AEG current metering board

[Gandolf]

Hi,

Here is what I was thinking of for the current meter:



http://extreme-fire.com/Engineering-Information/I-MeterSch.jpg

It just uses the standard parts I use now plus a few more:



A circuit board with screw terminal connectors that could be used for anything.
It would have a direct output for a fancy meter or scope at 125KHz bandwidth.
An average current output.
A peak current output.
Any meter could be used with it.
The output is +- 200 amps at 10mV/amp.
Accuracy would be around 2%.
I could be run by any power source including the gun or it could be totally isolated with a 9V battery.
Real universal and could be in kit form.  Some surface mount parts but they would be easy ones.
I am thinking it would be about $30 or $40 in kit form.

Any thoughts are welcome.

Gandolf

[salamander]

Nice job Gandolf. I never stumbled upon those allegro current sensors. One big advantage in contrast with normal hall effect sensor which you can put over the trace on PCB is that they are calibrated. One remark to the parts used. While it might work, the LM339 is technically a comparator and its outputs are not really designed to do anything else than switch from 0 to Vcc. Also it have open collector output, so the very minimum that should be added are some 4k7 pullups on the outputs. http://www.national.com/ds/LM/LM139.pdf

Just by quick search i found this http://www.national.com/mpf/LM/LMV934.html#Overview Quad amp with rail to rail in and out, while that is not completely necessary in this application i love R-R amps as you have nice voltage span when powered from 5V supply. There can be some switch added to adjust the gain maybe as well.

Edit: also there are programs that turn your soundcard into simple oscilloscope. I saw at least 2 or 3 of them in the past. Here is one for example http://www.zeitnitz.de/Christian/scope_en Iam not sure if the vertical resolution is callibrated on them thought as the the amplifiers in sound cards may wary, but you can do rough calibration using some voltage source with known value.

[Gandolf]

The Allegro sensors are also totally isolated too which is really nice and they have that big current range we like.  They are sort of expensive but they do the job.

Nice the LMV934!  I just pulled the 339 off the top of my head, but nowadays there are really nice new chips.  It would be nice if the amp was a 14pin DIP for a kit but DIP packages are quickly disappearing.  I could offer to solder it down for folks if needed.

The above is all public domain so if anyone else want to make them that is fine with me.  It's not like I need more stuff to do

I will try to get a board going and test it out and all.  I will probably be the biggest user of it for my work.  But I will sell them to anyone that wants them.  

Gandolf

The sound card scope things all are AC coupled if I remember right.  I am thinking that might get in the way. (?)

Obviously, AEG designed guys never sleep either

[salamander]

The sound card scope things all are AC coupled if I remember right.  I am thinking that might get in the way. (?)

True, i forgot about the AC coupling, it will deny you from seeing the value of the current. Still that scope might be usefull to see how the minimum and maximum values are far apart during the cycle and also to count the ROF. Depends on the highpass cutoff frequency at the inputs, but might work. When you offset it by the average value from voltmeter it should be some rough representation of current waveform.

Obviously, AEG designed guys never sleep either

Its 2 PM here 

[Gandolf]

I loaded the scope program you mentioned and it all looked oddly familiar.  And I already had sound card probes for it made on my shelf under the dust 

One thing about working late at night is that you forget things you have already done 

http://team-titanium.com/~airsoft/FreeFPSMeter.pdf

It all really works nice with these 2GHz laptops that are so nice now!!

But for seeing the current, the lack of DC coupling is a killer...  To bad, that would have been too cool!

[salamander]

Cool, that was awesome idea to make the meter that can be made by almost everyone without need of virtially any hardware. I am using simple tube with photodetectors and scope now, but if i would have seen yours few years ago i would definitelly have built it.

Thinking about the DC coupling now. If someone have some PCI sound card the coupling capacitors will be probably external and they can be removed. I know that it all gets integrated on the mainboard nowadays, but it might be possible to get some cheap old soundblaster and mod it.

[Gandolf]

I was wondering if we could have say 10KHz carrier signal like from an LMC555 timer that could be multiplied by the DC level.  Like AM radio does.  That would allow the DC signal to show up on an AC scope as an amplitude varying square wave.  Do you know of any multiplier chips that are easy to use?

Plenty of room on the board for another circuit to do this.  I don't know how well the voltage levels or anything are calibrated on sound cards though.  Could probably have a calibration level easily enough.

[salamander]

Variable gain amplifier can be used for certain, but those are not the most common parts around are used rather in high frequency circuits. Iam going to look for some other circuit.

I found AD633 for example http://www.analog.com/en/other-products/analog-multipliersdividers/ad633/products/product.html
Quite nice chip, only drawback i see is the need of rather high symmetrical supply voltage. Nothing that one 5V to +-12V DC/DC converter can't solve thought. But the parts list starts to pile up

[Gandolf]

Nice chip the AD633!    Not a bad price and very easy to get hold of!

We can just use a 9V battery for the negative supply.  It's cheap and easy.

Of course, these parts can be optional or no-loads for folks that don't need the capability.  But the board could have the pads so it could always be added later too.

I suppose a differential voltmeter might be a nice function to add too to measure battery droops and such.  I guess that would need a second AD633 if you wanted to use the other channel on the computer sound card for it

But that would be almost a full blown AEG electrical test system right there for not a lot of cash!  Full scoping on your computer is then available to anyone who would care!

I will try and whip up a schematic

[metallicafatcat]

In english please?   I cant understand. I really don't know how to word this stupid not enough words in post....

[Gandolf]

Here is a REAL Schematic for the mess!



http://extreme-fire.com/Engineering-Information/AEG-ETB-1.00.jpg

Early first try, mistakes will be abundant

No fancy AM modulation yet for the sound card folks, but I am putting in all the hooks to make it so...

In english please?  Huh? I cant understand. I really don't know how to word this stupid not enough words in post....

Sorry for the EE brainiac S***  But we are "in the zone" now   

Prolly should move this to the SW-Computer sub forum...

[salamander]

Sorry for the EE brainiac S***  But we are "in the zone" now   

Haha, yeah and nice schematic

We can just use a 9V battery for the negative supply.  It's cheap and easy.

I would personally preffer to use just single supply instead supply and battery or 2 batteries. Also if the design is not optimized for battery supply, the 9V batt which have quite small capacity will get discharged easily when you leave this running overnight accidentally. You can use TMA0512D DC/DC converter http://www.datasheetcatalog.com/datasheets_pdf/T/M/A/1/TMA1205D.shtml which provides +-12V 30mA output from single 5V supply. 30mA should be enough as the quescient current of the AD633 is 6mA max. That is when the amp is unloaded, but driving the sound card input should need minimum additional current. Part is not expensive and will pay for itself when you accidentally discharge few 9V batteries

Its good idea to put LC filters at the outputs of the DC/DC converter as they are little noisy. Outputs also don't like direct capacitive loading for some reason as stated in the datasheet. LC filters always worked for me. something like 22uH inductance and 10uF cap will do the job to get quite clean symmetrical voltage supply.

[jomarx]

In english please?   I cant understand. I really don't know how to word this stupid not enough words in post....

They are trying to cook up a device that can measure current for us, has an interface to a scope, a simple multimeter and (hopefully) a soundcard oscilloscope interface.

With this, we can finally know what current our AEGs are drawing. Which is beneficial for choosing lipos, AND/OR helps prevent bricking them (over current draw kills lipos  )

I'm definitely building once the schematics gets finalized.  

[Gandolf]

I was looking for a little DC-DC converter from Digikey (something that would be very easy to get).  I already have the 10uF caps on the lines so adding some surface mount inductors would not be a problem.  Still searching through all the available DC conveters that are inexpensive...  It could be run from the 9V battery side too instead of the 5V regulator.  That might cut down on noise.

I was thinking of this one:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=DCV010512DP-ND

A $12 part but you could just use two batteries or plug this chip in and use one battery
 Maybe a lower power part would be cheaper...

UPDATE:

I think the AM modulator for the PC scope should now be a separate board.  It is complex enough and useful enough to justify a stand alone board.  So that will now be a separate thing.  I am also thinking of having it do real time power as an output and also as a scope signal to the PC sound card...

Here is the metering board now with a few corrections:



http://extreme-fire.com/Engineering-Information/AEG-ETB-1.01.jpg

It has dual differential amplifiers for checking any voltages along with average and peak current.  It can easily be hooked up to do anything and it can take any power source. 

This is looking pretty good to me now.

[salamander]

Interesting how the stuff available on Digikey is so much different than what is availabe here in Europe. I picked some cheap ones for you on the Digikey.

RB-0512D -- 5V supply, dual 12V out
VAT1-S5-D12-SMT -- 5V supply, dual 12V out
VAT1-S12-D12-SMT --  10.8 ~ 13.2V supply, dual 12V out

I just searched there selecting the max power to 2W and +-12V out, there was over 100 results, so you can look for something more appropriate. Someting with like 5-15V input range would be cool, thought that last one i listed, i can imagine it can be powered from unused 9.6V AEG battery. The VAT1 series appears to be available with 5V, 12V and 24V input, so one might choose which one is best for the power supply he got.

or
VWRAS2-D12-D12-SIP -- got 9-18V input, but it is more pricey

One more thing why i don't like the dual 9V battery solution is that there will be uneven current draw from the positive and negative rail and one of them (probably the positive) will get discharged first, causing some odd results till you figure out whats going on