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# Slot Car System - current measurement

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#### Maciaz

##### Newbie level 1
Hey there,

I'm not a professional, but will try to explain the best I can. So me and my friend are running a small project that is a kind of promotion.

Basically, we have a slot car system. We need a connection to PLC, so we can track whether the cars are still on the track or got derailed. That along with a lap counter/timer needs to be shown on the screen.

The current that powers the whole circuit varies from 0-60 mA. It can only be connected to 4-20 mA or 10V PLC's analogue inputs. So, all we need is some kind of conversion. Keep in mind, that if we somehow lower the voltage on the track, the cars won't be able to run as fast.

So far, we've come with the idea to divide the current into 40 mA & 20 mA, using 2 resistors. The problem is that we lose the power.

Is there any other way to do this? Some kind of amplifier?

You don't say what voltage you need, but it sounds like you need an amplifier. The 4-20 mA output is just that, a CURRENT output, I think you need a voltage output. Are you measuring the current supplied to the slot cars to determine if you've lost a car? Are you using the PLC to control the cars or just monitor them? It's not clear what exactly is going on here.

If you insert a current monitoring resistor in series with the power feed, of say one ohm, then it will drop 60mV. This you can amplify by an op amp to give , say 10V output.
Frank

You can track current with 100mv drop max current sense, but motors consume ~8x Iavg on start. Rs of motor coil is critical for determining shunt value, to avoid drop in acceleration or current.

Thus for 100mV drop from 10V say shunt R must be 1% of Rs of motor and speed controller combined (?mOhm) then coasting or braking will drop current or even go negative.

So time interval of measurement should be ~1s and comparator threshold will be <5% of Imax or about 5mV. thus requires exceptional noise reduction methods of filtering, CM rejection, low ground shift error , and high CMRR.

You can put shunt on ground side near source to measure current rise above ground with gnd V sensing high gain OA or precision comparators with null offset adjust.

Lap counters are easy with IR photo detectors under or beside track using emitter/detector pair with logic level filtered, out and hysteresis.

This shunt may also reduce advantage of high torque motors if too large, so adding large bipolar cap across motor will boost a bit of acceleration.

if this not clear, you need more help from a pro.

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