Browsing Posts published on 22/09/2010

Momentous. 22/09/10

Ladies and Gentlemen, tonight something extraordinary happened.

At precisely 8.15pm BST, I sat on the bike, said a little prayer to whichever Gods may have been listening, and started pedalling.

Quite slowly at first, all the time looking for the tell-tale plumes of blue smoke and all the while sniffing the air for that unmistakeable smell of frying silicon.

Nothing.

So I pedalled a little faster…

…And the first green LED lit.  “…Huston, we have a green light on the board.  Ignition set for ‘Go!’…”

My heart was pounding, my stomach somewhere up near my throat.  Still no smoke or burning smell.

I pedalled a little faster still…

…And the other green LED to tell me that charge was being transferred first flickered, then lit properly. “…We have main lift off.  All engines running at 110%…”

I looked over at my multimeter, wired in series with the battery and set to read current.

It flickered, numbers rapidly changing.

So, I pedalled faster still, and kept that speed, and sure enough, both LED’s were still lit, and the meter had settled down to read a steady 0.5 A.  “Huston, we have achieved escape velocity…”

Okay, not earth-shattering numbers here, but its a start!

I had purposefully set the PWM width (…that’s how much I’m putting into the battery…) to be very, very slim, so this was entirely expected.

SO tomorrow, I’m going to pop down to Maplin’s first thing to get a ‘7805’ 5 volt regulator and a few de-coupling capacitors.  When properly wired up, this will mean that the unit is entirely free-standing.  No mains required.  The 7805 will power the little PIC processor and associated circuitry, and the power for that will come from the battery -it certainly won’t hurt any as it only takes a few milliamps.

This will mean that if all goes according to plan (…HA!..), I can take the bike, the circuit board and some lights down to LEAF on Saturday!  Volunteers will be able to ‘have a go’ at charging the battery, and we’ll be able to have lights to see by.

Synchronicity:

Just the other day, I bought a really ancient but otherwise fully working laptop in really gorgeous condition from ‘Taplin Computers’.  This will be used to show photos that we’ve taken while we’re down on The Plots.  The ‘synchronous’ part is that unlike many newer laptops, this one has a nine-pin serial port on it.   With a little initial ‘setting up’, this will connect straight into my little PIC programming board.  This means I’ll be able to make code changes on the site without having to come all the way back here to my main computer.

10.22pm News:

Just checked Maplin’s online, and they’re completely out of stock of ‘7805’ voltage regulators.

However, a good rummage through my parts inventory, and I’ve got a brand new one!

So, my mistress Lady Sleep will probably be a long time coming tonight.  I’d better get to work now then…

Oh, so nearly there.

The sensing resistor block.

The sensing resistor block.

A pretty momentous week, all told.

To the right here, you can see the resistor ‘block’ of ten 0.1 Ohm resistors prior to soldering which when wired in parallel make an astonishing 0.01 Ohms.  This is before they are soldered.

This low resistance is needed to be able to sense the current flowing into the batteries.  I had been using two 1 Ohm resistors in parallel which gave 0.5 Ohms, but found that when I wound up the power supply, the voltage across them -and hence the amplified voltage coming out of the new current sensing i.c. was a little on the high side -even though the current is meant to be limited to 2 Amps or under.

The bike generator will give out over a factor of ten of this, hence the lower value resistors.

The board in the middle of construction.

The board in the middle of construction.

Here you can see the board ‘as I’m building it’ last night.

On the right, you can see the PIC programming board I built so many months ago.  Towards the top of the photo, you can see the ‘new’ board with the little PIC and LEDs on it.  In the middle at the back, you can see the new 0.01 Ohm resistor block I built up last night.  This has been safely mounted using the engineers favourite -Superglue!

Between the two boards and below the new resistor block, you can see two FET’s that switch the power with signals from my new board.

Luckily, unlike normal bi-polar transistors, FETs don’t go into ‘thermal runaway’.  When this happens, a transistor gets hot, so its resistance goes down, therefore heating it up even further and the runaway happens.  The result? A little plume of blue, acrid-smelling smoke, accompanied by the immortal line, “Whoops.  That’s taken the chill off it…”

FETs are much more graceful when it comes to these vexing problems.  When a FET gets hot, its resistance increases, thereby limiting the current, thereby cooling it down.

SO, I’m using two here in parallel in all confidence that neither will blow, and it means I’ve effectively doubled the current-handling of the switching circuit.

Finished!

Finished!

Here we have the finished board.

You can see I’ve mounted the two switching FETs, and if you look really closely, you’ll see they are connected.

Beneath these two FETs is another heatsink with another device on it I haven’t previously mentioned.  This is the final ‘blocking diode’.  This prevents current going backwards into the circuit.  Without it, the cyclist would get a free ride as the pedals would turn themselves.  Not something we are keen to promote!

Anyway, thats it for now.  Wednesday afternoon is a LEAF session and as the weather’s so nice…

Okay, okay…  I can hear what you’re thinking! “When’s he going to get on the bike…???

I’m be leaving early this afternoon, and I’ll come straight back, and yes.  …I’ll get on the bike!

Don’t nag!


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