Trouble with ammeters shunt

[chemelec]

Alternately, I have a 0 to 150 Voltmeter, that I can easily Change to read 0 - 15.0 Volts.
Also Free, other than Shipping Cost.

I Have MANY NEW METER'S, and No One Wants these ANALOGUE Meters anymore.

 

[fezder]

Hmm, i wonder what shipping costs are to finland? i like analogue meters, they give better overall inspection of stuff than digital meters IMO, that's why i use analogue handwatch too

 

[chemelec]

If I just put it in a Padded Envelope for Least Amount of Weight, Than $11.00 Canadian or $10.00 USD.
Or the Equivalent of Your Money, Rounded Up to the Nearest Value in your PAPER MONEY.
SORRY, I ONLY Accept Actual CASH MONEY, Mailed to me in a Letter.

If I put it in a Cardboard Box, Than Depends on total Weight.

 

[fezder]

Sorry then, sending paper money is prohibited by local law and i don't pay otherwise too by sending cash by mail....
Thanks for offer, again!

 

[chemelec]

No I Find I CAN'T Convert this 150 Volt meter to 15 Volts.
It has a STRANGE TYPE OF Meter Movement.
But if you want the 0-30 volt meter, Possibly $12.00 USD, By PayPal

 

[MikeMl]

hmm, never thought of using micro-ammeter as currents can be quite huge
care to draw schematic just for sure Mike?

Goal is to have bi-directional alternator and load current between X-Y, but not starter motor current. You can set full-scale deflection by how far apart X and Y are on the wire, or by putting a 100Ω pot in series with the meter.

 

[fezder]

hmm, good thing with microammeter is that i don't need to cut or drag high-cauge wire all over the place. is that potentiometer calibration so to say?

 

[MikeMl]

hmm, good thing with microammeter is that i don't need to cut or drag high-cauge wire all over the place. is that potentiometer calibration so to say?

If you have the patience, you can calibrate it so that 50A=50uA. Since the "charge/discharge" indication is diagnostic, just a "relative" indication is likely "good enough"

 

[fezder]

well if i want absolutic precision i wouldn't use analogue meters, they tend to give better overall-idea for situations. If i got it right, if i take more from battery what alternator can produce, meter shows as discharge and vice versa?

 

[fezder]

Can it be, i can find 50-0-50 microammeters only from ebay? oh man...

 

[MikeMl]

Can it be, i can find 50-0-50 microammeters only from ebay? oh man...

I have several. They are available from "surplus" dealers in the US...

 

[MrAl]

Hi,

Another idea would be to just buy a new meter that does what you want it to do.
You can get a new ammeter and a new voltmeter. They sell pretty cheap these days on Amazon.

 

[fezder]

Al, you mean those 7-segment LED-meters? yeah, that's one solution, but i'd like analogue meters more in this application

 

[MrAl]

Hi fez,

Well, they have analog ones too. I am not sure what kind of current you are looking for here though. Is 100 amps enough?

I am also not sure how you want to handle the huge current difference between normal charging and starting. When starting the current can go very high so you either need a bypass switch (not as good) or a means to limit the input to the actual meter which is usually a high sensitivity meter. For example, if you were to use a 500 amp meter movement and starting was 400 amps, that would be up near the top end of the meter travel. While charging however, the current could be as low as 4 amps, so that means the reading will be almost lost on the other end of the meter face which will be harder to tell what it actually is.

The old way was to use two diodes back to back across the meter movement or before some set point resistors. That would limit the max signal that the meter itself would ever see even in the event of a large over range signal. For this though i am not sure if you want ot be able to read that 400 amps too or not, and just go with reading the charge current which might range 0 to 60 amps.

If you want to only read the starting current then a high amp meter would do.
If you want to only read the charging current then a lower amp meter would do with a means to protect it from over range signals.
If you want both, then you would probably need a variable scale where below a certain value (like 100 amps) it switches to higher sensitivity.

In the modern age almost anything is possible. For example, an op amp that reads the signal and provides over range protection to the meter movement, with possibly a comparator to automatically switch ranges.
Then there is also the most modern approach which would be a microcontroller. That would provide fast readings too, analog or digital or both.

I was in favor of an analog meter for my automobile voltage also. It's a simple 0 to 15vdc meter cost about $10 USD new. It has fast response that can catch dips when starting, which is important in order to determine battery health. I was going to build a much more sophisticated measuring system that even included a lower frequency scope type display, but realized i can see a lot of the behavior with the simple analog meter. The scope would display the battery voltage while starting over the first few seconds or so on a graphical output display.

So maybe you can indicate how far you are willing to go with this, and exactly what range of currents you would really like to measure.

 

[MikeMl]

...
I am also not sure how you want to handle the huge current difference between normal charging and starting. ...

We already talked about this. Dont run the starter motor current through the shunt...

 

[MrAl]

We already talked about this. Dont run the starter motor current through the shunt...

Hi Mike,

Ok thanks. My post also talked about possibly how this would be accomplished, after all we cant lift the hood to disconnect every time we start the car. So maybe some automatic circuit.

 

[KeepItSimpleStupid]

I just read some of this MESS. My take.

Usually the starter is fed separately off the battery with no metering. There is a separate feed that does all of the accessories.
In a Toyota I had it was actually easy. A BIG ring terminal sat at the battery to disconnect all of the other loads.

Way back when, the single fuse block got that feed with a few fuseable links here and there. Now, there is multiple fuse blocks. One car I own had 3 (one inaccessible without tools)

Do you really care about cold cranking amps? If so, two shunts. One to the starter (unidirectional 500 A) and one bidirectional to the rest of the car. The "dual scale" switching nonsense is up to you. Could be -800-0-800 / -150-0-`50 to make it easy with a zero centered microammeter.

Shunts are usually labeled with a voltage (usually mV) full scale with big lugs and small lugs. Between the small lugs is the precision resistor. so you might have 100 A, 50 mV as an example.

==

So converting a milliameter to a voltmeter requires two things:
1) Full scale current
2) internal resistance

1: Well, current is the same in a series circuit, so all you need is a way to do some varying. Variable PS and a fixed resistor. The voltage across a known resistor is current. figure out the steps.

2: the internal resistance "stumps" people. Cake. Use a variable resistor and a fixed power supply and adjust the resistor to 1/2 scale. Remove the resitor and measure it. that's the internal resistance of the meter. Simple. Figure out the steps that avoids emitting magic smoke.

Turning known characterized meter into a voltmeter is now easy too. Some resistor in series with the microammeter wil turn it into a voltmeter.

Make your own meter face. I can take a pic of an 8th grade science project (1970's) where I did just that.
changing the resistor based on starting/running shoud not be too bad. Just make "start" the normal state. The shunts alwats stay in place.

 

[MrAl]

Hello there,

Do i detect a subtle mockery in your tone buddy

I kind of like the idea of two shunts because you know what they always say, "two shunts are better than one" (har har).
That's if the starting current really has to be measured EVERY time the car is started, which i doubt.

I meant to test my car's starting current but never got around to it. I didnt intend to measure it very often though, just get some idea what it really was.

 

[fezder]

Well i don't really care measuring startupu current, if car starts, it is enought. If i want to monitor that, i'd need peak detetcor because it's so fast analog meters won't hold it long enought, i'd presume so
What i want to monitor is normal consumption so i can better monitor on-road whether charging is enought from alternator and such
I haven't yet put any of these voltage/ammeters, but i like more the idea of using microammeter over normal ammeter just because then i don't need to cut wires and all that....
What is funny, i yet have no idea what size of meter is enought, because i don't have any high-wattage woofers on trunk, i doubt i need 100A meter. Frankly, i haven't yet even located main fuse on this car which sounds ridicilous i know, every car owner should know where main fuse is.....unless it is some big wire what just snaps instead of fuse, which sounds like bomb to my ears.

So, only normal current is enought, no need to monitor startup-current

 

[KeepItSimpleStupid]

There are NO MAIN fuses in cars. They are usually implemented as fuseable links. They CAN be a wire (faston to faston) in a fuse block in the engine compartment. Otherwise they are hard to identify. Someplace there is a lower wire gauge, that will do a controlled burn in a specific place in the car away from stuff that fire would like.

What I'm familiar with:
External starter solenoid (ford 1965+)
Battery cable goes through the solenoid. 12 V continuous on the battery side.
Definately fuseable link. (non-removeable - part of harness - clearly marked)

Toyota (1982)
Ability to separate +12 and starter at the + terminal of the battery
Fuseable links inside engine compartment fuse holder.

Cheyy (2000) and Toyota (2000) i"d have to look.

The two they care about is the connection from battery to alternator and the feed to the rest of the car.

MOST continuous power is generated by the alternator and that could be 30, 60, 120 amp or larger when the engine is running. The battery acts as a big cap and discharges slightly due to loads on all th time.

so, you turn on your light, the alternator powers the Lights. If you turn on your megaWatt stereo system that overwhelms the alternator, your battery starts to discharge.

Don't expect indicators to work.
My car just stopped without warning.
the bond broke on the alternator rotor. No discharge light - nothing.

Fire wall harness corrosion (alternator circuit)
***** to find.
There are some tricks done here so there is no self discharge.

One car had a voltmeter,and a discharge light - really nice.

Ammeter tells you less than that.

I'm more interested in an "oil level" light. Now the dipstick is too thin.

FWIW: alternator/battery troubleshoting I normally do with:
1) Hygrometer - electronic tests miss stuff
2) Voltmeter - Don't believe it.
It will tell you your battery is dead.
the Hygrometer will tell you if it's cell related. If all Sg is the same you may have a chance.\Deep discharge kills automotive batteries.
Use voltmeter to test voltage drops ACROSS connections.

If you don't have a clamp on ammeter so this when the car is working.
Turn on high beams, heater, AC MAX, and known constant electrical loads. Measure the voltage across the battery to alternator cable. record it. later, when you have problems, do the test again. This test alternator output without numbers.

3) Clamp meter
4) Scope: shows blown diodes easy. Also shows other stuff.

Fly by wire is annoying when there is no accelerator cable in the engine compartmnt.