Using a switch with a mounted digital meter

[drzep]

I'd like to install a Datal Digital Meter on my RT. However, I don't want to look at the display 24/7 so i want to add a low resistance switch to the circuit to turn it on and off.

 

The switch (from action electronics) has a resistance of 4 ohms and the meter draws ~15 milliamps. This seems that it would create a voltage drop of 0.05 V at the meter.

 

Question is this: Is there something I could add to the circuit to correct for the delta-V induced by the switch? Thanks!

[NoHeat]

There's no component you can insert that will add voltage to replace voltage lost across a resistance, except another battery, and that would be a pain.

 

Since the meter displays voltage with only one digit to the right of the decimal, the 60 millivolt drop shouldn't make much difference. But if this still bothers you then the only solutions are a lower-resistance switch or a meter that draws less current.

[ShovelStrokeEd]

Zep,

Why should it matter? Exact battery voltage changes by more than that all the time as the voltage regulator does its thing with the bike running. When the bike is off, all that matters is >12.5 volts or so. Put a piece of tape over the 2nd digit.

[drzep]

Point taken - makes sense that I can't 'create' more energy without another source . I should of thought about that a bit more carefully.

 

Thanks for the input John and Ed.

[Ken H.]

Suggestion - Get the LCD one instead of the LED one. Then wire it so it is hot/on all the time. The LCD uses so little current the drain is not an issue, even over extended periods of time of bike inactivity, and it is an easy way to see your battery condition anytime you walk by the bike when it hasn't been used for awhile. Battery showing less than 12.5v or so? Time to put the charger on it.

[GoGo Gadget]

Suggestion - Get the LCD one instead of the LED one. Then wire it so it is hot/on all the time. The LCD uses so little current the drain is not an issue, even over extended periods of time of bike inactivity, and it is an easy way to see your battery condition anytime you walk by the bike when it hasn't been used for awhile. Battery showing less than 12.5v or so? Time to put the charger on it.

 

I don't see an LCD one listed there, or are you talking about another brand?

[Ken H.]

Suggestion - Get the LCD one instead of the LED one. Then wire it so it is hot/on all the time. The LCD uses so little current the drain is not an issue, even over extended periods of time of bike inactivity, and it is an easy way to see your battery condition anytime you walk by the bike when it hasn't been used for awhile. Battery showing less than 12.5v or so? Time to put the charger on it.

 

I don't see an LCD one listed there, or are you talking about another brand?

Sorry, it's not on that site, but available from other places. Here's one - http://www.metercenter.com/cgi-bin/websh...link--da144.htm

[ElevenFifty]

The LCD one is NOT waterproof ... my display is missing a few segments as a result of water intrusion.

[RFW]

Absolutely ANY switch will do just fine, provided that if you are using an LCD meter, the switch has gold plated contacts. This is obvious when you consider Ohm's law....

 

A typical LCD digital voltmeter has a current draw of a milliamp or less. Even a really crappy switch has a resistance FAR less than 1 Ohm. But lets say you have the worst switch ever made, and it has a resistance of 1 Ohm.

 

A 1 milliamp current flowing through a 1 Ohm switch resistance, results in a voltage drop of only 1 MILLIVOLT!! Obviously an error this small it totally insignificant ...and may not even be detected by a cheap digital panel meter anyway. If you are using an LED meter, its current is higher, but the voltage drop across any switch you could ever use is still too insignificant to matter.

 

And all this assumes a switch that has 1 Ohm of resistance. REAL switches have resistances of a tiny fraction of an Ohm.

 

Now about the gold plating. When the current being switched is very small (as it is in this case), you need to use a switch with gold plated contacts. If you are switching more than 10 to 20 milliamps (which is not the case with an LCD meter), then the small arc created when switching, acts to blast away any oxydation on the switch contacts and a good solid low resistance connection is made.

 

But when switching such low current that LCD meters draw, there is no tiny arc to "clean" the oxide off the contacts, so it is necessary to have switch contacts that do not oxydize in the first place (hence the need for gold plating). Electrical engineers refer to such a low-current connection as a "dry contact", and switches rated for "dry contact" switching are needed.

 

Failure to use gold switch contacts if you use an LCD meter, will result in erronious readings over time.

 

But if you are using an LCD meter, is is clear that the current is so low, adding a switch is pointless.

 

By the way, I attached a perminent LCD voltameter on my bike, and there is no switch. It has been on the bike for some years and the battery has no idea it is there ...its current is just too small.

 

The only precaution I took was to make a waterproof plastic housing.

[drzep]

Excellent. I just measured the resistance across the switch and it is indeed 0.04 ohm, not 4 ohm as I initially wrote. No excuse...

 

I'll go with the LCD as you and Ken suggested and use the gasket-lined bezel along with some type of water proof cover. Probably wont use a switch after all.

 

Thanks for the input.

[Ken H.]

The LCD one is NOT waterproof ... my display is missing a few segments as a result of water intrusion.

Good point I forgot to mention. I ran a tiny amount of clear silicon around the glass to bezel edge of mine and it has been fine.