Motorola with no voltage regulator

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Posted: Jan/15/2018 at 1:15pm

Heyas,

Electronics question for ya guys.

I have been trying to figure out how to make a Non-Obtrusive, Non-shunt type (and not digital) Ammeter for the car.
This is harder to do when measuring DC than AC.

Could I take a wrap of wire around the wire to the load I want to test and like put or pulse a current thru it and use a Hall-Effect transistor or IC unit to measure what I think I would get as a resistance to the flow thru the wire wrap? Or measure differential between what I send thru wire(wrap) to what is on other end with some sort of Comparator circuit?

I have seen some remote shunt type but they are way expensive.

I want to send final output to read to an aftermarket auto rally Voltage gauge.

Have not been heavy into the electronics like I used to be years back, a bit off and on.

last was restoring an old Hallicrafters S-38A Shortwave receiver a few years back, recapping and changing the Russian-Roulette 50-50 chance of a hot chassis to correctly re-wiring to a 3-prong plug, grounding the case.
Works much cleaner and safer!

Any low-cost ideas?

Thanks guys and Happy New Year!
Blitz

1972 AMC Hornet Sportabout
Not the one in the Avatar though, not as good looking unfortunately LOL.

6PakBee

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Posted: Jan/15/2018 at 2:08pm

The first thing that comes to mind is a saturable reactor but that is really old school.

Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

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Blitzman

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Posted: Jan/20/2018 at 2:37pm

6PakBee wrote:

The first thing that comes to mind is a saturable reactor but that is really old school.

Ha, I will have to look that up, never heard of it, thanks.
Do they use a Flux Capacitor with that? Wink

I would think somehow passing a wire thru a toroid wound with some wraps could provide some reactance. The pulse going thru the toroid winding would be disturbed/impeded by the current of the DC supply wire. Or maybe the opposite way, wrap a winding or so(more?) of the supply wire around the toroid, pass the sense wire thru the hole and toroid wraps flow would be impeded by supply current. Would probably be a very low amount of reaction that would need to be amplified or sensed by a sensitive Hall-Effect part.

Tom or Billd you guys have any input on this?
Thought for sure one of you two at least could maybe shed me a little light.

I can picture how it may work and seems like it should somehow but need a little more input from someone who has maybe had more hands-on with current measurement and Hall-Effect etc.

Thanks!
D

1972 AMC Hornet Sportabout
Not the one in the Avatar though, not as good looking unfortunately LOL.

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https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table

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Posted: Jan/21/2018 at 9:16pm

you can only measure alternating current with transformers or magnetic means, the magnetic field has to change for it to have any measureable effect. for cars, all DC, no-go.

shunts are just resistors, which are just "calibrated" wires of known resistance. you can use a length of existing wiring harness as the shunt -- a lot of modern cars and trucks do this; just connect a thin wire at each end of some long-ish run, and measure the voltage drop across the length of wire. OEMs can know the resistance of that length of wire to a fraction of a percent since they're assembled by machine.

it's most convenient to work with "round numbers" (one volt across one ohm means one ampere, etc) but that actually doesn't matter at all.

if you had 10 feet of 8-gauge wire from alt to battery, that's .006 ohms. each ampere through that wire would generate a voltage drop of (V= I * R) or .006V for each amp through the wire. that low drop is good for the alternator and battery but not much to measure directly. but a simple op-amp circuit can increase that voltage to something a store-bought panel meter can work with. with that "shunt" 50 amps would drop 0.3V.

the main problem rolling your own instrument panel gauge is... vibration destroys typical meter movements. lab type meter movements (which are mostly antiques now!) are D'arsonval movements, and don't survive the vibration of automobiles. car pointer displays used to be some form of taut wire or bimetallic gauge, which are also very slow to move, a good thing in a car, where the gas sloshes in the tank and the voltage regulator is an on/off bimetallic thing. (today they're mostly stepper motors and a lot of software, it's cheaper and "easier" once you have an ECU already.)

here's a wire gauge vs. resistance calculator:

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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Posted: Jan/22/2018 at 12:24pm

Hi Tom,

Thanks much for the info! Yeah I did not get down to trying to do the math(I kinda go by feel a lot, I see the workings of things in my head LOL and have a feel for some parts).

Great calculations. Yes in my observations of testing across a length of the battery wire just between the battery and where it connects to and joins with the Alternator wire, on the highest(most sensitive current) setting on my cheapie digital meter I got(if I remember right) up to like 25 millivolts of a reading due to the very small Resistance still present as you said. Even only just over two feet of battery cable will give some differential. Will also give a minus reading if discharging. :0)

Yeah a shunt could be only .001 ohms resistance even, low as possible for low current loss.

I used to use the old LM339(?) Quad Comparator for a lot of things, easy to make a simple level meter of four points(or more if cascaded) with them. I had a stereo back when with 8-track, took it out and made the slot a little storage area(record cleaner etc.) and changed out the track indicator lights for LEDs and made it a basic level meter LOL.

The LM339 may be discontinued now.

Anyway, I just have been trying to figure an induction(I guess it would be) method, I know you normally can only do this with AC as a field is energized or collapses but figured there may be some way to affect a coil that is being pulsed as its field goes thru energizing and collapsing.

I thought probably not possible till I came across this:

bayite DC 5-120V 100A Mini Digital Current Voltage Amp Meter Ammeter Gauge with Hall Effect Sensor Transformer .

Somehow they are doing it, I am thinking they have a wound toroid in the part the wire passes thru with a Hall-Effect transistor/IC next to the coil? Like you said you(normally) can only get a reading on a rise or fall so I don't know what they are doing. Maybe I could modify one of those to read on an Auto Voltmeter.

And yep right, panel meters would probably start going out of whack on just a trip to the store(with these awful roads). Would want to just power an Automotive Voltmeter with the circuit.

Input on all this?

OK Thanks Tom! Need to not overbear you with so much of my brain spew, I just will keep thinking of more and more...

Peace and best to you. Thanks for the Calc!(did you make the other Tire and gearing calcs etc? Way cool.)

Duane

Edited by Blitzman - Jan/22/2018 at 12:27pm

1972 AMC Hornet Sportabout
Not the one in the Avatar though, not as good looking unfortunately LOL.

Blitzman

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