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Old Jan 08, 2009, 09:47 PM
PLD
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kgfly,

I can actually go up as high as 16V without a problem at all (the input regulator has a 16V input limit, I can get others up to 37V but that's getting a tad out of hand ).

Yes, the 2 sets of pins (2.54 and 2.0mm) will be positioned so as to prevent accidental crossing. For avoiding reverse polarity situations of the input cell, I've got the input lines to the microcontroller protected. For JST-XH type plugged packs this is avoidable due to the plug pin holes being offset different amounts, so you can't actually get the pins to insert without forcibly bending them up to make the extra clearance.

I think I've also come up with a way to prevent anything happening if reverse-polarity cells are applied, I think that's a critical aspect as I'm sure it'll happen eventually.

.... tell you in 15 minutes


Paul.
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Old Jan 08, 2009, 10:20 PM
PLD
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Well, technically what I'm doing 'should work' *cough*, basically I'm planning on putting a forward biased diode between the MOSFET sources and the ground (not between ground and the cell under test, else that'll upset our open/closed voltage measurements).

Only thing is, with that one change, it's really made a mess of the board... oh well, at least it's just in a file at the moment and not an actual physical board.

Paul.
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Old Jan 08, 2009, 10:57 PM
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Sounds good I completely agree that reverse polarity protection is a must-have for a device like this, especially since there is no keyed socket body to reduce the chance of erroneous connection.
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Old Jan 08, 2009, 11:08 PM
kit
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Here's an ohm symbol you can use. Just copy and paste it.

Ω
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Old Jan 08, 2009, 11:21 PM
PLD
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kgfly,

Indeed, I had thought about keyed sockets but in the end all it did was mean that I'd have to cover all/most socket types or force the user to create their own looms, neither option was acceptable either from a cost/size factor or a technical performance factor.

I've managed to keep everything intact so far on the PCB, only barely using the 2nd side, which is good because I want to keep it mostly as a big groundplane (to keep any voltage wiggles across the board down to the absolute minimum).

Next considerations to think about are the case and the operational supply source. A 9V battery would work best in terms of easy connection. The LCD supposedly consumes ~5mA, the regulator would consume another 5~10mA, the AVR another 5mAh, so we've got about 20mA consumption which means a 'typical' 9V cell would last 5hrs continuous.

The other way of course is the DC-jack, though it does then kill the portability aspect.

Paul.
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Old Jan 08, 2009, 11:21 PM
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Why not place the load on the pack discharge wires and read the voltage from the balance taps. With no current through the balance wires the voltage reading would be much more accurate. I don't know diddly about designing something like this or if it would be lots more complicated or expensive, but it seems like a good idea to me.
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Old Jan 08, 2009, 11:34 PM
PLD
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Biskit,

Your idea is a good one (really it is, I had to think about it for a bit as to why I can't do it that way ), but the problem ends up being that because you're tying the -ve lead of the battery to some reference on the IRM (eg, ground), when you try to measure the higher cells you then have to start doing some fancy tricks to sample-hold or divide the voltage because the microcontroller can only tollerate ~5V inputs natively.

All in all, your idea is another possible way to do the task, though it brings about its own can of worms in the form of needing precision resistors for the voltage divider and/or sample hold circuits.

So far the simple "2-pin input" method seems to be winning in terms of outright simplicity and managable precision issues (namely the limits of the load resistor)

Paul.
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Old Jan 08, 2009, 11:36 PM
PLD
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Quote:
Originally Posted by kit
Here's an ohm symbol you can use. Just copy and paste it.

Ω
Kit, thanks for that - though 'R' is a perfectly acceptable ASCII variant is it not (I especially find it useful for doing 4R7 type values)? I might see if the LCD panel supports the Ω char in its table.

Paul.
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Old Jan 08, 2009, 11:49 PM
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Paul, Glad to hear that 15V is no problem. I wasn't suggesting/recommending keyed sockets for all the reasons you mention, I like your idea about using the simple 2-pin approach.

For the cost of a switching socket you could have both an internal 9V and an external supply to meet both scenarios, n'est ca pas ?
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Old Jan 08, 2009, 11:55 PM
PLD
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kgfly,

Yes... now, where to fit a DC socket onto the board. Actually a simple pair of 1N941/4148 type diodes will work fine to prevent any accidental reverses on that side (though plugging two things in at once, that could be chaotic).

Paul.
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Old Jan 09, 2009, 12:04 AM
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Other options to consider for DC power connection:

* Servo pins to allow a stock Rx pack to be plugged in
* JST socket to allow a small NiXx or LiPo flight battery to be plugged in
* USB socket (for power only, 20mA is well within the 100mA standard AFAIK)

Just throwing ideas into the mix
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Old Jan 09, 2009, 01:29 AM
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I am glad someone finally bring the Internal cell resistance issue to the front page. It's a good idea assuming if this become the "standard" way to test all battery. Now, RCer will no longer be fool by the C rating marketed by the battery makers.

However, I have to make a suggestion on price. Believe or not, there is already another product on the market that does a reasonably good job and measuring cell's internal resistance. That is CellPro 10S. Therefore, considering CellPro 10s also happen to be one of the best charger on the market for $190, a device like the one propused here probably will need to priced at less then US$40 to be marketable.

Just my 2 cent. I seen too many good idea that failed to become commercially viable product becasue it was priced way too high for the general public.

Brian
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Old Jan 09, 2009, 01:48 AM
PLD
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Quote:
Originally Posted by asm_
(regarding $40 price)

Just my 2 cent. I seen too many good idea that failed to become commercially viable product becasue it was priced way too high for the general public.
Sometimes the two extremes of the "willing to pay" and "willing to sell" ranges just don't meet up. Neither side can/will move, the manufacturer doesn't want to turn into a charity and the buyer doesn't want to take out a second mortgage.

At $40 USD I may as well just not even bother making it, seriously, it'd be pointless, I'd be better off standing on the corner of the street and jiggling a can for money, I'll make more money that way. It's not about trying to carve out a massive amount of profit, rather it's a case of trying to achieve sustainability.

This is going to now start another debate (oh well, I'm in for it ), but if we're to retain some more of our industry "onshore" then there is an extra margin we have to cough up because there's higher labour, freight and living costs. Up to until recently we've generally lived a very comfortable life with a glut of low priced goods, alas it has come at a considerable cost to our skills base and economy.

I'm trying to do my bit to turn that around along with many other cottage industries I know of (there's a lot in R/C it seems, that's great! ) and keep things alive. Consider also a callout fee for a plumber/electrician, it's a lot more than $40. It'll take about 45~60 minutes to build each one of these devices.

Right, that's my soapbox rant - I think $60 is about the mark to start with, component costs alone are already creeping up with the addition of a second load, power jacks, battery holders, switches (something I have to think about), more diodes (normal and zeners), not to forget the cost of the PCB being fabbed, there's also the need to use a larger uC (atmega88, maybe a 168) and we still haven't even got a case for the unit! There's no denying that you can collect and assemble all the parts yourself for less than $60, however there's a lot of costs associated with future developments, picking up new component lines (lots of 100 or 1000+ each) that a single unit doesn't need to incur. It's a false loss to not produce an item want that the community seeks if all it's going to do is cost you money. Sometimes you just have to move on.

I appreciate that it'd be -nice- to sell these at $40 USD, unfortunately it's not going to happen. $60 would be about right for a first-release batch.

Paul.
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Old Jan 09, 2009, 01:50 AM
PLD
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kgfly,

I managed to cram on a DC jack and a 9V plastic mounting assembly on the rear of the board, I think that'll do the trick in terms of options, anything more and it's going to get confusing and (more) costly. Still, nothing someone with a soldering iron can't kludge in

Paul.
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Old Jan 09, 2009, 01:58 AM
PLD
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Latest IRM board update

Here's the new IRM board with the DC jack and 9V clip/casing on the rear (notice the white rectangles, DC jack top-left).
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