8 Battery Lead Acid Slave Board

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dillond666
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8 Battery Lead Acid Slave Board

Postby dillond666 » Sat Jan 22, 2011 4:27 pm

This board is for 12v lead acid batteries. The board can look after up to eight batteries. I will be making a master board which will allow the connection of two slave boards, this means up to 192v packs will be usable.
The main bits are summarised as follows:

1. Fibre optic communication for the master bus. This should eliminate interference as my car is electrically noisy.

2. Onboard load resistors to shunt 900mA

3. Using precision resistors for the potential divider. No more pesky unreliable pots!

4. Using open source software to develop the board. I'm going for KiCad on linux, power to the penguin!

5. Decent input protection for reliability.

6. I'll try to fit it onto a 100 x 160 eurocard, then it should be easier to find an enclosure (I hope).

7. Still using the trusty Picaxe 08M for the slave board.

Initial schematic is available to view at http://www.dillonworld.co.uk file is called slavemulti.png D33 on the diagram is the led in the fibre optic transmitter.

All comments and suggestions welcomed :)

Derek

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Re: 8 Battery Lead Acid Slave Board

Postby JonSpence » Sun Jan 23, 2011 12:44 pm

dillond666 wrote:1. Fibre optic communication for the master bus. This should eliminate interference as my car is electrically noisy.


My experience with Fiber optic com's is quite the reverse. Sure the cable does not act pick up anything, the problem is that the optical receiver is very sensitive and picks up everything. It might be worth investigating grounding and screening techniques instead.

Termination resistors to prevent reflections, multiple screens, earthed at both ends to cut magnetic interference and ones "earthed" at only one end to cut out radio frequency interference. Twisted pair wiring with composite drive to remove common mode interference.

If you do chose the fiber optic route then think very carefully about the box that you put your board in and how it will act to screen the optical receiver from electrical noise. Don't ignore the fact that your sensor cables will enter the same box and carry noise with them. It might be worth designing shields around the receiver from the start.

I should also point out that fiber optic cable is quite restrictive on it's minimum bend radius. Check out what it is for the stuff that you intend using and check that you can route it with that limitation.

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dillond666
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Re: 8 Battery Lead Acid Slave Board

Postby dillond666 » Sun Jan 23, 2011 2:58 pm

Hi Jon,

Cheers for your comments. The datasheet for the fibre optic bits can be found here : http://www.avagotech.com/docs/AV02-1501EN
It says "A shield has been integrated into the receiver IC to provide additional localised noise immunity." this would suggest you are
spot on about the sensitivity of the receiver. When I do the master board I will make sure I use a metallic case and use that nice
screened cable for the current sensors etc.
Bend radius for the cable should not be an issue but I will be extra careful, erring on the side of caution.

Derek

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Re: 8 Battery Lead Acid Slave Board

Postby retepsnikrep » Sun Jan 23, 2011 6:54 pm

Sounds good Derek.

Could it not be done on the cheap with el cheapo opto transistors and led's heat shrunk onto ends of fibre? Does it need to be ultra sensitive and capable of high bandwidth? It really only needs low speed comms which would be less sensitive and cheaper ;)

I would be tempted to buy some cheapo fibre from e-bay and do a bit of tinkering :wink:

In one version of the slave software I sent timed pulses using pulsin/pulsout which is much simpler for low speed switching rather than serial data.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

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dillond666
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Re: 8 Battery Lead Acid Slave Board

Postby dillond666 » Sun Jan 23, 2011 10:05 pm

Peter,
I like the general tightness of your suggestions, do you have Scottish blood in you? :lol:
I've actually already bought two of the evaluation kits as I needed to get an electronics order above
the free postage threshold because I'm tight too :wink:

I had left my design using wires for the slave bus because it hadn't given me the same hassle
I had with the master bus. I reckon I could hack up something like you suggest for the slave bus, that
would be cool. Here's a pic of my board layout so far, I've omitted the values and airwires for clarity.
The outline is 100 x 160 but it feels pretty cramped though. :shock:

Derek
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Re: 8 Battery Lead Acid Slave Board

Postby retepsnikrep » Sun Jan 23, 2011 10:15 pm

You don't really need it for the slave bus though as it is less suseptible. As you mentioned.

How about simply using an infra red led for the master bus one for each slave. And place a detector somewhere above the board to pick up the signal.

I have found that putting an extra layer of opto isolation just before the signal reaches the master helped me. It needs another tiny dc-dc converter to power it but saved my bacon. Also the Master board really needs to be in a screened metal box.

The master bus does pick up lots of noise and could probably be redesigned much better now. We have learnt a lot in 3 years or so! Current loop etc would be a lot more reliable.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

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dillond666
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Re: 8 Battery Lead Acid Slave Board

Postby dillond666 » Sun Jan 23, 2011 11:22 pm

Just ordered some 3mm red leds and some infra red receivers. Red led light wavelength goes through fibre cable OK and the receiver should still work at red light too.

http://www.rapidonline.com/Electronic-C ... pair/29405
I ordered the unfiltered one as it sees red light.
http://www.rapidonline.com/Electronic-C ... -LED/29332
I ordered the HE model
A bit of 3.2mm heatshrink and we should have an experiment 8)

Derek

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dillond666
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Re: 8 Battery Lead Acid Slave Board

Postby dillond666 » Thu Jan 27, 2011 12:40 am

I've had a little play about without much success :(
Test set up was 15mA going through the LED shining on to the IR receiver
which would switch on another LED with 10mA going through it.
The test rig worked great when coupled with an optocoupler, so no problems there.

The LED driven by the receiver barely illuminated and when I tried it through the fibre optic cable
it wouldn't light at all, even though you could see that the red light does pass through the fibre cable.

I pointed the remote control of my camcorder at the test rig and the driven LED flickers with full brightness, so
I guess the receiver just isn't sensitive enough to saturate at red LED light wavelength.

The infra red remote control wouldn't communicate down the fibre cable, which I expected.

So conclusions:
Fibre cable conducts from 400 - 700nm wavelength
Red LED fires out light at 625nm
Green LED at 565nm

Just need a suitable sensor

Rapid do one that has peak sensitivity at 580nm cost £1.26 but unfortunately it's a 5mm diameter and 3mm would be better for heat shrinking to the fibre.
Farnell do one that has peak sensitivity at 570nm cost 71p and is a 3mm

Anyone for a green LED with a Vishay tept4400 from Farnell ?
I do hate getting reamed for postage though, when buying such cheap lightweight goods :x

Pretty much done with the slave PCB I reckon, it's got big ground planes on the top and bottom now. Just needs some mounting holes and lots of thorough
checking then I can generate the Gerbers and drill files.
Must check out this Tellymate thingy that Peter is using, I think I'll be using it for my master board. Busy busy busy :lol:

Derek

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Re: 8 Battery Lead Acid Slave Board

Postby retepsnikrep » Thu Jan 27, 2011 2:28 am

Did you use an IR led for Tx?

The pic inputs are quite high impedance so you don't need lots of current.

If you remove the led and have the opto reciever pulling the pic input high or low against say a 100k resistor you might find it is actually passing enough current.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

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dillond666
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Re: 8 Battery Lead Acid Slave Board

Postby dillond666 » Thu Jan 27, 2011 11:28 am

Did you use an IR led for Tx?


I tried the red led first.
The remote control uses an IR led, that's why I experimented with it.

I didn't try it on the pic inputs and as you point out they may be high enough impedance to change state with a meagre signal. It's just so much more reassuring to have a big strong signal like the optocoupler was able to deliver. In my head I always want to drive the transistor into full saturation even though It's probably not necessary. I tend to think of the transistors in a motor control/smps type setting where improper drive spells failure. I'll buy the other receiver and green led for testing, just for the science of it and I'll rig up a pic based test mule too.
Would I be correct in guessing that too high an input impedance on a microcontroller could be inviting interference pickup?

Derek


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