Measure Fwd/Rev DC Current?

Tcmtec, Bottler mate. Thanks for your help. Please see post reply to John.

To-morrow mate.

Mark N.

GA for the logic shown. If he wants to summarize the currents it is more a timely reading, synchronized to his rotation switch position.

Connect the shunt to the motor feed via a bridge (you'll obviously need diodes that can handle the peak motor current) such that the current through the shunt is always in the same direction. Then connect the meter to the shunt as usual. See here.

Thanks John, I see you had some fun. I'm still not sure where I stand. I have to sit down and read every ones reply in the "Link" I have just printed everything, plus some other stuff, so will have a feed, 8.00pm here, and a good read.

Will comment further to-morrow.

Thanks Mate.

Mark N.

Hi Mark,

You seem to have plenty of meters and shunts.

I cannot see any harm in running (less than FSD) reverse current through miving coil meters; they will indicate below zero (pointer against the backstop) and when the current reverses will just indicate as normal.

Connect the shunt in circuit with two meter movements each connected to the shunt's potential (small) terminals. They will show traction and braking currents in one travel direction and braking and traction current in the other direction.

The only problem is a tendency of the pointer to adhere to the stop so I would try a lashup to see if any of your meters suffer from this defect, and select the unaffected ones. It is usual to set the stop downscale so zero current will give enough torque to let the pointer move freely.

G'day Zaf.

Thanks your input. I bought a bucket full of meters ect, from China. Cheap as chips. They will be more about a "change of state" than knowing precise measurements. Though they seem to be OK.

When I get this beast finished wiring, I will be able to play and check your theory.

Thanks mate.

You can get zero-center meters. Digital meters are nice, and they'll show reverse polarity automatically.

As for Schottky diodes, yes they have a lower voltage drop than ordinary silicon diodes, but it's still far too large for your low-voltage shunts. The graph is the forward voltage drop for 1N5817 diodes from ON Semi. These parts are designed to handle up to 1.0A, so we'd expect them to have a pretty low drop at your low currents for a meter, but as you can see, even at the bottom of the graph, at only 20mA, they'll still drop over 220mV, which is way higher than your shunt voltages. So that can't work as you'd hoped.

Is we were to make an accurate electronic reversing circuit, it'd have a comparator to sense the polarity, followed by a CMOS switch to reverse the signal voltage. It'd also need a 5V power supply, etc., to operate from.

But once you get your hands on a small 5V power supply, etc., why not use it to run a digital meter, and be finished? Like this one on eBay selling for under $10. As you can see in the 2nd picture, of its backside circuit board, it wants +5V to operate. This meter is marked 100mV, but it's a 3-½-digit meter that'll display ±199.9mV with 0.1mV resolution. If you want it's possible to add resistors to change the scale such that it'll display amps directly. When you use it you'll need to connect the "-_ of your 5V supply to one of the meter input terminals.

Morning Win,

Mate the old voltage drop...... Ah well

Now Win, I am an electrician, but since the late 70's, I have worked mainly on the mechanical, steel, side of stuff. Brought up on 6bm8's ect, so am reasonably up on Analogue stuff, the present electronics, well I seem to understand, but I struggle, a LOT.

Yes, it seems as though I should have gone this way ( digital ) in the first place. I just wasn't aware of them, Well digital stuff,OK., just not this little package.

Thanks for your ideas Win.

Cheers

Mark N.

G'day Everyone,

My heartfelt Thanks for your assistance.

As you are aware, I am a little before "crossing the road". I am actually building the Tram elsewhere, and I "play" with the odd little "bit of trouble" at home.

I have just ordered 4 of "Win's" items/possibilities. But will use them, only after I have buggered around with the Analogue meters some more. See the Panel photo. I have cut holes to suit already

If I have to go Wins Way, then I will cut/shut and weld, and cut again...! At least I can use the meters on the next "simpler" Locos that I build. There are 3 more in the pipe line (gunna do).

Here is a mudmap of where I am metering.

So it is only the middle and right hand meters that will have reverse flow.

Thanks again, fellas, for your help.

Best Regards,

Mark N.

BTW, these meters use ICL7107 integrated circuits to take the measurement and run the LED display. The '7107 has an integrating A/D that typically averages the measurement over about 83ms, or five 60-Hz AC-line cycles. The conversion technique is dual slope with an auto-zero phase (good to 10uV), and updates the display about 3 times/sec.

The ICL7107 costs $4.48 at Mouser. That means the eBay price of $9.70 for a complete meter with free shipping is pretty damn good! I bought a few, and they're well-designed and well made. Recommended for your parts-drawer stock.

A US company called JCWhitney sells 50-0-50 and 60-0-60 ammeters for 21 and 46 USD respectively. No shunts needed, just suitable heavy cable.

I've used the 50 amp version for testing alternators.

Thanks JW., for your time. Funny, I hadn't really "thought" the direct meter completely through.

As it turns out, (yes, I didn't tell anyone) but the meters are remote, 4 meters 13', so really, the shunts work out as the best way. I will check the web for JCWhitney, for next time though.

Appreciate, Yours and everyone elses help. I will get back and advise the results of this EPIC. I'll be back at the Tram, next week.

Again.

Thank You All.

Mark N.

Hi Marcone,

Please consider a recipe to solve your problem. There are three parts to the proposed method.

First, feed a signal into the shunt/meter network which moves the pointer to half FSD (in the imagination at this stage).

Second, halve the meter's sensitivity by adding a resistor in series of equal value to the movement's resistance using a shunt and meter of the required FSD current.

Third, rescale the meter -A,zero,A; a temporary lable on the glass will do, but paint on the scale requires movement dismantling but can look and last better.

The centering signal can be generated by feeding a constant current into the junction between the meter and the resistor.

A voltage regulator type 317, a few resistors and a trimpot can provide the constant current generator. There needs to be an arrangement to turn off the constant current generator. A relay or a transistor and resistors can avoid 24/7 current flattening the battery.

Thanks Zaf. I will have a look at further. I am surprised, that I think I understand what you are saying. Maybe I'm not as think I dumb I am ass..(lol)

I now have another problem. I have had an idea, for about 10 years now. As of Tuesday , I now have a Patent Attorney on "A drip feed into his wallet". So I am now knee deep in getting my "cunning Plan" into operation. This idea, has nothing to do with this meter, measuring. I unfortunately, have little piles of "cunning Plans" scattered around my little 10 acre paddock, some approaching 20 years old.

So I will be a little delayed getting to my meter problem.

My digital meters, have turned up, I have blown one up already. Love the smell of burning money, better than baby shit.

Thanks, men. for all your help. I WILL get back,, when I get into the Tram again. Soonishhhh.

Regards.

Mark N.