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Hot Rods Ammeter at full discharge

Discussion in 'The Hokey Ass Message Board' started by vetteson, Aug 11, 2020.

  1. vetteson
    Joined: Oct 7, 2010
    Posts: 301

    vetteson
    Member

    Installed NOS but 70 Year old ammeter in car. I used 10 Gauge wire and a 20 amp fuse. The gauge would read zero (middle) with engine off. It worked briefly but I had the wires crossed. Anyway, the fuse blew. I reversed the wires and installed a 30 amp. Both terminals are getting 12 volts. But now it reads full discharge engine off and on. The gauge was originally for a six volt system and am running 12 volts. I thought voltage does not affect an ammeter, maybe it does and I cooked the gauge.
     
  2. Vimtage Iron
    Joined: Feb 28, 2010
    Posts: 561

    Vimtage Iron
    Member

    How did you wire it, wire in from Regulator or Alt other wire to batter cable on starter terminal, it sounds like you went to ground with it.
     
  3. vetteson
    Joined: Oct 7, 2010
    Posts: 301

    vetteson
    Member

    That's how I wired it. Reads full discharge all the time.
     
  4. Voltage doesn't affect an ammeter, but current does. Most vintage generator systems only put out 30 amps max or less with gauge capacity to match, if you've upgraded to an alternator with a output above that you've likely smoked the gauge. And keep in mind that a fuse isn't designed to protect the device it's connected to, it's there to protect the wire only. Under the right conditions a fuse can let through considerably higher current than its rating if only for a brief time. With a delicate gauge, a brief time would be enough...
     

  5. vetteson
    Joined: Oct 7, 2010
    Posts: 301

    vetteson
    Member

    Yep, I have a 78 AMP alternator. That must be what killed it.
     
    VANDENPLAS likes this.
  6. Very likely... Under 'normal' conditions, the gauge will live. But upon initial start, the alternator will spike current output before settling down to charging and that spike is what kills 'em.

    This is why ammeters have gotten a bad rep in our cars. You can't easily find any ammeter rated above 60 amps and most modern alternators exceed that in output, sometimes by double that amount.
     
  7. Ammeters give you valuable info. If you want one, try using a shunted ammeter with a fuse between the shunt and the meter. Very little amperage entering cabin.


    Sent from my iPhone using The H.A.M.B. mobile app
     
    LAROKE likes this.
  8. Actually, on an alternator system ammeters don't give you good info. Because of the differences in how alternators and generators work, they require different gauges to monitor system health. When generators fail, current output falls off. Alternators are the opposite; voltage falls off.

    And whomping up a shunt-type ammeter isn't something you can do easily in any case. The shunt resistance has to be calibrated to the gauge if you're going to have anything approaching accuracy and that generally requires lab-grade test gear to determine. The usual hand-held meters we use aren't anywhere near accurate enough. S-W sells a shunt-type ammeter, but it costs about $90 and is scaled 350-0-350 amps on a standard-sized gauge face so won't show anything other than gross problems.
     
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  9. brigrat
    Joined: Nov 9, 2007
    Posts: 5,615

    brigrat
    Member
    from Wa.St.

    I deleted my post, there's no sense in responding when crazy steve shows up, we are lucky to have him among others!
     
    VANDENPLAS, loudbang, clem and 3 others like this.
  10. vetteson
    Joined: Oct 7, 2010
    Posts: 301

    vetteson
    Member

    Thanks! Learn stuff like this all the time. Amazed I haven't burned a car up, well I almost did...…...o_O
     
    VANDENPLAS likes this.
  11. I’ve only dealt with generators. After frying a bunch of them I learned how to rebuild them, adjust a regulator and installed an ammeter along with voltmeter. (Found a 50amp ammeter with a paired shunt).

    But I didn’t realize this difference about alternators and ammeters. In fact I don’t know much about alternators at all. Good info. Thanks for the correction. I’ll try to learn more about it.


    Sent from my iPhone using The H.A.M.B. mobile app
     
    VANDENPLAS and loudbang like this.
  12. I've been meaning to do a deep dive into this (explaining why an ammeter is best for a generator system and volt meter for alternators) but it's pretty involved and I haven't had the time.

    Same goes for shunt-type ammeters. It's easy to build the circuit, but getting anything approaching accuracy is the back-breaker...
     
  13. Atwater Mike
    Joined: May 31, 2002
    Posts: 11,625

    Atwater Mike
    Member

    Witness the famous Italian Panteras. The main fuse was the ammeter....

    Dead in the water, come in on the 'hook';
    Ammeter burned, another one 'for the Book'.
     
    loudbang and 41 GMC K-18 like this.
  14. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    I've done shunted ammeter setups for few cars which have alternator conversions. First one was about 14 years ago last one about 5 years ago. They're all still working and none of ammeters have pegged even when starting up with a flat battery. Usually I just split the current, so I replicate the ammeter's resistance with a bunch of parallel resistors -this has the effect of halving the current that the ammeter sees. The important thing is to have sufficient power capacity in the resistors. This one was for a 40A alternator for an early car with 20A ammeter.
    [​IMG]

    [​IMG]

    This one is on my truck (68A alty) The (unmarked for current) stock ammeter went full scale @ 40A and had slightly more than 1/3 Ohm resistance.
    [​IMG]
    The three resistors in parallel here (ignore the little PCB it does something else on the truck) present 1/3 Ohm resistance. Pretty well all car ammeters are so-called "Moving Iron" style where high currents or ambient temperature change will alter the resistance of the meter itself. Even "Moving Coil" style ammeters found on early quality cars have some resistance change with current and temperature; so you don't need to be super accurate when setting up a shunt. Although your ammeter won't be showing true current produced, alternator output voltage reduction will certainly be reflected in a drop (or absence) of charge current when the voltage drops significantly.
     
    GeezersP15 and clunker like this.
  15. Not true. Alternators can lose 2+ volts of output while maintaining normal current output. That large of a drop will result in a battery that won't get charged. Remember, voltage and current are regulated separately, so while one will affect the other the regulator will 'mask' the problem. So while this is a solution to make the ammeter 'work', it still probably won't give meaningful information on the actual 'health' of the system.

    Another issue is those resistors significantly reduce charging system capacity. In your second example, you're losing about 13% of total capacity and worse yet it's a steady-state loss; this is a constant drain when the circuit is energized. It also produces considerable heat, as indicated by your heat sink.

    Typically, a shunt-type ammeter will be calibrated to read output at a small percentage of actual output, generally 10% or less.
     
  16. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    Interesting, you are saying that if an alternator drops 2v output, so now typically about 12.5v output, that the ammeter will still show as charging into a battery that is in a charged state -typically 12.5-12.9v

    Steve, can you explain how the efficiency of the charging system is materially affected by the reduction in resistance that will be seen by both the alternator and battery? the shunt is wired in parallel with the ammeter; the ammeter/shunt pair are then in series in the charging circuit. In both the real world examples in my previous post, the the overall resistance in the ammeter circuit has been halved. If there has been any reduction in charging efficiency, it hasn't been apparent to me, or the people I've carried out this modification for.

    Yes, in the case of a moving coil ammeter, there is an integral shunt, That's the type fitted in the vintage switch panel in my previous post. It's more likely with one of these that the manufacturer will have used a material for the shunt that has a reasonably stable TCR (Temperature Coefficient of Resistance) as do quality resistors. But that means that the TCR of the ammeter and shunt are likely to be a better match than for the more common moving iron ammeter setup which usually have the internal current passing through a Brass frame.
     
  17. Interesting, you are saying that if an alternator drops 2v output, so now typically about 12.5v output, that the ammeter will still show as charging into a battery that is in a charged state -typically 12.5-12.9v

    Yes. Ammeters register amps, not volts. It's entirely possible for an alternator to show 'normal' amp charging with low voltage. Again, this has to do with the mechanical/electrical construction differences between alternators and generators.

    Steve, can you explain how the efficiency of the charging system is materially affected by the reduction in resistance that will be seen by both the alternator and battery? the shunt is wired in parallel with the ammeter; the ammeter/shunt pair are then in series in the charging circuit. In both the real world examples in my previous post, the the overall resistance in the ammeter circuit has been halved. If there has been any reduction in charging efficiency, it hasn't been apparent to me, or the people I've carried out this modification for.

    Under 'normal' conditions it probably won't be noticed. And it doesn't 'reduce' the efficiency of the charging system, it simply reduces the available output to the remainder of the electrical system by the power consumed by the resistors. In the example noted, about 10 amps worth. If you used similar-sized resistors in the first one you showed, the loss will be about the same but will be a larger percentage of the available output.
     
  18. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    Steve, the analogy often used for voltage or EMF is electrical pressure, in this case the analogy holds true, in my example in a previous post where alternator output voltage has dropped to an at or below battery voltage level the ammeter will show no charge because there would be no current flow in the direction of the battery as the alternator (or a generator for that matter) has to output a higher voltage than that existing at the battery if there is to be current flow to provide charging.


    Steve, this diagram shows a simplified "before and after" charging setup with an alternator and ammeter in the battery circuit. I the "after" version The shunt resistor array presents the same resistance as the ammeter -so about 1/3 Ohm; there is a 50-50 current split as a result. The ammeter will now read half of whatever the alternators nett charge happens to be. As the ammeter and shunt array are in parallel the resistance that the alternator sees is reduced slightly because the resistance of the parallel array of ammeter and shunt array will only be half that of that where the ammeter is on it's own in a circuit.

    I can't see (and haven't experienced in a number of conversions) any situation where the resistors would be consuming any significant energy let alone 10A -I really think that the resulting 120W of heat would be rather noticeable.
    [​IMG]
     
    Baumi likes this.
  19. Steve, the analogy often used for voltage or EMF is electrical pressure, in this case the analogy holds true, in my example in a previous post where alternator output voltage has dropped to an at or below battery voltage level the ammeter will show no charge because there would be no current flow in the direction of the battery as the alternator (or a generator for that matter) has to output a higher voltage than that existing at the battery if there is to be current flow to provide charging.

    You're correct as far as you go, but you're forgetting that the alternator load doesn't consist of just the battery; you've got the rest of the electrical system connected also. The purpose of the charging system is to supply however much power is being consumed by the total system plus some extra to charge the battery. If the battery isn't being charged, the rest of the system won't care.

    Steve, this diagram shows a simplified "before and after" charging setup with an alternator and ammeter in the battery circuit. In the "after" version the shunt resistor array presents the same resistance as the ammeter -so about 1/3 Ohm; there is a 50-50 current split as a result. The ammeter will now read half of whatever the alternators net charge happens to be. As the ammeter and shunt array are in parallel the resistance that the alternator sees is reduced slightly because the resistance of the parallel array of ammeter and shunt array will only be half that of that where the ammeter is on it's own in a circuit.

    I can't see (and haven't experienced in a number of conversions) any situation where the resistors would be consuming any significant energy let alone 10A -I really think that the resulting 120W of heat would be rather noticeable.
    [​IMG]
    Again, you're not showing the connection for the full system. Now by your own admittance, you're shunting 1/2 of the current that would normally go through the gauge through resistors. Where does that current go? It just doesn't disappear, it's converted to heat. And remember, an ammeter isn't showing the total current flowing though it, it's only showing the extra needed for charging. If driving down the road you need 10 amps to power the system consisting of ignition, lights, etc, the alternator will output that 10 amps plus enough to charge the battery but the meter will only register the 'extra'. Add the resistor load, and the alternator will increase output to compensate... and will still only show the small 'extra'.

    There's other factors involved that I'm ignoring. You're assuming a static resistance of 1/3 ohm for the meter but it's actually an inductive load and in operation its 'resistance' (resistance plus inductive reactance, better known as impedance) can vary widely, further complicating actual circuit values.
     
  20. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    Steve read my previous post again: Nett Charge -what goes to the battery. But the point you stated originally, and now seemed to have forgotten was that the ammeter would not reflect the lack of charge when alternator output voltage drops, when in reality a significant drop in voltage output will be reflected in a drop of current flow to the battery which would be indicated by the ammeter.

    It should be obvious from the diagram that the resistor shunt array is in series with circuit, but in parallel to the ammeter. The current splits equally and all of it ends up at the load -the battery and ignition etc. Resistive losses won't be any/significantly different in the resistor array than in the ammeter.

    Yes, There will be some inductance in the meter and also some Capacitance in the wiring loom, I'm well aware of them. However, this is some simple power wiring for a car or truck; not a top end HiFi system, so the minuscule effect these other primary constants can be safely ignored here.

    Over to you Steve, I have one more post on this thread, which I hope might be of some help to the OP. I won't be replying to any more of your posts, as to me they seem to be simply contrarian and ignore inconvenient electrical facts. Anyone who reads the whole thread will be able to make up their own mind on the subject.
     
    Last edited: Aug 15, 2020
  21. Stueeee
    Joined: Oct 21, 2015
    Posts: 305

    Stueeee
    Member
    from Kent, UK

    Vetteson. Your ammeter which is pegged is probably quite repairable. Here's the one on my truck when I bought it in the USA 15 years ago. Pegged at full charge as you can see: 68A Alty; this ammeter's full scale deflection is 40A. When there's a flat battery, this will happen every time you start the motor.
    [​IMG]

    But these instrument are quite tough internally, a "Moving Iron" ammeter is most unlikely to be burnt out because the frame which the current passes through is usually made of a fairly chunky brass pressing. If you remove the glass on the instrument and gently dislodge the needle from the edge of the scale it is likely to return to it's central point. Even if the needle is actually bent, as long is the pivot mechanism isn't broken, you can "bend it straight" so that the needle rests back in the centre. It should work properly again after this treatment. That's what I did on my truck ammeter, it's still working fine 15 years later, along with the apparently controversial shunt setup which ensures that the needle can't get pegged again even when the Alty is giving its all.
     
  22. ... But the point you stated originally, and now seemed to have forgotten was that the ammeter would not reflect the lack of charge when alternator output voltage drops, when in reality a significant drop in voltage output will be reflected in a drop of current flow to the battery which would be indicated by the ammeter.

    No, I didn't forget. I'll repeat; voltage and current are regulated separately and the gauge won't care what the voltage through it is. By your logic, increasing the voltage from 6 volts to 12 should cause it to read higher and that's not happening is it...

    It should be obvious from the diagram that the resistor shunt array is in series with circuit, but in parallel to the ammeter. The current splits equally and all of it ends up at the load -the battery and ignition etc. Resistive losses won't be any/significantly different in the resistor array than in the ammeter.

    ... Yet you felt the need to install heat sinks...
     
  23. garyf
    Joined: Aug 11, 2006
    Posts: 283

    garyf
    Member

    That's a mistake, you could have brought something valuable to the post !
     
  24. vetteson
    Joined: Oct 7, 2010
    Posts: 301

    vetteson
    Member

    Yow :eek:, I've learned more about auto electronics in the past week than I I've learned, well, ever.

    I bit of clarification, I wanted to retrofit original Corvette style gauges in my '56 Corvette. Very expensive. Then I learned that the gauges installed in early 'vettes are actually '49-50 Pontiac gauges, slightly modified to fit Corvette dashes. First I bought a fully gauged 1950 Pontiac gauge cluster for $50 to see if it was true. True. I bought two NOS Pontiac gauges for $30, one was the ammeter, the other water temp. I was able to "restore" the original Pontiac oil pressure gauge face and installed it with the ammeter. As we know the ammeter lots its mind, but the 70 year old oil pressure gauge works, and no leaks! Happy dance. Solution, I found and bought a reproduction voltmeter (yes, someone makes it for dopes like me) that appears similar to the other period gauges (in a '56 Corvette).
     

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