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Unread 02-18-2013, 10:15 AM   #21
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

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Originally Posted by Volt_Ampere View Post
I think you need an isolated supply for that meter. You cannot power it from the battery pack. A 9V will last a long time though. I have several of these meters - use them to monitor the battery voltage in the radio system for my model airplanes. I think the 9V lasts a couple years for me.
My DC to DC converter that I attached the spec sheet is isolated to 1kVDC.
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Unread 02-18-2013, 10:34 AM   #22
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

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Originally Posted by Volt_Ampere View Post
One thing I forgot to add here: Make sure that your sense wires from the shunt are at least twisted. Could also be shielded. This is a very small signal and there is lots of RFI generated by a golf cart controller when it's not at full throttle.
What kinds of numbers are you seeing on your ammeter? Are they too high or low, or sporadic?

Also, where is your shunt located? Maybe placing them in the battery interconnecting cables helps.
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Unread 02-19-2013, 04:07 AM   #23
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

I put an o'scope on the shunt sensor wire and ran the cart on jacks at part throttle. Man that thing is noisy, it covered maybe 4 divisions of the scope screen at part throttle. When I applied full throttle it was a solid line, clean. No wonder my indicator is not accurate at p/t.

I have yet to test installing the capacitor (if interference is on the line) or relocating the shunt far away from controller (if interference is being radiated).

I've got bigger issues now if you haven't seen the UGLY picture yet.
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Unread 02-19-2013, 05:38 AM   #24
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

Bummer! How'd you manage that?

I successfully repaired a 350 cid Chevy engine that had a hole in it the size of a quarter. I cleaned it up real good and used JB Weld with fiberglass cloth. Held up for years.
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Unread 02-19-2013, 08:25 AM   #25
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

The noise on the shunt isn't due to the location it's due to the current waveform when the controller is at part throttle. (and the inductance of the shunt) An RC filter will cure that. R in series with one of the leads and the C across the two meter connections. Put the RC at the meter not at the shunt.
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Unread 02-19-2013, 08:34 AM   #26
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

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Originally Posted by Volt_Ampere View Post
The noise on the shunt isn't due to the location it's due to the current waveform when the controller is at part throttle. (and the inductance of the shunt) An RC filter will cure that. R in series with one of the leads and the C across the two meter connections. Put the RC at the meter not at the shunt.
My attempt at fiding suitable values for the RC came up short. Johnnie suggested a standalone capaciator of 1uF, if I use that cap what value resister should I use, if at all? How many watts (1/8, 1/4, 1)?
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Unread 02-19-2013, 08:35 AM   #27
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

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Bummer! How'd you manage that?

I successfully repaired a 350 cid Chevy engine that had a hole in it the size of a quarter. I cleaned it up real good and used JB Weld with fiberglass cloth. Held up for years.
This one isn't "fixable". The ring gear is pushing the case outwards.
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Unread 02-19-2013, 09:53 AM   #28
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

Any resistor wattage will work. It isn't going to draw any measurable power at all.
R*C = 0.1 seconds If C=1uF (1e-6) then R would be 100K ohms. This is a basic low pass filter.
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Unread 02-19-2013, 10:29 AM   #29
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

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My attempt at fiding suitable values for the RC came up short. Johnnie suggested a standalone capaciator of 1uF, if I use that cap what value resister should I use, if at all? How many watts (1/8, 1/4, 1)?
Since placing resistance in series with the meter alters the voltage drop across the meter, hence the meter reading relative to the voltage developed across the shunt, perhaps using the meter's intrinsic impedance as the "R" value might be more logical.

Neither the R or the C value is all that critical. It is simply a low pass filter for a waveshape whose fundamental frequency is in the high audio range. (See attachment)
If C is too small it won't filter all the ripple on the DC through the Shunt.
If C is too big, the meter response will be slow.

I'm not sure what the waveshape through the shunt looks like, but I'm fairly certain it isn't rectangular since batteries act like huge capacitors.
Perhaps installing the shunt between two batteries in the middle of the pack would have a cleaner waveshape.
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Unread 02-19-2013, 02:58 PM   #30
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Default Re: ebay 500 amp ammeter inaccruate at part throttle

Basic current waveform should look the same everywhere since everything is in series. It will be ugly due to the inductance in the circuit. Those meters ususlly have a large input resistance but it must be quite large relative to the resistor in the filter to prevent a significant calibration error. (I would shoot for R not more than .01 times the meter input resistance) For that reason, I would go with R around 10K and C around 10uF. Try that and see how well it smooths out the meter reading. Again, the C goes across the meter leads and the R goes in series with one of the meter leads.
The calibration error is RMeter / (RMeter + RFilter) and you want this close to 1.00
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