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Discussion Starter #1
Hello all,


I haven't been on the forum for a while due to being so busy with half a million things. But in January, out with a certain someone filming for a cable TV show, I gave this great community a shout out! Hopefully they include it! More info on the show coming later...


I have a new build I have been working on, 74' VW Super Beetle! Been a lot of fun with this one, but I'll write up another post with all of the info on this build later...


I am trying to make a custom display as the Orion BMS which I am using in this car outputs deadly accurate SOC on 0-5V output as well as has a hall effect current meter which the system outputs the current on 0-5V scale as well. This is great as I will be using an arduino Uno and a display to make a little guage screen for the car. This will have a gauge for current as well as have a SOC meter. Only thing I am missing is the voltage!



My only issue is that I need to scale down the pack voltage to measure it with the arduino (microcontroller). My pack is about 240-328VDC. I know with the TBS units they have a 10:1 prescaler so that would bring the max voltage of 328V down to 32.8V, but too much for the microcontroller.



I was wondering...can I take two of these prescalers wire them in series so that it divides it by 10 once and then by 10 again? The max voltage would then be at 3.28V max which would be perfect! With that being said, I am unsure how these prescalers work internally and if this setup would realistically work or not. Now in theory to me it sounds great, but I really don't know if it would be that easy.. If anyone has any insight on this, any advise would be wonderful!


Thanks,


Adam
 

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Discussion Starter #3
Great idea! Those are expensive though... any other ideas? How do they do it inside of chinese voltmeters? There has got to be a way to do it for less right?


Thanks again,


Adam
 

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How about a voltage divider circuit? For example to reduce a pack voltage of 500V down to arduino level of 5V would be a 100:1 divider ratio.

So let R1 = 10 MegOhms and calculate what value of R2 is needed such that when Vin is 500, then Vout is 5.
 

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Discussion Starter #5
Thanks for the reply! Alright this is making sense!...i think.


Please stay with me here as I am only 18 and trying to make sure I understand it all so I don't end up blowing something up or hurting myself lol


In regard to what you said before, R2 would have to be 101k ohms to have a Vout of 4.999V (the upper end of the measuring spectrum with a 500V input). In this case, if I change it to a 100k ohm resistor which is actually more common, the Vout becomes 4.95V (new upper end), so essentially instead of 0-5V measuring band, it is now 0-4.95, and 4.95 would still be proportional to 500V input. Hope I have it so far! (Actually later on I found a voltage divider calculator online to check my work, so i know the calculation is right, woo!)



Now my other question has to do with the wattage of the resistors. Now I hope I did the math right here, but I calculated that the circuit would consume 0.0000495 amps @500V input, and that would be 0.02475248 watts. Does this mean I can just use your small typical electronics resistors? Is this safe to do at high voltage and how accurate would this measuring system be? I just want it to be generally pretty accurate.


Also, does this mean I would have the arduino microcontroller ground connected to my battery negative? Is this the only way to do it or is there a different option to that? To me in the case of something going wrong, it seems like an opportunity for something not good to happen if you are connecting it to your pack's ground. I am not sure, that's just what I am thinking.


All help is so appreciated! Thanks :)!


Best,
Adam
 

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Discussion Starter #6
Also, I have heard an opto isolator helps to keep a high and low voltage circuit isolated, would that be something that I would need as well?


If so, what do I look for, or do they all do the same thing? How are they hooked up? Is it something that is important to add?


Thanks,


Adam
 

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Yes, you'd need isolation between your low and high voltage circuits- all of them. You do NOT want to tie your high voltage negative from your pack to the ground of the car (which is what your microcontroller will be connected to since you're driving it from your low voltage 12V battery).

So you'd be looking at building a voltage divider to measure your pack voltage, plus another one to provide the supply voltage to whatever optoisolator chip you're going to use. That one will also need a linear voltage regulator on it.

Isolation of digital signals is easy enough, but building a circuit to provide your own isolated retransmit of pack voltage as a 0-5V (better 1-5V) signal would take a little effort. Or you can push the "easy" button and buy one, like just about everything else you'd like to do on an EV!
 

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Discussion Starter #8
Yes it is more effort to buy one, but it’ll be worth it to have a nice little display since the Orion BMS is so accurate with the other data! Plus, I love learning this kind of stuff, especially at a young age, it’ll allow me to go and make maybe more affordable displays for other DIY EVers out there.

I can get a little adjustable DC converter to power the opto isolator chip, spending a couple bucks extra will make that part easier.

But I don’t think I can just get an opto isolator that costs a few cents and use it right? Is there one I need to find that says a certain spec about high voltage?

And how do you go about installing it? Since I know the goal is to isolate the negative leg of this circuit, and you apparently as you said have to power the chip with x Volts, what else is there to connect to it?
 

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You need two separate circuits, connected to one another by light only.

The "sender" circuit would be powered from the full pack voltage, either via a voltage divider or a small high voltage input DC/DC converter. The power draw would be very small so a voltage divider would do it, followed up by a linear voltage regulator like a 7805 or 7812 etc.

The "receiver" circuit would be powered by your 12V battery that also runs your Arduino etc.

The sender circuit would use a voltage divider on your total pack voltage as an input, and would illuminate an LED on one side of an optoisolator chip in proportion to your pack voltage.

The receiver circuit would use the phototransistor side of the optoisolator chip to produce a voltage proportional to the brightness of that LED.

What you get is an isolated retransmission of your total pack voltage.

A typical optoisolator chip is used just for digital signals, ie. to retransmit zeroes and ones- but there are chips which can be used to transmit analog signals, likely supplied by Analog Devices or whoever owns them now.

The Orion will give you total pack voltage- I'm sure of it. So I'm not sure why you're doing this at all.

Look in the Forrest Mims beginners electronics books- you can find them online now for free. They are old but still have the clearest explanations of electric circuits you can find anywhere for beginners- and they give lots of example circuits to breadboard and try out.
 

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Discussion Starter #10
You need two separate circuits, connected to one another by light only.

The "sender" circuit would be powered from the full pack voltage, either via a voltage divider or a small high voltage input DC/DC converter. The power draw would be very small so a voltage divider would do it, followed up by a linear voltage regulator like a 7805 or 7812 etc.

The "receiver" circuit would be powered by your 12V battery that also runs your Arduino etc.

The sender circuit would use a voltage divider on your total pack voltage as an input, and would illuminate an LED on one side of an optoisolator chip in proportion to your pack voltage.

The receiver circuit would use the phototransistor side of the optoisolator chip to produce a voltage proportional to the brightness of that LED.

What you get is an isolated retransmission of your total pack voltage.

A typical optoisolator chip is used just for digital signals, ie. to retransmit zeroes and ones- but there are chips which can be used to transmit analog signals, likely supplied by Analog Devices or whoever owns them now.

The Orion will give you total pack voltage- I'm sure of it. So I'm not sure why you're doing this at all.

Look in the Forrest Mims beginners electronics books- you can find them online now for free. They are old but still have the clearest explanations of electric circuits you can find anywhere for beginners- and they give lots of example circuits to breadboard and try out.

MoltenMetal,


Thanks so much for the great write up, makes it all so much clearer now. I will definitely check out some of those books, they sound awesome!


Oh shoot, I just remembered reading the part in the manual about it, and how it sums all of the cells to get the total pack voltage. But..all that is done through CAN. I know I have heard of some CAN shields for Arduinos so maybe I could read all of this info that way instead of using the analog 0-5V outputs. Now, this takes it all to a whole new much more difficult level...but is CAN easy to do? Any experience with it? I've learned the basis of how it works, but as far as physically setting it up and such..that I do not know.


Thanks again for all of the help
 
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