Re: (ET) DC Shunt Question

["David Roden" <etpost drmm net>]

On 13 May 2017 at 1:38, weinreich frontiernet net wrote:

> He purchased one of those $8.00 China made hard wired
> digital volt meters to install to see how much current draw he has ...

My first thought was that this made no sense.  You bought a voltmeter to 
measure current (amperes)?  

But of course you can and do measure current with a voltmeter.  You pass 
the 
current through a shunt, and the shunt produces a voltage drop you can 
measure.  The typical shunt produces 50 or 100 millivolts (0.05 or 0.1 
volt) 
at its maximum rated current.  I've also seen 60mv and 75mv and there are 
no 
doubt others.  

So you just measure the shunt's voltage drop and mathematically convert 
that 
to amperes.

> the shunt is rated at 6 VDC ...

I'm pretty sure that's not the case.  Shunts are rated as I described 
above. 
The voltage of the circuit you put them in doesn't matter (unless you are 
using the same power source to power a digital voltmeter that reads the 
shunt).

> if placed it between the two battery's you would get readings from only
> one battery 

You want to measure current, yes?  Current is the same everywhere in a 
series circuit.  No worries.

That said, It sounds to me (guessing) like you bought a voltmeter to use 
with a shunt that you already have.  That can work, but you have to more 
or 
less match the meter with the shunt, and as I said above, mathematically 
convert the voltage to current.  

As for the math conversion, in the days of analog meters we just took the 
meter apart and put a piece of paper behind the needle with the current 
readings marked on it.   :-)

Today with everything digital, I dunno.  Maybe you can scale the voltage 
reading to read the actual amperes with some kind of meter tweak, but why 
bother?

If you're going to buy a Chinese Cheapo, why not get what you really need? 
 
There are gobs of digital combination (voltage, current, maybe even amp-
hours or watt-hours) meters all over Ebay.  Just get one that includes the 
shunt and measures in the current and voltage range you need.  

It will come with a wiring diagram.  Follow it.  Treat the two 6v 
batteries 
in series as one 12v battery. It'll work fine.

I have one similar to this: Ebay 111981546136

It was cheap.  It reads instantaneous voltage and current, and also power, 
in case you don't feel like multplying the two together. :-) .  It has 
high 
voltage and low voltage alarms, though they amount to just flashing the 
screen at you.

It even tracks actual energy usage in watt-hours, saving the value when 
it's 
powered down, which is kind of nice.  That's mainly what I got it for. 

As I said, it's cheap.  I won't guarantee it will last more than a couple 
of 
years, or be all that accurate; but for casual use, it's probably OK.  

Oh, one thing more, note that this meter is for indoor use only.  I was 
planning to use it on my ER8-36 equivalent rider until I read that in the 
datasheet. :-(

There are similar combo meters, no doubt from the same Chinese sweatshops, 
that measure just voltage and current.  They're a little cheaper.  Here's 
one: Ebay 331898442021

I mean no endorsement of any vendor or product mentioned above.  This is 
just for your information.  I hope it helps.


David Roden - Akron, Ohio, USA

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = 
Note: mail sent to the "etpost" address will not reach me.  To send 
me a private message, please use the address shown at the bottom
of this page : http://www.evdl.org/help/
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =