Mat's BX Blog

Post by **mat_fenwick** » Fri Mar 04, 2011 9:30 am

Another reason not to go LED just yet, with the technology still in its infancy. There's reports of an 800 lumens per watt LED in development, which is considerably better still!

electrokid · Post by **electrokid** » Sat Mar 05, 2011 10:48 pm

I would guess the resistance is less at a lower voltage and current?

Very good question - one I don't know the answer to. Funny things FETs - they tend to change their characteristics in the first few hours of operation but that shouldn't bother you at 40m? RDSon though there are some around 7m? if it does. I suggest putting in the extra cap - my calculations

suggest 10nF (with the 47M?) should give about half a second startup time - leaving it out might not increase bulb life by much.

The latest in LED technology always seems to cost over the odds - but as soon as the new stuff is on the market the prices for the 'previous' technology seems to go down quite quickly.

Post by **mat_fenwick** » Mon Mar 07, 2011 10:49 am

I had a feeling you might reply Brian

I don't suppose you'd have a part number (to hand - don't bother to look it up) for the lower resistance FETs would you please?

electrokid · Post by **electrokid** » Mon Mar 07, 2011 9:31 pm

well it's leccy stuff init

I don't have a part No. in my brain but I'm going to move my technical library soon - hopefully by the end of this week - and I'll welcome the distraction of looking something up

At my last place of work I remember having a batch of FETs quarantined for having an on resistance greater than 10m? but I don't remember the type number of that one.

You should be fine with the 40m? ones though - I'd only think of changing to a lower R if it's problematic in any way. If you're going to buy one anyway then check out the specs at CPC - for intermittant operation you should get away with a 10 or 15A device but get the highest spec you can as long as it will fit in the space - filaments are incredibly low R when cold so the initial current will be much higher than the 4.8A running.

Post by **mat_fenwick** » Tue Mar 08, 2011 5:36 pm

No need to worry about the part number - I've ordered one from Farnell. It's rated at 26A continuously so should be fine...

I've already been playing around with the 11w bulb and it's very impressive compared to the standard bulb. However, it's (obviously!) giving off a fair bit more heat now. I believe that in the 'good old days' the early Maglites had a glass lens instead of plastic, so I've managed to source one of those. I've also managed to find a polished aluminium reflector that should fit, so just need to put it all together and see what happens!

electrokid · Post by **electrokid** » Tue Mar 08, 2011 10:46 pm

Yes - glass better than melted plastic

The 26A device should be fine - the instantaneous current at switch-on may well exceed that but the size of junction needed for 26A continuous should cope with the higher momentary pulse.

electrokid · Post by **electrokid** » Wed Mar 09, 2011 8:38 pm

PM sent

Post by **mat_fenwick** » Mon Mar 14, 2011 4:23 pm

And received! Looks like I might be busy making another circuit in the near future…

Changed the alternator/compressor belt on the BX today - I had been experimenting with running it without the tensioner and idler pulleys (which increase the wrap around for the alternator and compressor). Although it worked, I wasn't entirely convinced, as I found that to completely prevent all squealing I had to increase the belt tension to a level I didn't really feel happy with. So I've been living with an odd squeal till I got round to modifying the tensioner to fit the (slightly different) mounting hole spacing on the 16v sump. Now silent, even when cold and everything turned on (including the glowplugs which remain on for a minute or so after starting). Funny how little things like that can affect your enjoyment of the car!

Post by **mat_fenwick** » Mon Mar 14, 2011 5:36 pm

In other news my trusty chainsaw isn't very well.

I was cutting some (fairly hefty) trunks and it lost power, so I backed off the throttle and it stalled. Tried to restart it and it wouldn't - very little compression and a nasty rattle (is there a good kind of rattle for an engine?) Not stripped it down properly but a peek through the exhaust port has revealed a scored piston. Should be repairable though, and we have enough firewood to split by hand to last us.

electrokid · Post by **electrokid** » Mon Mar 14, 2011 9:47 pm

Looks like I might be busy making another circuit in the near future…

Me too

I can easily send you a parts pack (for the cct on P22) if that helps - it's all stuff I keep in stock in large quantities and let me know if you need a piece of veroboard as well.

Post by **mat_fenwick** » Fri Mar 25, 2011 1:43 pm

Stripped the chainsaw down last night, and the piston has seen better days...

Out of interest (before I looked inside) I was looking at a few secondhand models that my local specialist has in, and was struggling to find one with a similar power to weight ratio. I had a go with one that was slightly lighter but only 2/3 the power, and a couple that had more power, but weighed considerably more! It seems like it wouldn't be easy to replace it with a current saw that weighs under 5kg but puts out over 3bhp. Plus the cost to change would be more than twice that of a rebuild.

Decision made then, it will live again!

The barrel is absolutely fine as far as I can tell, so will get a new piston, ring, and crank seals (for peace of mind). Maybe the main bearings too but I'll see how they look with more stripdown. I found a split seal on the inlet manifold, so that may have let it run lean and overheat.

toddao · Post by **toddao** » Fri Mar 25, 2011 9:19 pm

scarecrow · Post by **scarecrow** » Sat Mar 26, 2011 12:33 pm

Talk about deja-vu, Mat! Mine's still in bits waiting for me to get in the mood.

Post by **mat_fenwick** » Sat Mar 26, 2011 10:42 pm

Yeah, kind of ironic - I wonder which will be fixed first?

scarecrow · Post by **scarecrow** » Sun Mar 27, 2011 9:36 am

There's no need to rub it in!