meter is drawing that kind of current your readings could be off. I assumed these things were battery driven. Then again if it is just for monitoring the battery and not for measuring other things then not much loading will occur if it is very close to the battery.
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Old 12-13-2012, 05:22 PM   #44
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If it's multiplexed, in a dark room, barely. More importantly if your meter is drawing that kind of current your readings are way off. I assumed these things were battery driven.
These don't have a battery, they just plug into the cigarette lighter socket, so all power to run them comes from the source being measured.
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Old 12-13-2012, 09:25 PM   #45
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I measured 10 milliamperes, but that is using my cheap Chinese Harbor Freight multimeter.
My "hunch" was based on observation. I left one of these things plugged in for a few hours. Everything was turned off except the voltage gauge, but the voltage gradually dropped.... From 12.6V down 12.2V. I figured it was the voltage gauge drawing out power. Perhaps the batteries are just gently aged, only partially charged, and cooling off after a short drive. Perhaps the gauge gets dopey after a few hours. The truck has so many gremlins, as long as it still goes when I fire it up I'm not going to worry too much. Im still curious if any theories?

As for the impact of power draw on the actual voltage reading, cant you just correct for this mathematically? Or is it enough to even bother worrying about it?

Derek
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Old 12-13-2012, 09:43 PM   #46
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My "hunch" was based on observation. I left one of these things plugged in for a few hours. Everything was turned off except the voltage gauge, but the voltage gradually dropped.... From 12.6V down 12.2V. I figured it was the voltage gauge drawing out power. Perhaps the batteries are just gently aged, only partially charged, and cooling off after a short drive. Perhaps the gauge gets dopey after a few hours. The truck has so many gremlins, as long as it still goes when I fire it up I'm not going to worry too much. Im still curious if any theories?

As for the impact of power draw on the actual voltage reading, cant you just correct for this mathematically? Or is it enough to even bother worrying about it?

Derek
You left it plugged into what? A truck?
Assuming truck, the computer in the your truck draws more current than Tom reported on gage and it's on all the time. 10 miliamps isn't enough to worry about. My truck battery drops to about 12.2 after sitting a 2 to 3 days.

Somebody mentioned multiplexing on the display. Yes, it would need to be multiplexed. Otherwise the current required for the display would be just plain too high to practical. Figure it out, each display has 8 segments (7 plus a period) each segment draws around 10 miliamps. All 8' would mean 36 segments plus probably 1 period. results in 3.70 amps. Multiplex is the only way.
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Old 12-14-2012, 08:08 AM   #47
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Ferget the voltmeter (even the $2.50 variety) as a major current draw. There is something called "Surface Charge" that will allow a battery to show a higher voltage right after charging that will quickly drop with even the slighest load an/or time passage. This is more noticed on older batteries that have become sulfated from excessive use.

I can run my TV, DVD player, a hand full of lights, water pump and what ever else I need for two nights and not see than much drop on a fully charged battery

I think it might be time to investigate battery condition as well as your charging system.

All this talk about current draw from the voltmeter is 99% theory and stems back to the days of needing vacuum tube voltmeters to read voltages in electronic circuits. Not really a concern with what RV's are using.

BTW: The voltminders I install are usually hard wired to stay on all the time and are disconnected only when stored for extended periods.
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Old 12-14-2012, 09:55 AM   #48
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Ferget the voltmeter (even the $2.50 variety) as a major current draw. There is something called "Surface Charge" that will allow a battery to show a higher voltage right after charging that will quickly drop with even the slighest load an/or time passage. This is more noticed on older batteries that have become sulfated from excessive use.

BTW: The voltminders I install are usually hard wired to stay on all the time and are disconnected only when stored for extended periods.
Its likely my batteries are getting tired. I should probably investigate further.

For what its worth, voltminders are not currently available (unless you have some old stock hidden away somewhere). Details here:
Voltminder.Com-Buy Now!

Apparently voltminders were but will nolonger be made in china. That discovery should stir the pot a little.

Derek
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Old 12-14-2012, 10:08 AM   #49
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Its likely my batteries are getting tired. I should probably investigate further.

Derek

You probably know this, but you can check the specific gravity of each cell and get an idea if the battery is on its way out. Of course, that would start a new round of arguments over which hydrometer is sufficient.
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Old 12-14-2012, 10:47 AM   #50
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Quote: "For what its worth, voltminders are not currently available (unless you have some old stock hidden away somewhere). Details here:
Voltminder.Com-Buy Now!"

AU Contraire....
They are still available at BatterySpace.Com:
12V DC Volt Minder (71730)
For about $23
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Old 12-14-2012, 11:56 AM   #51
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If it's multiplexed, in a dark room, barely. More importantly if your meter is drawing that kind of current your readings could be off. I assumed these things were battery driven. Then again if it is just for monitoring the battery and not for measuring other things then not much loading will occur if it is very close to the battery.

I thought I remembered LEDs specified at 1mA. So I did a bit of looking and found specifications for 7 segment LED displays that operate at 1mA per segment. The light output is pretty low at that current per the data sheet. However at 5 mA per segment the light output was pretty good. Properly multiplexed the current required for the display could be quite low.

Loading, not likely enough to be detectable. You're dealing with a 20 Amp circuit, and drawing 10 mA. You're dealing with a very low impedance source, the battery near 0 ohms. The meter at 10 mA is approximately 1.2K load impedance. If the battery even had a 100 ohm internal resistance the meter would still be beyond the point of significance. The rule is 10x, in this case the load impedance is at least 12x of the source. If calculate the significance by using current it's even higher. The circuit is designed to handle 20 Amps. 20 Amp vs 10 mA is 200x.
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Old 12-14-2012, 08:36 PM   #52
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I thought I remembered LEDs specified at 1mA. So I did a bit of looking and found specifications for 7 segment LED displays that operate at 1mA per segment. The light output is pretty low at that current per the data sheet. However at 5 mA per segment the light output was pretty good. Properly multiplexed the current required for the display could be quite low.
The only multiplexed LED readouts I am familiar with are digit multiplexed. I have never heard anyone doing segment multiplexing. I would think readability would be an issue. LCD is a much better solution in my opinion. Why they are not doing that here is odd. I also think 10 mA might be a little optimistic but Tom is a nice fellow and I didn't want to hurt his feelings.

Loading, not likely enough to be detectable. I think I said that. For every ohm of resistance between the meter and the battery, the reading is off 10 mV or 1 volt for every 100 ohms. Any meter that effects the measurements it is taking is a poor design in my opinion. But in this case adding a battery would drive the price up so even Tom would have passed and we'd have nothing to talk about.

The meter at 10 mA is approximately 1.2K load impedance. If the battery even had a 100 ohm internal resistance the meter would still be beyond the point of significance. It sounds like you are saying a voltage divider between 100 ohms and 1.2 kohms is insignificant? I get a little over 8%?? That said EE is the sloppiest of the engineering disciplines. We build things with 5 to 500 % parameter variation and not only do they work but they are reproducible. Try building a wall or an engine at + or- 5%.


I would prefer to spend my money on a DVM rather than one of these. A multimeter is much more versatile. But then I like to measure my tires as opposed to a TPMS. Each to his own. Take care, Raz
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Old 12-14-2012, 09:21 PM   #53
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Originally Posted by P. Raz View Post
I thought I remembered LEDs specified at 1mA. So I did a bit of looking and found specifications for 7 segment LED displays that operate at 1mA per segment. The light output is pretty low at that current per the data sheet. However at 5 mA per segment the light output was pretty good. Properly multiplexed the current required for the display could be quite low.
The only multiplexed LED readouts I am familiar with are digit multiplexed. I have never heard anyone doing segment multiplexing. I would think readability would be an issue. LCD is a much better solution in my opinion. Why they are not doing that here is odd. I also think 10 mA might be a little optimistic but Tom is a nice fellow and I didn't want to hurt his feelings.

Loading, not likely enough to be detectable. I think I said that. For every ohm of resistance between the meter and the battery, the reading is off 10 mV or 1 volt for every 100 ohms. Any meter that effects the measurements it is taking is a poor design in my opinion. But in this case adding a battery would drive the price up so even Tom would have passed and we'd have nothing to talk about.

The meter at 10 mA is approximately 1.2K load impedance. If the battery even had a 100 ohm internal resistance the meter would still be beyond the point of significance. It sounds like you are saying a voltage divider between 100 ohms and 1.2 kohms is insignificant? I get a little over 8%?? That said EE is the sloppiest of the engineering disciplines. We build things with 5 to 500 % parameter variation and not only do they work but they are reproducible. Try building a wall or an engine at + or- 5%.


I would prefer to spend my money on a DVM rather than one of these. A multimeter is much more versatile. But then I like to measure my tires as opposed to a TPMS. Each to his own. Take care, Raz

No bodies arguing about using a 3.5 or 4.5 digit DVM vs a cheapo 2.5 digit meter. How accurate do you think you can get with 2.5 digits. That's .1 Volt already. So of course our DVMs are going to be much more accurate. I'm with you on that point. However, how much accuracy do you need. Are building a fence or inner workings of a jet engine.

My point was that loading will NOT have a noticeable effect on the reading.
There's at least one case where in order to conserve heat we multiplexed both segments and digits.

Oh yah! I want to see that wall that all dimensions are within 5% of design.
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Old 12-15-2012, 07:57 AM   #54
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......... but Tom is a nice fellow ..........Raz
While I vigorously deny this, to Derek's question as to whether the LED display on this cheap voltmeter is a significant drain on his camper battery, it seems the answer is "no".
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Old 12-15-2012, 09:30 AM   #55
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While I vigorously deny this, to Derek's question as to whether the LED display on this cheap voltmeter is a significant drain on his camper battery, it seems the answer is "no".
Perhaps "Tom" is... Well... Let's open a debate about multiple online identities, shall we?

If the power consumption is indeed this low, what is the explanation behind the fuse selection? It seems to me that the fuse is rated a little high, considering the power draw. Then again, maybe I'm not reading the imprinting correctly.

This has been a pretty detailed discussion of a $2.50 electronic product. I think about the only thing left is to convince someone to cut theirs open to analyze what is inside. I would cut mine open, but I wouldn't know what I was looking at.

Derek

PS. For the uninitiated, the metal cap on these voltmeters is thredded on. Remove it and a spring plus a glass fuse will fall out.
PPS. No offence Tom. Online, sometimes you have no idea who you are really talking to.
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Old 12-15-2012, 09:51 AM   #56
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.........
If the power consumption is indeed this low, what is the explanation behind the fuse selection? It seems to me that the fuse is rated a little high, considering the power draw. Then again, maybe I'm not reading the imprinting correctly..........
The job of the fuse is to make sure that the thing doesn't burn up if it shorts out somewhere. The fuse chosen probably was based on what what was available to fit their five cent manufacturing cost goal.
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