Yeah I know the discussion amp vs volt gauge but trying to use what I have. The gauge I am using is from a 34 Buick, not a lot of requirements for power those days so rated at 20 amps. The alternator on my nailhead is a one wire setup and I have no way, at this stage, to determine the output. How can I limit the amps to the gauge so I can use it ?
Make a "diode shunt" that bypasses the Amp Gauge when the current exceeds 20a Normal usage [< 20a] the Ammeter has less voltage drop than the forward drop of the diode [so the ammeter reads as normal ] when the ammeter is maxxed out the voltage drop equalizes with the diodes so the shunt acts like an "electrical relief valve" [this is usually the battery recovering from excessive cranking] And if you totally Chernobyl your ammeter, you won't be left stranded on the roadside.
Thanks for the reply. Being totally inept when it comes to anything above basic wiring, what diodes would I need and what do they look like ?
Hey, Kerry; Thanks for the info & drawing - I also have long wondered about this. Since I also have some old ammeters, that I wouldn't mind using, but won't until I know *exactly* what to make. What diode(s) does he need, & is it a one-size-fits-all? Like 1 set for 12v, one set for 6v, or does the max possible amps need to be figured in? If not, would you expound/expand on the actual parts needed? & the formula if needed? I can build stuff, & work w/electrickery, but electronics & I don't get along well... TIA. & dp32; Those are some *nice* gauges... . Marcus...
Thanks for chiming in and need all the help I can get. I really like the gauges also as the original 32 Ford you need a microscope to see them plus am not in any position to buy the duplexes that fit the panel.
Use 2 Vishay diodes mounted "back to back" on a phenolic block.[using bolts and spacer tubes with wires that have eyes on the ends] It doesn't matter which wire goes where because it flows both ways. One wire direct to the Alt and the other wire direct to the battery. On my friends old Chevy pickup wr mounted this inside a gutted out external regulator box. https://nz.rs-online.com/web/p/schottky-diodes-rectifiers/8184553 This has a forward drop of 0.83v You need to measure the voltage drop across your ammeter [with lights / heater fan etc on] If the voltage drop across your ammeter is greater [eg 1.0v] use a 150a Vishay diode. The 80a diode is actually safer but the ammeter wont reach full readings before the diode equilibrium occurs
Am I missing something, please let me know if I am? I just ran a wire between the two terminals of the ammeter on my AVATAR to carry part of the current around the ammeter. I selected the wire heavy enough to carry all of the load in case the ammeter failed. I made the wire long enough to have the same resistance as the ammeter resulting in a reading that is half of the actual amperage, thus the loop in the wire. I just double the reading in my mind when I look at the ammeter. I am running a generator (I don't like the looks of an alternator), if I had to allow for more current I would just have changed to length (thus resistance) of the wire. Charlie Stephens
I like Charlie's solution, it's simple and does what's needed. The numbers on the ammeter won't be correct, but that doesn't really matter, you just want to know if juice is going into or out of the battery, and (relatively) how quickly.
Why ???? When there is a shitload less work making a shunt [and you don't really need to crawl under the dash] ----------------------------------------------------------------------------- @deucepickup32 @Charlie Stephens @squirrel The diode shunt does not alter the values of the ammeter. It acts like an "electrical relief valve" when the amp draw/flow is greater than the ammeter's capability A simplified explanation here.............. The Diode has a fixed value forward drop [eg: 1v] so 12v in = 11v out or 13.5v in 12.5v out Whereas the ammeter has a non-linear voltage drop. And usually the max voltage drop is about 1v to 1.15v at full values. 99% of the time the ammeter current is below 10a so the voltage drop is very little [so it's business as usual] The diode will not flow any current because it is met with a greater voltage on the output side when the amps across the ammeter increase , so does the voltage drop. It gets to a point of equilibrium where both the ammeter AND the diode flow current. As the amps increase even more the ammeter stays at this value and all the extra excess flows via the diode Electricity takes the path of least resistance. With normal usage [20a or less depending on the ammeter] the ammeter shunt might as well not be there. It is only needed for protection in high current flow situations like recharging a flat battery etc. @Charlie Stephens an ammeter has a built in shunt, by adding a wire you have altered this resistance. Whereas a diode acts like a fixed value on/off switch [simplified term] The diode is only used for its forward drop values [voltage needed to allow flow] not for it's one way blocking values We made one on my friend Chris's Chevy pickup [he didn't want to butcher the dash AND he likes all his gauges to be working] If you can buy a cheap "off the shelf" voltmeter solution , I would always recommend this [but some of us don't want to butcher our "orphan" vehicles
so...if the diode is bypassing the ammeter, you have no indication that there is way too much current flowing through the wires....
The diode isn't bypassing the ammeter until the voltage drop across the ammeter is equal / exceeds the forward drop of the diode. [the path of least resistance is via the ammeter] Usually this happens when the ammeter is near maxxed out [this voltage drop can be measured] We found the max voltage drop on an early Chevy pickup ammeter was 1.15v drop. The forward drop of the diode we used was 1.12v to "open" it to allow flow. So just shy of the ammeter being fully pegged to the limit, both were sharing current. Any more current would go through the diode [that could handle 100a] In a low current situation [eg: a 0.3v voltage drop] there could be 12.5v on one side of the ammeter and 12.2v on the other. But the diode would have 12.5v on one side and 11.38v on the other [1.12v forward drop] It cannot flow current [11.38v into 12.2v] 99% of the time the ammeter would read it's normal charge and discharge. We didn't care if the ammeter would read the excess current, we just didn't want to fry it. Putting a larger alternator on a vehicle will not fry the ammeter unless there is load [eg: after being jump started from a flat battery] This example the diode saves the ammeter [and wiring] Electricity always flows the path of least resistance
I used the same circuit without the diodes. The ammeter still works and it doesn't peg itself when the amp output is high. It's just a parallel cicuit with the same gauge wire so you are cutting the amperage through the ammeter in half. In my case that worked fine. You can add more shunt legs (parallel circuits) to cut the current through the meter. You really need to find out what the max output of your alternator is. For instance; if the max output is 60 amps, you would want two circuits parallel to the ammeter with the same gauge wire. Each wire would then carry 20 amps at max alternator output.
So you have to do math in your head while driving to figure out the charge or discharge through the gauge. With a diode shunt it does not affect the values of the ammeter while the voltage drop is less than the diode forward drop. Eg: if a 20a ammeter is reading 15a charge, that is how much is flowing back to the battery. If it gets to 20a is flowing it is reading 20a BUT when 40a is flowing to the battery the ammeter is still only reading 20a. When the battery gets recharged, and the load drops back to less than 20a the ammeter reads as per normal.[eg: 10a] The only true way to get 100% accuracy is to have a matching ammeter to the charging system. So chose your poison?? Do you want it 100% accurate within a normal range [most of the time] Or do you want it partially accurate 100% of the time The diode shunt irrelevant to the size of the alternator [as long as the shunt wire can handle the excess of amp load] . Also an 85a alternator doesn't rob any more engine power than a 35a alternator when the battery is fully charged. But the larger one will recharge a lot faster when the demand is placed on it [this requires power]. I observed this on my Lotus Cortina [a lot of these old timer gurus would not run a charging system on 4 cylinder Fords] I needed a charging system for electronic ignition. So I installed the smallest alternator that was available. Eventually I swapped to a larger one . I noticed that from the time I started the engine in the pits to the time I was sitting at the start line was enough to fully recharge the battery The lap times were consistent between both.
Wow….lots of great responses. As per my initial post, I want to use the gage and don’t need a fire under the dash. I have researched many options and ran through many scenarios. I bought a used early 60’s ford gen, which I was hoping to use but it’s trashed internally, armature bearing end is undersized. I would rather use a gen then the alternator, thus alleviating the amp gage problem. Plus the gen would fit the look that I want. Living in Canada and on an island does limit resources. Thanks for the input.
I don't think I would risk using that ammeter without some circuit modifications with an early 60's Ford generator. That generator will probably exceed 20 amps at startup or when you are trying to charge a low battery. I shudder to think how difficult it would be to find a replacement ammeter. Charlie Stephens
That's true if you want a really accurate gauge. However, as long as the gauge is on the plus side, the battery is being charged. That's all I worry about. If it goes negative and stays that way, you have a problem. Remember you are watching current being fed to the battery or taken away.
I like the idea of the diode shunt. But I can look at that Vishay diode and have no idea even how to connect it to the wire. How about a tech article as far as putting it together, and what amp measurements or voltage drops need to be taken into consideration, and where. Or can we assume that if the alternator output is less than 100 amps, those two diodes are the ones to use?
OK........so tell us how to wire that up! (please). Most electric stuff with me...always starts with a head scratch. Sorry Thx 6sally6
??????????????????????? I'll do a tech article if I ever make one again. [the last one I made was in 2017 ] I got the idea from a friend who played around with Chryslers , he was a bit of a "mad scientist" that played with this sort of thing. [originally he made a manual bypass shunt with an on/off switch in it, for excess cranking] But the wiring [connecting ] is very simple. A wire at each end of the diode that is connected to the Alt output / Battery. You can use one diode as long as it is flowing towards the battery. We used 2 diodes back to back to make the shunt not polarity dependant AND as a safety backup if the ammeter totally failed Ammeters usually NEVER go full discharge [eg 20a] 20a x 12v = 240w so if you had 2 x 120w sealed beam headlights on full beam it would show 20a discharge with the engine not running. If you look at the schematic in post # 2 the Alt / Gen goes on the upstream side of the ammeter, so any heavy loads [eg heater, headlights etc] are being fed /maintained by the charging system. Apart from the shunt, this is a standard layout for automotive ammeters. The Ammeter only measures battery charge/discharge. So any major battery discharge [eg cranking a stubborn engine] is the only time the ammeter sees excess current. I used 2 x phenolic blocks approx. 1/4" thick and 2" x 3" size. In one block I drilled 2 holes [countersunk] through and used countersunk cap screws up from underneath. The 2 screws had the diodes [back to back] with small tube spacers. Under the nuts [of the screws]the wires were attached with "eyes" on them. The 2nd phenolic spacer was glued under the other to prevent the cap screw heads short circuiting It is important that none of these components short circuit.
Stewart Warner made high amp , amp gauges that we used on Firetrucks with 200+ amp alternators. The current actually goes through a shunt, which was really metal bridge. Then two small wires went to the gauge and the gauge measured how many amps went through the shunt/ bridge. One gauge wire was mounted on one end of this shunt , the other wire was in a slot on the other end and could be adjusted for accuracy! It appears to me a person could make that slot longer or drill a hole closer to the first wire to get your 20 amp reading while actually putting 40/50 amps into the battery. And it would indicate amps, small or large! Just a thought……might work! It kinda looks like this, I got this picture off Amazon…..it may even work. Bones
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