Free Amps...

[R_W_B]

Wow I just went there and bought this thing with free shipping for $4.50. Amazing, just don't usually see stuff like this. It ain't gonna be "Free Amps" but for $4.50 I ain't complaining.

Gives the mVolts right on the nomemclature and it's good for 50amp circuits.
Pretty good deal.

 

[R_W_B]

Jim (Hoople) I sent you email with my latest study of trying to get under what you've been trying to show me. I think I'm a little closer to getting what you said now. I was confusing the way line current is added into the circuit from a 2nd source as opposed to another load.

Anyhoo, I want to ask this question. I've already spent my $4.50 on my prospective amp reading shunt resister (which is a steal in any case at fee shipping) but I need your input on this? The one I bought is a 50 amp shunt. Will that be enough to withstand the heat of the starter cranking over ? Do I need to buy one with a higher amperage (for car or bike).

 

[Hoople]

The one I bought is a 50 amp shunt. Will that be enough to withstand the heat of the starter cranking over ? Do I need to buy one with a higher amperage (for car or bike).

It all depends on what your trying to measure with the shunt and where you insert it in the circuit. If your measuring starter current draw, the shunt is not large enough. But if your wanting to measure the total running current of the bike or the output of the charging system you will be fine with a 50 amp shunt. That is because the starter motor current will not be going through it.

Does the starter motor current go through the Maxi fuse? No it does not. So if you insert the shunt at the Maxi fuse (or at the voltage regulator output), will the starter motor current go through the shunt? Once again, no it will not.

I am starting to understand the way you think. Let me guess. Your planning on inserting this shunt directly at the battery terminal aren't you? No, that will not work. Besides the reading will be meaningless. Your going to be reading the differential amount current between what is required by the bike, minus the charging current. A somewhat useless number.

That value shunt is perfect for measuring charging current or total bike current draw. Also since it is a 50mv shunt the ratio is easy to remember.
1mv=1 amp. If you read 22mv your know it is 22 amps.

 

[R_W_B]

Hoople ->{{If your measuring starter current draw, the shunt is not large enough. But if your wanting to measure the total running current of the bike or the output of the charging system you will be fine with a 50 amp shunt. That is because the starter motor current will not be going through it. }}
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Ok good.
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Hoople->{{ Does the starter motor current go through the Maxi fuse? No it does not. So if you insert the shunt at the Maxi fuse (or at the voltage regulator output), will the starter motor current go through the shunt? Once again, no it will not.}}
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Ok good again, only I thought in your later post you said the Maxi Link would not work due to the way the bike was wired, (sketch 1, post #28) but I probably misunderstood that also.
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Hoople->{{inserting this shunt directly at the battery terminal? No, that will not work. Besides the reading will be meaningless. Your going to be reading the differential amount current between what is required by the bike, minus the charging current. A somewhat useless number.}}
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Exactly ( I see now, before I did not), only I now would have guessed it to be reading the charging current actually going into the lesser resistance (lower voltage) of the battery, but maybe that's what you just said and I failed to grasp it again.

I think you are finally teaching me this. I know you will be glad to be done with me. But I will be even more glad if I can learn it enough to boost up my troubleshooting skills on my vehicles. Will have to buy you some beer someday to pay you back for all the head scratching you did at my crude descriptions. Ha I can hear Bodeen now, there he goes again promising beer that never materializes.

But aside from my crazy backwards analytic brain, I have a good understanding of basic electric, it's just the ramifications of line current reactions that I've never gotten fully under yet. I do appreciate all the help.

Now before you go would you also please verify that I have the calculation assumtions correct to actually use this shunt (in the correct ) location.

I am understanding this shunt is designed such that the voltage drop across it, is Xv value of milliVolts when the current flowing through it is Xa value of Amps.

Therefore I can calculate the 'design' Xo value of ohms resistance of this shunt to be design voltage Xv divided by design current Xa. Correct ?

So then = Xv / Xa = Xo design ohms

Therefore if doing an incircuit test a voltage drop of say 28mV is measured across the shunt using a standard multimeter, I can use a ratio math calculation so that the 0.028mV / Xo (design ohms) = Test Scenario Amps. Correct ?

And further I suspect that overheating could damage this shunt. Should I attempt to never measure more than WHAT ? percentage of amps of it's rated 50 amps ?

Thanks again.

 

[Hoople]

And further I suspect that overheating could damage this shunt. Should I attempt to never measure more than WHAT ? percentage of amps of it's rated 50 amps ?

I wouldn't be expecting much quality or accuracy for $4.50. But with high quality shunts, you can run them all day at their 100% rating and push them much harder in short bursts.. Also hi quality shunts are individually calibrated by hand at the factory against known standards. Remember you also have to factor in the error of your multimeter.

When working on bikes or automotive applications, you need not be concerned about the resistance of the shunt having any effect on the circuit. Inserting the shunt in the circuit will not change or influence the circular currents within the complete circuit. Treat the circuit like it is not even there.