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Please check my math.

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SirSparks

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It is many years since I have performed these calcs. I want to use a 0 – 100 amp scale meter as a 0 -100% PV insolation meter. The reason is that I cannot normally measure insolation as a simple factor of total current out from my PV array as the charge controller is constantly changing that value to suit battery charging needs. However I have another very small array of 11.25 amps max which is directly connected to the battery bank so as not to increase the already maxed out amps of my controller. The beauty of this is that it appears to be always a direct ratio of amps out to % insolation (irrespective of main controller output) with a max ever noted of 11.25A (presumably 100% insolation) So I need a shunt for this meter that will give 100% FSD at 11.25 amps, the meter is a 75 mV coil.

So my very rusty math tells me; I need a shunt which is 0.075/11.25 or 0.00666 ohms.

Am I right or did I screw up?:)
 
Re-thinking this I don't think I will be able to get a resistance wire to make .00666 ohms it will be way to short! Another thought is;

I get a 0 - 1 amp meter with built in shunt, measure the Resistance of it and then add a shunt of 11.25 - 1 or 1/10.25 of the meter Resistance.

Am I getting whacky here? I think this resistance will be larger and thus easier to do.
 
So my very rusty math tells me; I need a shunt which is 0.075/11.25 or 0.00666 ohms.
Am I right or did I screw up?
Your calculation is quite correct.

Am I getting whacky here? I think this resistance will be larger and thus easier to do.
Yes you are getting whacky.
You will just end up with another very small value resistor.

Have a look at my attached picture, you will see that the ammeter shunt is basically a very short piece of wire between two large brass blocks.

The large bolts are for attaching the main current carrying wires, the screws are for the connection to the meter.

The protective plastic cover which has been removed from over the wire, shows that the shunt will produce 100mV at 10Amps.

If you look carefully at the wire, you will see that the shunt was calibrated by filing the wire to make it thinner.

If you make something like this, you shoud be able to use your meter without any problems.

JimB
 
JimB, I love the idea of filing to tune thanks. Whacky is OK I guess as long as I realize it.
actually have one of these blocks but it is 200amp my arm might get tired filing LOL! So I get a 10 amp 75mv shunt and file away about 10% of the metal?
 
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Shoot! I am confused; Or does it need to be 15amp with some filed away? My mind is boggling!
 
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Ease your boggling mind there SirSparks!

Removing metal from the shunt will increase the resistance and hence make the shunt more sensitive.

So, to modify a shunt by removing metal, you would have to start with a shunt of higher current rating.
As you suggest a 15amp shunt could make a good starting point.

JimB
 
Thanks, on a side note I have not had real Scrumpy Cider for 30 years! I just figured out how to simulate it; Publix (chain store) non alcohol cider (Thick!) with a good splash of vodka! I shall celibate my new found Insolation meter tonight! Shame I don't have McKewans too.
 
I have two alternative suggestions for an 0.0066666 Ω current sense resistor.

You could use three of these 0.02 Ω current sense resistors in parallel. As Jim points out, you need a big bit of copper or brass to connect them to, so the there is little effect from any additional connection resistance.

Or you could use one of these: 0.01 Ω 4-terminal current sense resistor. That is a 4-terminal resistor, so like the shunt pictured in this thread, it has current terminals and sense terminals. Now i realise that you can't get 0.006666 Ω, but that can be worked round. 0.1 Ω is 1.5 times too large, so if you increase the resistance of your meter by 50%, it will work fine. If you measure the resistance of the meter, or it might be written on it, and just find a resistor of half that value to put in series with the meter.

(There is a 4-terminal resistor in that range that is 0.008 Ω, which would be closer to what you want. You would need a resistor that is 20% of the resistance of the meter. However, 0.01 Ω is a lot more common, and Newark have that in stock)
 
Divere300.
I like those solutions too. Let me say that at present, simply because it was so cheap, I ordered a 10 amp meter with shunt from China with airmail for $5 total. I have no idea about the shunt because it doesn't show a picture of it BUT perhaps it is something I could modify, if not I could just throw it away or maybe try to mess with the moving coil. I don't need terrible accuracy for a sun insolation indicator maybe even set mid scale 5 amps as true with the movement spring adjuster. and accept the 5% inaccuracy at both ends.

I'll let you know how it finally turns out. :D
 
You could use the adjustment on the 10 A meter that I suggested for the 0.01 Ω 4-terminal shunt. Disconnect the moving coil meter from the shunt, measure the coil resistance and add 1/8th of that in series.
 
The trick is measuring the resistance. It has to be measured indirectly with a voltage source and variable resistor. Adjust for 1/2 scale. Measure the resistance of the resistor. That will be the resistance of the meter.
 
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