How would you measure current ?
jbalat
Posts: 96
I have heard of people using a very small resistor eg. 2 ohms to somehow measure the current ?
Does anyone have a circuit and code as to how this can be done for currents up to 30 Amps (12-14v) using the propellor ?
I was thinking of using this in my car as a PWM to power my Hydrogen Generator
Post Edited (jbalat) : 6/21/2010 4:54:37 AM GMT
Does anyone have a circuit and code as to how this can be done for currents up to 30 Amps (12-14v) using the propellor ?
I was thinking of using this in my car as a PWM to power my Hydrogen Generator
Post Edited (jbalat) : 6/21/2010 4:54:37 AM GMT
Comments
Try googling for a "shunt", for example this one: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_162309_-1
What kind of hydrogen generator are you using in your car? I've read about some devices that are supposed to electrolyze water into hydrogen which is then sucked into the intake and burned along with the normal combustion, but I'm pretty skeptical that those devices produce a meaningful amount of hydrogen.
for measuring large currents. It looks like nothing
more than a short metal rod bent into a U shape
with 2 pins on it to plug it in.
Yep that would be the one... Still only achieving minimal savings < 10% but trying hard
www.youtube.com/watch?v=lo4w7Z5WJI0
At a fixed duty cycle on the rc speed controller / pwm, the current will tend to runaway as the water/electrolyte in the cell warms up. I then wanted to reduce the duty cycle to maintain a constant current of say 15 amps..
So back to measuring the current with the propeller.. Would I use similar code to measuring a 10k pot and a similar circuit with a capacitor that charges and discharges ?
search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=DN6849SE-ND
I prefer surface mount and running a trace up the middle of the IC. Page 14 of the data sheet
search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=391-1001-ND
The shunt I linked had a voltage rating of 1mv per amp. So 50 amps = 50 millivolts difference from one side of the shunt to the other. I would think to get the accuracy you're looking for that you might need an ADC converter and/or an op-amp. It's been so long since I had to work with measuring such small voltages that I don't recall the specifics.
I once had a friend who was working on designing a battery monitor for his RV. I don't recall the specifics, but it seems like he found specific chips that were designed to interface with the shunt to do D/A conversion and voltage monitoring.
One such device is this one: http://focus.ti.com/lit/ds/symlink/ina209.pdf
It monitors the shunt and the microcontroller can talk to it via i2c. I've never used one, but it looks promising.
Post Edited (smbaker) : 6/21/2010 5:38:30 PM GMT
www.fuel-saver.org/Thread-How-to-make-a-PWM?pid=30186#pid30186
Or download it here:
www.keithley.com/knowledgecenter
Up to now I wasn't able to find the downloadable pdf... this is great news... I'll post the sandbox..
To create a current to voltage converter will cost about $1 in components using a shunt and a voltage amplifier, possibly with a quad op amp like a LM324. It is not hugely complex, but it does involve some basic electronics and physics. The only problem is, once you understand that sort of basic electronics and physics, you will also understand that HHO does not work. And if you don't understand the basic physics, then HHO seems like the best thing since sliced bread. But you won't be able to get it to work because you don't understand it. Especially if you can't measure anything properly so you will be always be able to convince yourself that it does indeed work. And if you do understand it, you will have the skills to measure things properly and prove to yourself it does not work.
Sorry to be confusing, but I've just finished reading "Catch 22". A truly brilliant book.
Ok, so as to be mildly helpful, not that I want to help perpetuate the myth of HHO in any way, grab a cheapie multimeter with a 10 amp range, get a car battery, get a 2 ohm 10W resistor in series with the multimeter on the 10 amp range (usually a special socket), put it across the car battery and measure the amps. You should get around 6 amps. Now get some thick wire, maybe some mains wire, get a nominal length, put it in series with your resistor, and measure the millivolts across the wire with another cheapie multimeter. You now know the volts and the amps, so you can calculate the resistance of your shunt with V=IR. All connections should be soldered, as screw connections vary too much. With your new shunt (which will have cost you only a few cents) knowing its resistance, you can generate millivolts from amps, without throwing away too much energy as heat (which your 2 ohm resistor is very good at, and throwing away energy is not the point of this exercise, right?). If you can get that to work, you can use an op amp to multiply the millivolts to volts, suitable for measuring by a propeller or similar.
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www.smarthome.viviti.com/propeller
Post Edited (Dr_Acula) : 6/22/2010 12:23:25 PM GMT
2 ohms across a 12 volt car battery is 6 amps. (Ohms Law I = V / R)
Power dissipation in a resistor is P = IV that's 72 watts in this case.
That 10W resistor is going to get mighty red !!
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For me, the past is not over yet.
Browns gas could be a component of the INFINTIE IMPROPABILITY DRIVE and therefore there a chance of 10^-3.154.580.184.215.522 it will work. So why not find a way to measure small currents with the propeller.
So, how to make a precise shunt?
On every old pc board, hard drive, cd-rom you can identify a lot of resistors with 56 or 33 Ohms, or in that range. The are uses to control the inpedance of the driver lines. Normally 4 resistors in one SMD housing.
The show a quite well precision and nov, as there are so many, You can parallel as much as necessary to have any resistance it needs to measure every current. Also, in paralleling them, the power dissipation capability increases.
So, in the end, a lot of experience can be gained by building the H-thing, and that justifies all the efforts necessary to fail!
The way is the goal!
ErNa
See everybody at UPEC
Inpired to google I just found: en.wikipedia.org/wiki/The_Hitchhiker%27s_Guide_to_the_Galaxy_%28novel%29
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Hello Rest Of The World
Hello Debris
Install a propeller and blow them away
Post Edited (ErNa) : 6/22/2010 1:02:06 PM GMT
For the purposes of a quick test for a few seconds a 10W resistor should be ok, but, yes, it will get hot very quickly. The problem is that if you are trying to measure 30A you need something in that ballpark to measure the shunt voltage. A 100R may not give enough current to produce enough millivolts to give something meaningful, and if you do go for something that does give enough millivolts then the eventual shunt will end up with a higher than necessary resistance.
12 ohms maybe?
Or - you could buy a proper shunt...
Hmm - one answer might be to use a 1 ohm 10W resistor and a fully charged nicad battery.
I'm just thinking back to the many experiments I did building an active glow plug controller for a model plane, where you pulsed the plug, then measured the resistance in a bridge and calculated the resistance with respect to a fixed resistance and used resistance=temperature. It worked very well. The 'shunt' was 30cm of automative wire, and it took a lot of experiments to find that.
Actually, thinking of a 'safe' resistor, a 12V 60W 5A car headlamp could work quite well as it is designed to dissipate 60 watts. Put in series with the multimeter on the 10A range and the shunt wire.
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www.smarthome.viviti.com/propeller
Post Edited (Dr_Acula) : 6/22/2010 1:36:19 PM GMT
Attention: H is extremely explosive when collected.
The best idea is: find another reason to work with the propeller
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cmapspublic3.ihmc.us:80/servlet/SBReadResourceServlet?rid=1181572927203_421963583_5511&partName=htmltext
Hello Rest Of The World
Hello Debris
Install a propeller and blow them away
v=voltage, I= current, R= resistance
Else use Ammeter.
"To create a current to voltage converter will cost about $1 in components using a shunt and a voltage amplifier, possibly with a quad op amp like a LM324. It is not hugely complex, but it does involve some basic electronics and physics. The only problem is, once you understand that sort of basic electronics and physics, you will also understand that HHO does not work. And if you don't understand the basic physics, then HHO seems like the best thing since sliced bread. But you won't be able to get it to work because you don't understand it. Especially if you can't measure anything properly so you will be always be able to convince yourself that it does indeed work. And if you do understand it, you will have the skills to measure things properly and prove to yourself it does not work."
Not to flame you but nothing like being a little condescending, sure glad we don't burn people anymore for heresy. In my experiecne people in white coats due tend to feel superior to the rest of us mere mortals. You probably believe that the universe is also electrically neutral and the sun is a big hydrogen fusion machine also. What a joke.
dp
Personally, I don't think there's anything wrong with the OP's HHO experimentation, even though I also believe this HHO stuff is mostly hype. Experimentation always provides the opportunity for a learning experience. The problem in this particular case is to get an objective measurement of what (if any) improvement is made by the HHO contribution. Since the vehicle is driven by a human, and that human has a vested stake in seeing the experiment be a success, that the human's natural inclination will be to drive more conservatively. The same is said to happen with Prius owners who fixate on the fuel economy meter while driving.
It'd be nice if someone could throw together an Engine and Dyno with one of these HHO generators attached and conclusively disprove (or "prove", I'm open-minded!) the effectiveness of the HHO generator.
I just want to make sure I know that it doesnt work because it really doesnt work and not that I didnt understand enough to make it work...
Anyway this is what I am proposing for my pwm, and I have some adc code to measure voltage, just need to know where to add the shunt and whether I need a transistor to boost the voltage to no more than 3v?
I want to believe it works but need to prove it for myself. Have a look at some of the emails I got back from a HHO Guru 'Les'
www.fuel-saver.org/Thread-Tuning-for-Performance?pid=31944#pid31944
Post Edited (jbalat) : 6/23/2010 4:47:06 AM GMT
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Why did I think a new, more challenging, job was a good idea ??
But the propeller is real! It is know for a long time, that parallel tasks can be serialized, that serialization is a challenge and therefor the mainstream hardware and software engineers focus on handling this challenge and are proud on that.
Chip went onother way: He had the vision and experience that the natural way to solve parallel tasks is a parallel processor array. And he spend the money to have is realized.
So that is the difference: someone asks You for money to give you something valuable. And someone asks you just to fool you. In that I would prefer to give money to a beggar. If he needs it.
To measure current is a challenge on its own. If you master this challenge, its worth any investment. But don't give money to those, that promize you a little part of a perpetual mobile!
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cmapspublic3.ihmc.us:80/servlet/SBReadResourceServlet?rid=1181572927203_421963583_5511&partName=htmltext
Hello Rest Of The World
Hello Debris
Install a propeller and blow them away
Please feed the bear! A full bear will run your lawnmower longer and it'll be far less dangerous and unpredictable than an empty bear!
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
Is there a problem placing a multimeter set for Amperage placed in series with the portion of the circuit under study?
30 amps is too much current to try and pass through most multi-meters. You need to have some 14g wire or better to handle that kind of current and most meter leads are waaaay too tiny.
Responding to the original question, I’ve had excellent results making analog ammeter shunts from brass shimstock. Brass has a specific resistance of 33 ohms/mil²/ft, or 2.75 ohm/sq-mil/in.
For a standard 50mV movement installed in an experimental 12V aircraft charging system, the shunt resistance would be 0.050 / 30 = 0.00168 ohms.
A three-inch long piece of .020 brass shim stock, trimmed to be .250 inches wide, would produce a 50 mV drop at 30 amps. I would always test these home-made shunts by powering up a 250W landing light to make sure that the thing didn’t burn up or anything, and I’d also fan-fold them to sit neatly across the meter terminals and exposed to free air. If you use this idea in this application you’re going to have to insulate it which could create heat buildup.
The shunt could be installed in any of the high current lines. I’d be inclined to try putting it in the negative supply to the gas generator, with one end of the shunt attached to the motor controller negative output and the other side attached to the gas generator negative input and the input network of the ADC on the I/O pins of the propeller.
, this just wont be viable. Its a simple matter of scale. The whole point of this HHO (which is a BS name - HHO scientifically is read as H2O)·system is to electrolyze water into 2 parts hydrogen and one part water. funny thing is that when the two gasses are exposed to one another theres a chemical reaction·+ electrical discharge·that recombines them (whole opperating theory behind fuel cells),·so all your really doing is injecting water vapor into your engine which in itself may make the engine run slightly different and have the effect of absorbing some heat from the intake charge which reduces knock.
· You say this HHO device draws 30A? How much power is supposed to be gained when using this thing? Your HHO maker consumes 432W? ([url=mailto:30A@14.4V]30A@14.4V[/url])·no wonder it gets hot. So it consumes 0.58 HP.. unless it can generate more HP than its consuming your not gaining any fuel mileage. And in any case it's not making your car more efficienton gasoline, its just displacing the gasoline and oxygen with·another fuel.. personally if you wanted to sell me one of these, show me first that this HHO gas being produced even burns in a labratory test tube, then show me its producing enough CFM of this HHO gas mix to even suppliment combustion @ speed.
· My $0.02, your more likely to change your diving habbits to be more efficient than with this thing installed (read·sugar pill). And if you want to drive around for fnearly free, go get yourself a veg oil kit·for a diesel and then sell your gasoline car and buy a diesel, run the diesel off of processed deep fryer oil. Then you'll achieve 100% gasoline efficiency.
As far as measuring current via voltage across a shunt.. Since you need to produce a voltage drop across whats basically a piece of wire the shorter your shunt, the more precise your·shunt voltage amplifier has to be in order to calculate the current through that shunt. A differential amplifier or even an instrumentation amplifier would best suit your needs, since a piece of wire (read - shunt) say 24" long, even at 30A will likely only produce a few millivolts at the ends. Why a differential amp? CMMR - Common Mode Rejection Ratio. What happens to both inputs is canceled at the output, ie- noise. So this amplifier is better at amplifying accurately your microvolt/mV DC signal from your shunt.
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Quicker answers in the #propeller chat channel on freenode.net. Don't know squat about IRC? Download Pigin! So easy a caveman could do it...
http://folding.stanford.edu/ - Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.
·Of course with under hood electronics, you have alot of things to compensate for, large temperature swings (over 220F durring the summer), large ammounts of RFI, EMI, and also moisture.. The best mounting position is outside the engine bay on an upper frame rail inside the fender and behind the splash shield inside a sealed metal enclosure. The location provides good temperature stability, RFI protection (the steel of the upper rail is double/tripple layered in areas - serves as a shield), and at least alittle bit less moisture exposure. If you were to run the small gauge wires from your·high current wire in a twisted pair and/or shielded·into the interior, your enclosure can be a simple aluminum one, you would still get good temperature stability (climate controlled), and you wouldn't have to worry about moisture so much.
Just a thought..
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Quicker answers in the #propeller chat channel on freenode.net. Don't know squat about IRC? Download Pigin! So easy a caveman could do it...
http://folding.stanford.edu/ - Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.
Post Edited (RinksCustoms) : 7/9/2010 4:38:49 AM GMT
Not sure where you got those numbers, but I looked up the resistance of various gauges of copper wire and calculated the voltage drop for one meter of each gauge at 30Amps. For 30A I would use AWG12 which would give you a 0.162V full scale signal.
I used AWG14 for those 5V supplies and had about 0.078V signal to work with. Don't recall the op amp used but it had better specs than a 741. The gain (~20) was adjusted to give 1.5V at 15A (.01A/bit with an 8 bit ADC). The sense wire was unshielded twisted pair.
AWG R/KM R/M V@30A
12 5.4 0.0054 0.162
14 8.54 0.00854 0.2562
16 13.0 0.013 0.39
18 21.9 0.0219 0.657
Post Edit - @kwinn, looking over the wiki i was looking at, i was looking at the wrong resistivity tables, sorry.
This subject interests me because i have a few uses for measuring currents, in particulary how much rms/peak current my Memphis audio amp, which in its manual says to include a 150-175A fuse/circuitbreaker on the main power cable (4ga). for components/circuits controlled by a mosfet, couldn't you measure the Vf of the mosfet switching the circuit? A power Mosfet will have a few milliohms of resistance when its turned on.. Couldn't a resistance in a length of wire be measured with a·wheatstone bridge and an oscilliscope? Using kirchoffs laws and ohms law to find the resistance of the wire?
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Quicker answers in the #propeller chat channel on freenode.net. Don't know squat about IRC? Download Pigin! So easy a caveman could do it...
http://folding.stanford.edu/ - Donating some CPU/GPU downtime just might lead to a cure for cancer! My team stats.
Post Edited (RinksCustoms) : 7/9/2010 6:47:03 PM GMT
My friend was super excited about this a couple years back, so he asked me for some help making something for proof of concept. We made a very simple device: two metal rods about 4 inches away from each other in a buck of water. Powered by a single variable voltage power supply we got it to draw about 9A @ 15V with good results. We then combined a couple power supplies to make it draw 28A @ 30V. At this point it was boiling like a cauldron. My friend got excited and put a lighter near the top of the bucket. The escaping gas was quite explosive.
The problems with our experiment were the following:
840W of power is a lot to control, and a lot to produce, as said before, a motor would have to produce more than the 1.13HP required to break even.
Such massive amounts of heat were generated that our water was nearly reaching boiling temperature.
The electrolysis quickly (15 minutes) created a thick layer of corrosion (oxidation) on one of the two poles (don't remember which). This increased the resistance on the rod and the reaction would slowly die down to a simmer.
Mixing the correct amount of electrolyte (we tried a few different ones, but ended up just using baking soda) was a guessing game, the amount changed drastically with voltage and ambient and water temperature.
It was a fun experiment and I was surprised that the gas stayed separated long enough for the gas to explode as it boiled up. We didn't test to see if the gas would stay separated or re-condense.
Another reason I heard that this doesn't work in cars as many have hoped is that the sensors in the vehicle detect the increased level of introduced gasses and try to compensate by going to a more fuel-rich mode and thus negating the benefits. But successes of vehicles with carburetors (thus fewer or no sensors) are not unusual to find on the net. As well as a few who claim to have fiddled with their sensor settings to keep their vehicle in a normal operating mode.
I like the idea of all this, but I remain skeptical as well. Check out the guy who invented a water powered welder and vehicle (google it).
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
No offense taken. I now see you had used the resistivity tables of materials (Cu = 1.68x10-8) for this calculation rather than the resistance per meter of a particular gauge of copper wire as the basis of your comments. I agree that temperature would affect the reading, but in my case the instrument was in a climate controlled room so the temperature did not fluctuate by much. As long as the calibration was done after the internal temperature stabilized the reading was reasonably accurate.
The resistance of a piece of wire certainly could be measured using a Wheatstone bridge. The Vf of a mosfet could be measured over the range of gate voltages and source/drain currents but it is not a linear relationship so would be difficult to use to measure current. While the resistance of a length of copper wire varies with temperature the effect is relatively small, making it a much simpler choice for measuring currents in most applications.