* * * The Minimal Ammeter? * * *

[safe]

imo a racer would prefer a buzzer that warns of HIGH MOTOR TEMP

That would be good too...

I really want to know if this simple "trickle" approach could work. Why wouldn't this work?

It wouldn't inhibit the current across the direct path... it would not increase the voltage drop... but it might allow a small current to light up the LED...

 

[Matt Gruber]

LED NO WAY.
do u have dislexia?(sp?)
i am at loss as to Y U Cant read a DVM.

 

[fechter]

You can't get ANY current to go through the led unless the voltage across it is over ~2v. If you used an incandescent lamp that was made for 1/2v, it might glow dimly. An analog meter only takes millivolts to read full scale.

Now a good motor thermometer is something I don't have.

 

[safe]

You can't get ANY current to go through the led unless the voltage across it is over ~2v.

So an "alternative path" will get NO CURRENT at all even if the main "shortest path" the "line of least resistance" already has a large voltage and current in it? (it's "full" so to speak)

I figured that some "excess" would develop as the voltage and current increased. At some point (using a river analogy) if the main channel for a river is flooded then eventually the tributaries get a little "surplus" water flowing into them. When the main channel drains out (current and voltage goes down) then the tributaries would empty. Sort of the "flood plain" concept.... the light goes on only during a flood...

Is this analogy incorrect?

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html#c1

 

[Matt Gruber]

perhaps u need bifocals?
i had custom single vison glasses made years ago. took 5 tries to get it right. i can only use 2 of 5 pairs. 1 for driving, 1 for tv.

 

[safe]

perhaps u need bifocals?

I'm trying to understand the principles and you're making jokes.

If the analogy of electricity to water is a good analogy then if a wire is holding a high voltage and current then one "might" make a comparison to a river at flood stage. If the wires were configured so that the "alternative path" went AROUND the motor then you would have a "short circuit" and you would be pretty sure to get the LED to light up! (and probably blow up) But what I'm trying to figure out is why it's impossible to get a 2 Volt LED to run off of a "shunt like" device.

Do you see the theory? (or the misunderstanding on my part?)

It "should" flood if the analogy is good and there "should" be some kind of "trickle" current that wants to find a "less crowded" path.

Aren't there physics to wires that are already loaded? (that are different than a wire that is unloaded)

What's a semi-conductor?

 

[safe]

Could a Hall Effects sensor generate 2 Volts?

 

[Matt Gruber]

Why cant safe read a DVM?

He has electric coming out of his bathroom sink, and water squirts out of his battery!

When he tries to wash his face, he gets a shock.

When he tries to ride his bike, he gets his face washed.

 

[Lowell]

Could a Hall Effects sensor generate 2 Volts?

Hall sensors are usually higher voltage than that, but I don't think there's much current drive.

I was just installing some aftermarket engine electronics in a customers car, and it has multi hue lighting that can change shade through the whole color spectrum. Might be an interesting way to do an ammeter by color temperature...

 

[fechter]

Matt, I sense your frustration here.
Safe is like a lot of folks I deal with on a daily basis at my work. Things have to be explained in a way that makes sense to them, not necessarily the way I see them. Human nature.

Safe, to use the water analogy, the path of the LED circuit has an uphill section of 2 volts that must be overcome before any water will pass over the top of the dam. 2 volts worth of voltage drop is an unacceptable loss for a shunt. A graph of voltage vs. current for a typical LED would be good to look at (sorry I don't have one).

A hall effect sensor would work great. Zero additional loss in the wire, just pass the wire through the sensor. Output levels vary, but most are powered by 5v, so it might be able to drive a LED directly. It does require a 5v supply to operate, but most controllers have a handy 5v supply that runs the throttle.

 

[Matt Gruber]

Tip: to make a $2.99 DVM more ev friendly, solder a short piece of 8 10 or 12 AWG directly on the internal shunt.it can read to 50a(or whatever) instead of just 10-20a. Now, that's a minimal ammeter for $3.
Calibrate the shunt just like with a controller.

 

[fechter]

Ahh.. good idea. For $3, that's much cheaper than an analog meter.
You could put the meter on the mv DC setting and measure voltage drop across the shunt.

 

[safe]

DVM - "Dangerous Viper Machine"?

No, just kidding... how about "Digital Voltage Meter"?

I definitely wouldn't want that.. at least not in the context of this thread which is about finding the simple way to get an LED light to go on.

So it sounds like a Hall Effects sensor is the better way since it makes no sense to put in great wiring to lower resistance and then blow all that with a shunt that increases it.

Something still doesn't "feel right" with the answer about parallel wires... for some reason it just seems that if you have parallel wires that even though one path has an LED in it you would get some "trickle" of energy, but I guess it's an "all or nothing" situation. (sort of like a levee breaking during hurricane Katrina) Oh well...

 

[Leeps]

http://www.allegromicro.com/hall/currentsensor.asp these are some nice hall effect devices, they work on dc. Im not sure what the output is i never checked.
Fechter is right but its worse than he's making it sound, for example if you have a led that takes 2.5 volts hooked up in parallel with a shunt like you have (assuming the shunt is capable of dropping full volts, its imaginary). You would have no current at 2 volts, microamps maybe nanoamps (in any case no light) at 2.4 volts, at 2.5 volts it will come on, at 2.6 volts too much current very bright light. Somewhere from 2.6 to 2.8 volts the led will catch fire. Its a diode drop and diode drops are very very steep its not an ohmic drop.
What your saying is very doable if i were you i would amplify the output of the shunt, hall effect, whatever. Then i would pass that into the base of a transistor with the collecter going to the cathode of the led and the emitter going to a resistor to ground. This would give you proportional current through the led based on signal, exactly what you wanted in the original post. But hey thats me.

Joe

 

[Matt Gruber]

From

.......Use in LED to the shunt and you would know while riding that if it glowed really bright that you are using a lot of current and need to either back off the throttle or downshift.

YOUR PREMISE IS FALSE:

Out of the corner of your eye you would know exactly what was going on.. ******this is a pipe dream
[/b][/color]

Need a DVM to know exactly.......

 

[Leeps]

Hey if thats what he wants, well thats what he wants, a light that gets brighter or dimer based on current. It is an ammeter, it is minimal, you dont know exactly whats going on, but you do get an idea. Its justifiable
Joe

 

[bobmcree]

if you really want a cheap simple indicator of current, that can be read at a glance out of the corner of your eye, how about a single led that flashes at a rate controlled by the current? 10 times a second for 10A, etc...

you could use a shunt and an amplifier or a hall sensor to produce a 5v signal from your max current, then use that voltage to charge the cap in a 555. the flash rate would be controlled by the current draw.

-bob

 

[Matt Gruber]

Hey if thats what he wants, well thats what he wants, a light that gets brighter or dimer based on current. It is an ammeter, it is minimal, you dont know exactly whats going on, but you do get an idea. Its justifiable
Joe

imo it could work at night only. and then a 10 year old could ride better/smarter without it(after DVM training).
But, if that's what he wants....(every race needs losers) :?

 

[xyster]

every race needs losers

And the more mistakes Safe makes, the fewer the rest of us will. So go Safe! You're idea is the best! You win this thread! Just don't forget to show us how well it works!

 

[safe]

Whatever happened to the spirit of "Boldly Going Where No Man Has Gone Before?"

The whole point of a discussion thread it to think and exchange the possibilities that those thoughts might suggest. There are no "sacred" technologies... all things pass away with time as they are replaced with something else or if they are really good sometimes they stay the same. It's called "evolution". Only that which "fits in well" (is useful) survives.

Anyway... I like that idea of the blinking light. When you are using very little current it blinks very infrequently, but as the current draw increases it speeds up to the point that it's just a bright light. With no current it's off completely. Now that's what you call "good thinking". Who the heck is this "new guy"? He seems pretty smart. 8)

 

[Matt Gruber]

last year i finally put a DVM on my psev. i wrote down what i learned. it took ~1/2 hour. then i took the DVM off.
safe, we all wasted more time typing, than the actual test takes.

Safe, u ignorantly harped for 3mos about how all motors need gears, then stumbled upon a bad controller, only
because u are unwilling to use a DVM.
enough is enough.

 

[TylerDurden]

Whatever happened to the spirit of "Boldly Going Where No Man Has Gone Before?"

It has turned into:

Baldly Going Where No Man Has Gone Before

and

Badly Going Where No Man Has Gone Before


(Let's not have an age poll, thank you very muchhhhhh...)

 

[xyster]

Let's not have an age poll, thank you very muchhhhhh...

From your avatar you appear pretty young. Can we have a "guess TD's age" poll?

 

[bobmcree]

Safe's quote= But what I'm trying to figure out is why it's impossible to get a 2 Volt LED to run off of a "shunt like" device.

part of the answer to this question that has to do with the fact that an led is a band gap device. in a normal light bulb a tiny bit of voltage will force a tiny bit of current and it will light up just a little. of course a light bulb cannot be used with a shunt because the low impedance of the light bulb would cause a big error, and because it takes too much voltage to run a light bulb.

in a band gap device there is a wall across which the carriers cannot jump until they achieve an electrical potential higher than the other side of the wall by the band gap. a regular diode has a gap of a volt or so, a schottky diode .5v, and zeners have a wide range of voltage. the point here is that you cannot get current to flow through an LED until you exceed the band gap. There are LEDs with a gap of 1.1v, but that is still really too much of a shunt to introduce into your system. you would not want to throw away a volt to measure current, and the LED would not start to light until the drop across the shunt is over 1.1v.

When the current increases so that the voltage across your shunt would get a bit higher, there is no mechanism to limit the current through the LED and it would quickly burn out. For this reason, you need a current limiting stage or resistor in series with the LED. When you calculate the current in an LED, you use Ohm's law, but you have to subtract out the band gap voltage when there is a semiconductor involved.

For example, let's say you want the LED to get 10 ma when the voltage hits 5v, and you are using a 2V LED. I=E/R so .010A=(5V-2V)/R 1/R=.01/3 R=333 ohms. The resistor does nothing until the voltage exceeds 2V (except for very small leakage current that does not result in any light). Once the voltage exceeds 2V (or whatever the LED voltage) the resistor will limit the current.

it is possible that at high current level you could have enough voltage from one end of your battery cable to the other to light an LED. You can find out by putting your meter on the positive battery terminal and the positive input to the controller and measuring the voltage when you feed power to the motor. If you see more than 1.1v your idea of using an LED as a simple ammeter could work, and this will be an example that might help understand current flow in a semiconductor circuit.

You only need a very small wire to the LED from the ends of the positive battery cable. you are in effect using the battery cable as the shunt. the small current used by the LED will not load down the shunt or require wire any larger than the smallest you have. #28 would be adequate. When the voltage drop in the battery cable is less than the band gap of the diode, only the leakage current will flow through the small wire, and almost all the current will go through the battery cable. Once the voltage drop exceeds the LED drop, current will start to flow through the LED, only a few milliamps at first, but as the voltage rises a few tenths of a volt the current will climb quickly without a limiting resistor and burn out the diode.

If you measured 2V drop in your battery cable at maximum current, and you have a diode with 1.1v drop, you will want the diode to be bright at 2v so if we let it have 20 ma that will be 20ma. through r ohms with .9v or
R=E/I R=.9/.02 R=45 ohms. 47 ohms is the standard value so if there is 2v drop in your cable that could make your meter LED idea work. It would also mean your cable is not heavy enough. This example was more to help understand what is happening than to make it work.

A semiconductor is a material like silicon or germanium that can be "doped" with other chemicals so that current flow can be controlled by current or voltage control. A diode is the simplest semiconductor device, and a LED is a diode that liberates photons of visible light when the carriers jump across the band gap.

-bob





Do you see the theory? (or the misunderstanding on my part?)

It "should" flood if the analogy is good and there "should" be some kind of "trickle" current that wants to find a "less crowded" path.

Aren't there physics to wires that are already loaded? (that are different than a wire that is unloaded)

What's a semi-conductor?
[/b][/color][/size][/quote]

 

[lemmiwinks]

Tip: to make a $2.99 DVM more ev friendly, solder a short piece of 8 10 or 12 AWG directly on the internal shunt.it can read to 50a(or whatever) instead of just 10-20a. Now, that's a minimal ammeter for $3.
Calibrate the shunt just like with a controller.

I burned the track off my cheapo DVM and jumpered it like you describe, however it can only read 19.9A before it displays OL which I take to indicate overload.