Increase shunt resistence?

Thread Starter

Poor old sod

Joined Jul 25, 2017
88
I'm stilltrying to sort this averge current drawn by gopher. Perhaps if I increase the resistance of my commercial shunt by 2.5x I will get up to a workable voltage for a meter? I doubt it would be enogh to cripple the gopher with the losses, and It would work, as long as the shunt didn't overheat. How would I do that, or who could do it for me? what would they need to know? the shunt appears to be a strip of metal with 2 notches cut into it. I would hope to widen the notches, rather than deepen them. Ideas pls?

mod:
https://forum.allaboutcircuits.com/threads/pwm-variable-amplitude-i-p-getting-analog-average-i-drawn-readout.146386/
 
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mcgyvr

Joined Oct 15, 2009
5,394
If you have another post it would be helpful to link to that here..
I have no idea what this gopher you are talking about and think you may need to up your meds because you sound like a crazy person without more context :p
I envision some Caddy Shack experiment and you talking to a Gopher in its hole..
While some people will remember if you had a post here already not all of us will.. So you are really limiting your audience too without providing more context..

I also find its best to state the problem and not what you think the solution should be.. You get much better results that way..

In general though if you have a shunt already there is usually no need to do anything to it.. Maybe you need to amplify the signal coming off it to achieve full scale meter reading though..
Or get the correct shunt.. They can be adjusted though by removing/decreasing material however this also will cause them to dissipate more heat which can be a problem at some point..
 

ebeowulf17

Joined Aug 12, 2014
3,139
I'm stilltrying to sort this averge current drawn by gopher. Perhaps if I increase the resistance of my commercial shunt by 2.5x I will get up to a workable voltage for a meter? I doubt it would be enogh to cripple the gopher with the losses, and It would work, as long as the shunt didn't overheat. How would I do that, or who could do it for me? what would they need to know? the shunt appears to be a strip of metal with 2 notches cut into it. I would hope to widen the notches, rather than deepen them. Ideas pls?

mod:
https://forum.allaboutcircuits.com/threads/pwm-variable-amplitude-i-p-getting-analog-average-i-drawn-readout.146386/
Two (groups of) questions:
  1. Did you ever try the simple RC circuit into your original meter, but with larger RC values? Without knowing your PWM frequency, the original values (in my schematics from the first post) were just guesses. If the PWM frequency was lower than i guessed, you'd need larger RC values to get the ripple down to acceptable levels. I still think this is the simplest solution, and I'm not sure you've tested it enough to rule it out.
  2. Assuming you have ruled out option 1, we need to know your current setup. Are you using an op amp like you were considering in your second thread? If so, are you using a split/dual supply so that you're not relying on perfect rail-to-rail performance? If you're using an op amp, why do you need to change shunt resistance? Why not change amp gain? We need schematics, at least one, maybe several. What do you have now? What are you thinking of building next?
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
88
r/c cct was tested, unsatisfactory but better. op am discarded, suspect lack of linearity at low voltages. trying to boost voltage across shunt by increasing R. Jusy wondering if better to widen the notches or deepen them? @50A a 0.25V drop should be ok. My motor won't feel it. Getting a 15W 0.005 Ohm shunt isn't easy and it might need to be even more for reliability.. I am afraid of overheating existing shunt if I mod it.
 

ebeowulf17

Joined Aug 12, 2014
3,139
r/c cct was tested, unsatisfactory but better. op am discarded, suspect lack of linearity at low voltages. trying to boost voltage across shunt by increasing R. Jusy wondering if better to widen the notches or deepen them? @50A a 0.25V drop should be ok. My motor won't feel it. Getting a 15W 0.005 Ohm shunt isn't easy and it might need to be even more for reliability.. I am afraid of overheating existing shunt if I mod it.
Sorry, I don't have the experience to help with the shunt modification idea. Hopefully others can help with that.

Cheers!
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
88
I could get anothe shunt and connect them in series, and only increase rhe notched area by an half on one if it tested out insufficient volt drop? that would spread the thermal loading, or spend better having 2x 50A and 1x100A shunts, each at 75mV at full current and daisy chain the sampling points to a 1mA fsd 220 Ohm analog meter.
 

Thread Starter

Poor old sod

Joined Jul 25, 2017
88
Well, after fitting a new fwd/rev pot, I've increased the max speed to at least 8kph from 6kph by adjusting the max speed pot under the handlebar cover. The control module is very early, being analog. Circa 2011. My bicycle speedo has fried in the summer sun. I've installed an analogue meter across one 75mV==50A shunt, and to avoid problems when charging, I set neutral point at 0.08mA on a 1mA 220 Ohm movement. Along the flat absolute readings of 0.18mA, up hills 0.2-0.3mA. Downhill I get down to 0.08mA. The meter is mounted on the floor, immediately behind the front fenders and centrally across the machine. I used mastic to secure it. I hope the bearings survive the rough routes I drive. Further, a self selecting 12V/24V 120W charger which had died has been resurrected by cleaning the cct board, tidying up some soldering and removing a mounting screw which was shorting a small electrolytic capacitor. I've also removed the cover over it's full charging current lock switch. I'm now awaiting delivery of 2 more shunts to ultimately x2.5 the available voltage at 50A to nearly match the meter fsd voltage. Errors such as extra losses from joining 3 power shunts end to end are anticipated, and countered by one shunt being 100A==75mV. Rough, but near enough for me.
 
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