Amp meter - Ammeter general topic.

[uniberp]

Will a Subaru run when the alternator fails, until the battery dies? Will the alternator light, light up?

 

Not driving the Subaru, I got stranded by an failed alternator yesterday. (In my S10). Stupid truck, no dash light that I saw, voltmeter, well I didn't notice. Turned on the air and it died with NOTHING left, hardly enough to run the emergency flashers. Had to get TOWED. <frowny embarassed face> Charged battery, cleaned terminals (not too bad plenty of clean copper), replaced alternator, all fixed. Test stand confirmed dead alternator under load when I returned it for core. ALT light shines merrily in "key on-engine off" position. Volt gauge sits dead on 14v, well, as it always has. It may as well be a dummy gauge.

 

The humiliation makes me seek out a monitoring system (ammeter or "amp meter") . Carrying a spare alternator would proibably be cheaper, but I want INFORMATION.

 

The only "clamp-on" type ammeter I can find is by Dakota Digital, and I won't run fire hazard 8 gauge 100 amp wiring into the cab of any car.

 

Anybody have any other thoughts? (Besides carrying full set of tools and spares in every car. which is what I can do now, after buying the tools to fix the crank pulley on the Forester last weekend.)

[nipper]

Cars will run with a dead altenator. They run off the battery untill there is no more power. Don't run a car without an altenator hooked up for any length of time.

 

The batery light in GM's tend to be useless. In subarus you may not always get the battery light (or be able to see it when it first starts) but you will see the brake light. MOst cars will have the brake light and battery lights come on as they are grounded through the altenator. If the altenator is undercharging, it will be dim, if it lost a diode (or thre) it will be brighter.

 

There are lots of threads here on "why is my brake light on (and or abs and or battery) " . My honda and datsun were the same way.

 

and tsktsktsk not watching your battery gauge.

 

 

nipper

[nipper]

ALT light shines merrily in "key on-engine off" position. Volt gauge sits dead on 14v, well, as it always has. It may as well be a dummy gauge.

 

 

PS

 

A good battery can recover from being run down over night or a few hours, depending on how far down it was drained. Thats why you would show 14 voolts till you ran it down again.

 

Just get a aftermarket battery gauge if you dont trust yours. Amp meters just tell you the flow of electricity, not the state of the voltage output. Ideally you should have both, but its not needed.

 

nipper

[uniberp]

The batery light in GM's tend to be useless....

 

and tsktsktsk not watching your battery gauge.

nipper

 

I will go along with that first part. There should be a bell.

And I have to watch this crazy traffic so closely I am not surprised I missed it.

 

I'm cooled off now, as my GF would say, no longer "wrapped around the axle". Gekko just called back with instructions for reimbursement for the tow, so I may not be out all that towing fee. Put in a new tensioner and serp belt while I was at it.

 

Thanks

[porcupine73]

Many vehicle ammeters have a 'shunt' (resistor) that you place inline with the load you want to measure. Then just minimal gauge wiring runs from the shunt to the gauge.

 

If you get a meter that is 50mV at FS (full scale), and you want it to read full scale at 100 amps for example, you would get a 50mV/100Amp shunt = .0005 ohms

 

Or if you get the cheapest multimeter you can find that you can get a clamp on ammeter probe for that might be an option.

[uniberp]

Many vehicle ammeters have a 'shunt' (resistor) that you place inline with the load you want to measure. Then just minimal gauge wiring runs from the shunt to the gauge.

 

If you get a meter that is 50mV at FS (full scale), and you want it to read full scale at 100 amps for example, you would get a 50mV/100Amp shunt = .0005 ohms

 

Or if you get the cheapest multimeter you can find that you can get a clamp on ammeter probe for that might be an option.

I've heard of the shunt. With 100+ amps is there any danger of the resistor fusing/shorting and going full current to the gauge and again, burning the wire? I've read about that possibility. I hate messing with the hgh current lines.

 

I like this setup (see the install instruction), but not the instrument (the sensor sold seprately) or the price:

http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=360/category_id=243/home_id=59/mode=prod/prd360.htm

I just want a small readout. (one that I can ignore :-\)

[porcupine73]

If the shunt were to short, no worries; the gauge would just read 0. If the shunt fails open, that could fry/damage the gauge. A very low milliamp fuse inline with the gauge wiring might pop it then if the shunt fails open.

 

The power dissipated by the shunt at 100amps, 50mV would be five watts.

 

Yes the dakota digital gauge you showed does have a nice setup. That is the CT (current transformer) sensing method. The current induced in the CT is proportional to the current flowing through the wire. These are very common in industrial equipment where you might want to measure 1000Amps or even more.

 

The only risk I know with CT's is that the secondary CANNOT be operated open circuit. It will probably damage the CT and can present electrocution hazard. The secondary voltage will climb to thousands of volts if current flows through the CT with its secondary circuit open. If it needs to be disconnected, the secondary should be shorted. This is a common safety issue in industrial equipment.

[frag]

In my car I installed a cheap volt meter with parts from Radio-shack. It's connected to the cigarette lighter receptacle. I'm sure it would give me advance warning if the alt shoul fail.

But of course an audible alarm would be best, same thing with the oil pressure and water temp. You don't always keep you eyes on the gauges. Failed alt gives more time to be aware of the problem than zero oil pressure or overheating though.

Good luck!

[nipper]

In my car I installed a cheap volt meter with parts from Radio-shack. It's connected to the cigarette lighter receptacle. I'm sure it would give me advance warning if the alt shoul fail.

But of course an audible alarm would be best, same thing with the oil pressure and water temp. You don't always keep you eyes on the gauges. Failed alt gives more time to be aware of the problem than zero oil pressure or overheating though.

Good luck!

 

Sadly an audible alarm on the oil pressure is still one of those that would be "i just want to tell you its too late but you now own a paperweight" things.

Altenator is hard to do too unless the threshold is set at 10.5-11 volts.

Technically you dont read the gauges, you scan them. Your supposed to be scanning your spedometer to check your speed, and you scan the gauges at the same time. Race car drivers have far more to concentrate on then we do, and all their gauges are set up to be scanned by the eye (ever wonder why all the gauges point in different directions...so when everything is normal all the gauges point the same way).

People dont listen to the buzzers the cars have now (key in ignition, seat belt) as it is.

The only one that can have an honest threshold is the engine temp gauge.

I scan my dash fairly regularly, but dad was a professional driver, my drivers ed class (way back when), and my enginnering background have trained me to do so regularly.

There have been cars with buzzers for oil pressure (back in the 60's) but they were shown to be too touchy. Now with 100 watt and higher sound systems, i dont think anyone can hear anything in a car. I dont even hear my cell phone ring most of the time.

 

I will admit that the idiot lights have gotten so small sometimes it doesnt matter if they come on or not, no one can see them.

 

The other trick is to put the gauges in your line of site. Thats usually the best thing to do.

 

Sometimes its just really hard to engineer around the carbon based interface. :-p

 

 

nipper

[ron917]

The '86 VW Golf that I used to own had an oil pressure buzzer along with the light. It was a very different sound from the other chimes (keys, etc). The sender had a habit of going bad in those cars, so they often buzzed and lit the idiot light when the oil pressure was fine.

 

My brother had a Jetta from the same generation. His oil pressure light/buzzer started alarming, and a new sender fixed it. A few years later, it started alarming again so he ignored it, figuring the sender had gone bad again. Minutes later, it threw a rod.

 

Reminds me of one of Aesop's fables...

[RallyKeith]

The battery light in a subaru actually comes on when the alternator is not charging. This seems to be reguardless of the voltage on the system. Crappy cars like the mid 90's neon on the other hand have a battery light that is nothing more than a low voltage indicator. My soon to be sis-in-laws Noen had an alternator that went bad. Her dad (I guess my soon to be pop-in-law) thought he had found a wire that had come loose. Fixed it and charged the battery. He started the car and drove it around the block and the light never came back on, so he told sis-in-law it was fine.... She gets in and 5 miles from home the light came on again. Stupid thing was mearly looking for voltage above 10 volts. Freshly charged battery was 13volts until a few miles of driving with no alternator!

[OB99W]

[...]Now with 100 watt and higher sound systems, i dont think anyone can hear anything in a car. I dont even hear my cell phone ring most of the time.[...]

Easy engineering fix: Have an alarm condition mute the sound system. That will get noticed.

[uniberp]

 

The power dissipated by the shunt at 100amps, 50mV would be five watts.

 

That's a pretty big resistor, no?

 

The only risk I know with CT's is that the secondary CANNOT be operated open circuit. It will probably damage the CT and can present electrocution hazard. The secondary voltage will climb to thousands of volts if current flows through the CT with its secondary circuit open. If it needs to be disconnected, the secondary should be shorted.

 

Does this mean: if the "Send" wire is disconnected from the gauge, the CT windings will build up and hold a charge? Like a capacitor?

[porcupine73]

Five watts isn't a whole lot; many nightlight bulbs are seven watts or so. It does have to be able to carry the 100 amps or so. Also under average conditions maybe the vehicle is drawing what maybe 30 amps or something when running. Of course high loads such as lights, defrost, power windows, etc all add to it.

 

The CT's I'm thinking of want to output a current on their secondary proportional to the current flowing through the primary. The CT's are usually rated like 75:1 or 200:1 or whatever, so 75 amps flowing through the wire the CT is around results in 1 amp flow through the secondary.

 

If the CT secondary is disconnected, it still wants the current to flow. The resistance is basically infinite, and since V=(I)®, you can see the voltage becomes very high. The limiting factor is usually the insulation in the windings; maybe it can handle 5kv or 10kv or whatever.

[OB99W]

[...]

The CT's I'm thinking of want to output a current on their secondary proportional to the current flowing through the primary. The CT's are usually rated like 75:1 or 200:1 or whatever, so 75 amps flowing through the wire the CT is around results in 1 amp flow through the secondary.

 

If the CT secondary is disconnected, it still wants the current to flow. The resistance is basically infinite, and since V=(I)®, you can see the voltage becomes very high. The limiting factor is usually the insulation in the windings; maybe it can handle 5kv or 10kv or whatever.

You seem to be discussing the "true" current transformer (CT). They have turns ratios as you described, dividing the current by a certain factor. These devices only work with AC, and are indeed prone to outputting high voltage if the secondary is open-circuited.

 

However, DC current measurements are not done with a true CT, but rather a "pseudo-CT" that encorporates Hall-effect sensors (known as an HCT). These don't have the same transforming effect, and there isn't a high voltage developed if the "secondary" connections are left open.

[uniberp]

You seem to be discussing the "true" current transformer (CT). They have turns ratios as you described, dividing the current by a certain factor. These devices only work with AC, and are indeed prone to outputting high voltage if the secondary is open-circuited.

 

However, DC current measurements are not done with a true CT, but rather a "pseudo-CT" that encorporates Hall-effect sensors (known as an HCT). These don't have the same transforming effect, and there isn't a high voltage developed if the "secondary" connections are left open.

 

How accurate can a Hall effect DC ammeter be?

 

With all the variations of cable size and current density, it seems like it would have to be calibrated to an inline ammeter.

[OB99W]

HCTs do indeed have some accuracy issues; Hall-effect devices even drift with temperature changes. However, for the application being discussed (assuming the basic goal is to determine that a charging current is flowing), it seems that accuracy is secondary to spotting a trend. Although it should be possible to obtain accuracy within a few percent, it would appear that even 10% would easily be sufficient for the purpose.

 

Some further info: http://www.multimic.com/clump_e/index.html

Click on the links at the left of the page under "CT SENSOR FOR DC LINE CURRENT" for typical specs.

[Cougar]

How accurate can a Hall effect DC ammeter be?

 

With all the variations of cable size and current density, it seems like it would have to be calibrated to an inline ammeter.

 

The test equipment I have used in the past using a Hall Effect device have been very accurate. Certainly more than enough to do what you are looking to do.

 

I wouldn't discount the value of having a voltmeter to monitor the charging system with. I prefer it over a current meter as it can tell you a little more on how the charging system is working. As long as the voltage stays in the prefered range you should be safe. When the battery died on you last time you can bet that the meter wasn't showing 14 volts at that time.

 

If I did use a current meter I would install one that uses a shunt in the circuit so the meter then only passes a very small fraction of the total current through it.