Panel meter and current shunt

[(unknown)]

Hi folks,

I'm building a box to house a voltage and current meter for the output of a battery bank. So I bought a combined voltage and current panel meter and a 75mV current shunt. eBay of course.

I was quite surprised by the suggested wiring plan for the supplied current shunt. To my mind the shunt should be in series with the load (and probably in the negative line of the load). But the diagram seems to be showing the load's positive supply line being shorted through the dump straight to the load's negative supply line.

I may have misinterpreted the battery shown in the diagram in that I think it is not the battery bank but a standalone battery for driving the meter. I think this must be the case because the meter can monitor up to 50V but must be driven by a standalone battery that provides no more than 24V.

So, based on that, what really appears to be missing from the diagram is any indication of where you are supposed to wire in the up-to-50V supply voltage that the panel's voltmeter is able to monitor.

What do you think?

Thanks!

LeeK

[Madscientist267]

LeeK -

I believe you are correct, something's not right - the shunt shouldn't be appearing across the load. One of the leads (maybe green?) it would seem, has to be a voltage sense line. It's almost as if the grey between the shunt and the negative of the load/measurement shouldn't be there, but would lead to the positive side of the bank instead, with the green sensing the negative potential at the load... ? I think it's all there, its just not indicated correctly in the diagram.

I believe you are correct however on your assumption that the 5-15V battery in the diagram is to power the meter itself. Look at the TO-220 case just to the right of the red wire in the picture. If that says 7805 (or something very similar), it is a voltage regulator. The 78xx series I believe to be good up to 30V input, which is why they are calling it 24 - There's room for charging/equalizing a 24V battery bank to drive the meter directly from it.

Don't hold me to any of this, but that's my gut feeling on it. Hopefully this helps? Best I can do without any specs on the meter itself...

Steve

[scoraigwind]

I have altered the diagram to how I think it must have meant to be.  I tried to upload it to my user files but it seems to have ended up here instead and I do not know how to insert it in this message.

http://www.otherpower.com/images/scimages/212/shunt_sketch.jpg

This might be right.  or wrong ...  :-(

[(unknown)]

Thanks guys for that fast, considered feedback and Hugh for taking the time to redraw it.

Madscientist, you are correct - it is a 7805 that is sitting to the right of the connection header.

I had emailed another meter vendor to ask if he had a wiring diagram for his very similar-looking meter and he replied this morning. Of course, I'm not certain it is the same meter. Going by the images, there's a tiny difference in resistor layout on the rear PCB but in all other respects the component type and position appears identical right down to the manufacturer and model.

They showed a single battery diagram and a separate two-battery diagram in what was a much more fully fleshed out technical spec. Here's how their two-battery system looked:

I think they have probably supplied the answer. If it's correct - I'll give it a try this week - then I'm inclined to buy any more of these from them regardless of the slightly higher price simply on grounds quality tech spec and their willingness to supply it up-front.

LeeK

[Madscientist267]

Its hard to say who is right; Hugh or the alternate from the other vendor... One has the shunt on the positive side, the other on the negative.

I think I would start out with an experimental layout to figure it out for sure before I hooked it up to the intended loads.

Use a 9V battery to power the meter logic, and then use high value resistors (say 100k?) to probe the other connections, sourcing from the 9V. You'll find out which ones do what by how the meter reacts, without risking doing any damage.

I'd imagine you can tie the two negatives together (if they aren't already this way on the meter's board), and then probe the other two connections. One will make the volt meter respond, the other will go to the ammeter.

Do the probing without the shunt; you won't likely see any reaction from the ammeter if it's there since only about 90 uA will be flowing. You'll also be able to confirm polarity across the shunt this way, and determine which diagram is right.

One thing for sure, the original is dead off...

Steve

[Madscientist267]

Addendum -

After really looking close at the vendor diagram, I think it is probably the correct version.

The meter would be getting it's negative reference for both meters directly from (more or less) the negative bank terminal, with the only error being in the drop between the actual terminal and the negative side of the shunt.

The only thing left for you to decide with this configuration is whether you want to:

A - Monitor the terminal voltage as seen at the battery (connect "5. +Voltmeter" directly to the battery terminal)

or

B - Monitor the supply voltage that the load sees (connect "5. +Voltmeter" as close to the load's supply terminal as possible).

Steve

[Opera House]

Does this meter actually read both current and voltage at the same time?  If not, you would either connect 4 or 5 only.

[(unknown)]

The meter does read both voltage and current at the same time. You press a button on its front panel to change which one its display is actually showing.

I am setting up a test and appreciate the advice to use 100k resistors for protection.

I've been thinking about that 'measure battery voltage' or 'measure load voltage' question. That kind of choice always prompts me to chuck in another switch to allow a real-time choice.

I really appreciate all the advice and the thinking points this has given me.

I'll report back on how the testing goes!