Member postings for Robin Graham

I'm trying to get to grips with simple electronics for workshop projects. I came across this design for for a bistable multivibrator:

The accompanying text says:

"Switching between the two states is achieved by applying a single trigger pulse which in turn will cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative half of the trigger pulse. The circuit will switch sequentially by applying a pulse to each base in turn and this is achieved from a single input trigger pulse using a [sic] biased diodes as a steering circuit."

I don't understand what the author means by 'a steering circuit' and no explanation is given.

If anyone can explain more clearly how this circuit works I'd be grateful. The diagram comes from an electronics tutorial which I have found generally useful.

Robin.

PS - I should have said that I know this cat can be skinned other ways, but my enquiry is specifically about understanding this circuit.

PPS  - also, I don't understand what the writer means by 'the negative half of the trigger pulse'. It looks pretty negative throughout to me. I'm aware that the falling side of a pulse can be steeper than the rising side, so might get through the 1nF capacitors more easily, but I'm just guessing.

Edited By Robin Graham on 19/02/2023 22:59:23

Edited By Robin Graham on 19/02/2023 23:15:04

Edited By Robin Graham on 19/02/2023 23:23:26

As I get older and more aware of my mortality I sometimes muse on things which might have done for me in early in life. Quite a few, but I think one of the stupidest thing I ever did was (at the age of nine maybe) was to get one of those plastic caps for toy pistols, stick it in on the end of a nail, mount in an empty .303 cartridge case, fill with gunpowder from bangers, crimp the top of of the case about the nail, rest the assembly on the top of a wall and hit the top of the nail with a hammer. The bang, my Dad told me in the course of a difficult interview afterwards, was reported to the police . Miracle was that although there was a crater in the top of the wall and bits of brass embedded elsewhere I was unscathed. I couldn't see what the fuss was about - I was 'doing an experiment'.

Anyone got closer?

Robin.

Back in the day when I was working at a University I would sometimes pop into the campus newsagent for a packet of papers. They would always give me the King Size (spliff rolling) variety, Students eh!

Robin.

Robert - apologies for not responding earlier, I have only just got back to this.

I have now made experiments along the lines you suggested. My procedure and results:

- set PSU to 12V in CV mode

- adjust voltage pot on convertor to give 24.5V no load - PSU is indicating 10mA at this point.

- attach load (definitely 3 x 27 ohm 5W resistors in series) and wind down current pot on convertor to give 24.0 V

- attach voltmeter over the central resistor in load - 8.05V as expected. The PSU is supplying 660mA.

- short out one of the other resistors, momentary flicker, but 8.03V across the central resistor.

So that seems good - it's current limiting.

However...

Removing the short then turning the current limit from the PSU right down I find that:

- if I turn it up again straight away (within a couple of seconds) the PSU voltage / current return to 12V / 660mA immediately and all is as before.

- if I leave it for ~15 minutes with the PSU current down to zero, then turn it up to max (3A) the PSU supplies the full 3A. At this point the the voltage from the PSU is about 6.9V and the voltage across a single load resistor is about 7.5V. After approx 70 seconds in this condition the PSU suddenly switches back to 12V and 660mA and the convertor output jumps back to 8.05V per resistor.

Does that give any clue as to what is going on with these modules? I haven't tried connecting to 12V lead acid battery (which was what I'd hoped to do) because I suspect that the convertor would suck up too much startup current and self-destruct, as Les found.

Robin.

Edited By Robin Graham on 04/02/2023 23:38:19

Edited By Robin Graham on 05/02/2023 00:02:58

Edited By Robin Graham on 05/02/2023 00:47:30

I did actually start the thread with the tap held in a drill press chuck which is what I normally do - then go over to to a flat bar type wrench when the tap seems established to get a better 'feel'. Normally I buy plug taps because they seem OK for both blind and through holes provided they are guided at the start. In this case I had only a taper tap in my box, and that might have been part of the problem - inexperience in using the things. Because my smallest tap wrench didn't grip well I used small drill chuck held in my hand - which was a mistake, too heavy to give good control and feel. I'll make a knurled wheel as you suggest. Though Taptites may the practical answer in this case I'll plod on with tapping out of interest. With paraffin! I'll also make a guide, easy enough to do.

I was surprised to see how expensive pet ether is these days - I have a supply left over from my working days when it was sloshed about pretty liberally

Regards, Robin.

Edited By Robin Graham on 02/02/2023 22:49:07

Thanks Tug. I'm awaiting a set of M2.5 taps to replace the the one I broke and shall have a go with paraffin. Your suggestion took me down memory lane - I don't think I've bought paraffin since I was ten, when I converted my dad's garden weed sprayer into a flamethrower. It worked well, but he wasn't pleased when he found out. Back then you just took a empty pop bottle to the ironmonger and bought by the pint. It was an enticing pink colour - I remember passing a group of 'street bullies' with a bottle - 'giz a swig'. I relinquished my bottle, stopped long enough to see the chieftan take a pull, then ran for my life.

To return to the point - it seems that it's still available so I shall buy some.

On cleaning, petroleum ether (a light petroleum fraction) is even better than acetone for degreasing - the 40-60 degree stuff has a similar volatility to acetone but being composed of non-polar hydrocarbons is generally a more powerful solvent for workshop oily/greasy stuff in my experience.

Robin.

Thanks to all for replies - forgot to say I'd drilled 2.1mm.

Clearance drilling for a screw / nut would be difficult because of the geometry of the heatsink, otherwise I would have just done that:

My reason for posting this query was, at least partly, that I wanted to understand why I f..f..fouled this up. I find it difficult to say 'well, that didn't work, I'll try something else' without trying to understand why it didn't work.

From replies it sound's like it's doable, but in this material not as straightforward as tapping normally is. That's reassuring. I'll carry on trying, taking heed of advice - I don't like giving up! The heatsinks actually have some M3 tapped holes (not in the right place obv) as supplied, so it's possible.

For practical purposes it sounds like Taptite is the way to go. Didn't know of them, thanks Robert.

Robin

Ah, I did indeed miss something - an all too frequent happening these days. My intention was to give a source for compatible leddynamics drivers in the UK if anyone was interested.

I'm possibly alone in fretting about heatsinks, but in the spirit of publishing ones findings whatever I bought some round jobs from eBay:

Perhaps overkill as it runs at about 65 degrees at the centre of the array (without the dreaded thermal paste) but it'll do for my needs.

Now I've got the arrays up and running (It's been journey into new territory for me! ) my mind turns back to Michael's original post - the point of these things is that they give a directed beam without ancillary optics. Which they do!

Robin.

Edited By Robin Graham on 29/01/2023 00:23:41

I tried to tap some M2.5 x 0.45 holes in an aluminium heatsink today and it didn't go well. I'm assuming that the material is pure (ish?) aluminium because it didn't drill or tap like 'ali' at all - it was horribly gummy. Despite copious wetting with WD40 it just didn't seem right - I didn't get the 'crack' which you normally feel when you reverse the tap half a turn. I managed two threads then had to resort to caustic soda to clean the flutes because the swarf was so solidly impacted. Then broke the tap on the third hole of course.

I'd welcome any advice. I'm wondering if it might go better with a spiral point tap or if I'm being over-ambitious in trying to through tap 8mm at this size. I'm pretty sure I've done it in brass, but a different animal obviously.

Robin.

Edited By Robin Graham on 28/01/2023 22:36:05

On the LUXdrive units (they look nice!) I couldn't see a Vout on the site Michael linked to (may have missed something?) but the LUXdrive A011-D-V-700 is available in the UK from Digi-Key for £20.50 and they give a maximum Vout of 48V - so they should be fine for this application.

On other (cheaper) solutions I, like Noel, ordered a 400W booster (£8.49):

Reading Les' account I went very carefully! With 10V input and no load I adjusted the output to 12.5V - the module drew ~10mA. Connecting an 81 ohm load (shown) the voltage remained stable and it drew 220mA at the input . Cranking the current pot right down the voltage across the load dropped to 11.5V, so I guess it's working as expected and I'm hopeful that it will be OK to drive the LED array..

A slightly odd thing is that the +ve output appears on the leftmost heatsink in the photo. Something to be aware of if going down this road.

Robin.

Edited By Robin Graham on 26/01/2023 21:40:12

Edited By Robin Graham on 26/01/2023 21:42:35

Edited By Robin Graham on 26/01/2023 21:43:21

Edited By Robin Graham on 26/01/2023 21:57:24

HI Andrew, I have the same machine which (like you from your photo I suspect) I use mainly for woodwork. I like your mod to the depth gauge - the two-nut arrangement is a pain.

I quite like the machine in that the spindle runs true enough and there is minimal slop in the quill, but mine 'shakes' quite a lot. I improved things significantly by skimming the drive pulley on the lathe - it was quite badly out of balance- but have yet to get round to doing the intermediate and spindle pulleys. Just wondering if you have any such problem and if so how you have addressed it.

Robin.

Excellent! It is indeed informative. Interestingly, the third-party heatsinks they mention don't meet their claim of a <25mm total depth - for example the MechaTronix unit they show has a height of 68mm. Maybe the ones they designed in-house are much more efficient.

On thermal paste, the application note doesn't actually contradict the datasheet. Looking at the (small print!) footnotes to table 2 the use of pastes, gels and adhesives is not recommended.

I've been looking around for cheap heatsinks which might do the job and came across this offering on eBay:

Diameter - 76mm
Base height - 8mm
Pin height - 59mm
Pin diameter - 2mm
154 pins.

The seller doesn't give the thermal resistance and hasn't yet responded to my enquiry, but it may be that there is someone out there who can just look at it and say it'll be fine (or not).

I too grabbed some 240V drivers before the price went up, so thanks from me as well Les.

Robin.

Edited By Robin Graham on 17/01/2023 23:39:09

For me, I think I have power supplies sorted but I'm still unsure about heatsink requirements. One of the things that attracted me to these arrays was the possibility of making low-profile lights - from the data sheet:

"One example of an innovative application of the OrionTM is Plessey’s ability to re-create an
LED version of the AR111, for a track spot, at less than 25mm in depth including LEDs, optics
and a heatsink. Further information can be found in the OrionTM application notes."

It sounds like other contributors to this thread already have heatsinks which are up to the job (possibly bulkier than Plessey suggest), but I haven't, so will have to buy or make them myself. Probably the latter.

I haven't yet found Plessey's application notes, which I had hoped might shed light.

Robert, that sounds like you have a formula for the heat dissipation from a flat plate of aluminium given (I'm guessing) a point source. I've been making some simple calculations of radiative dissipation because that's all I have in my toolbox. For your putative 23cm square sheet at 140C and ambient 25C I get a dissipation of around 120W, but that's assuming that the whole sheet is at 140 degrees, and ignores convective heat loss. So unrealistic assumptions, but not a crazy result. If you have a formula it would save a lot of head scratching.

It may well be that I'm worrying unduly - Paul's interesting photos (welcome to the forum from me too Paul) don't show a heatsink other than the housing.

Robin.

Edited By Robin Graham on 17/01/2023 00:47:08

Edited By Robin Graham on 17/01/2023 01:00:21

Edited By Robin Graham on 17/01/2023 01:02:40

Thanks for the heatsink calculation Robert - I thought it would be more complicated than that!.

Can you - or anyone else - rationalise this from the datasheet:

"Do not expose the module to liquids such as adhesives or thermal paste. No thermal paste
should be used as a TIM between the module and the heatsink. Do not remove the product
barcode".

My bold/italics. It seems common sense to me to use thermal paste. Am I missing something?

Obviously no one in their right mind would remove the barcode. One shudders to think of the consequences...

Robin.

PS. I guess the fact that the array didn't blind the camera at that angle shows that the 'optics' work pretty well in directing the beam - the overall illumination comes mostly from reflection from the ceiling, about 6ft above the device.

Does anyone know how to calculate heatsink requirements for continuous running?

Thanks for sharing this discovery Michael - I think I might have to order more if they're still available at this price.

Robin.

This is a 60 degree version running in an otherwise unlit room (white ceiling):

It's pretty bright!

It's running from a lab PSU in CC mode, current clamped at 700mA , voltage was 43.4V at that current.

Not having a suitable heatsink I just perched it on top of a piece of 2.25" Ali bar, fresh from the workshop so at about 10C. I ran it for maybe 2 minutes - the aluminium back of the LED was stone cold after that but the metallised plastic housing was warm.

I'll rig up a thermocouple and get proper measurements anon.

Robin.

Edited By Robin Graham on 09/01/2023 22:12:38

Sorry John, image of Wolfram calculation still not visible (to me at least). But no matter, in the formula I gave in my OP I indicated that that my result was approximate and in a later post gave the order of the approximation. I can't remember, but probably a Taylor expansion on the back of an envelope or summat - sadly I don't have Wolfram or anything like these days.

Anyhow, I think it's sorted (for me at least) - but thanks again for contributing.

Robin.

Never one to turn down a bargain I snapped up four of the 60 degree 4000K for a paltry £8.44 - £12+ apiece at RS!

But ... how do you propose to power them Michael, and what is your plan for heatsinking? I have a bench top power supply which will supply the voltage and limit current for experiments, but something more permanent would be good. At 43ish volts and 700mA they're about 30W so will get hot.

Tangentially, I lit on on an RS press release when looking for datasheets. I was glad to hear from RS COO Mike England that:

“We see continued consolidation in the customers and competitors landscape and we are confident we can grow organically and inorganically”

Can anyone translate?

Robin

I got back to my computer tonight to see a barrage of MEW forum notifications - thought I must have sleepwalked and posted something controversial! Phew, just an old thread which I started.

I think (like many of the questions I have posed on this forum) my opening post was answered in the first few replies, but it's always good to discuss stuff.

Unfortunately I couldn't see John Vigger's Wolfram calculation (which reignited the thread), perhaps because it it's John's first post. I'd be interested though. Anyhow, thanks for your contribution John and welcome to the forum.

I think JasonB's simulations are pretty much definitive and rather more accessible than the raw algebra. In my opening post I did say that that I suspected that conical sections might be involved and the link that Duncan gives tells us that 'A modern view of the unification of the sphere and hyperboloid uses the idea of a conic section as a slice of a quadratic form. ' , which I'm sure clarifies things.

I propose that there should be a prize for the thread that wanders most off topic - if adopted, I'll throw my hat in the ring - from misaligned lathe parts to ATP synthase animations. I love it, the way one thing leads to another.

Robin.

Thanks Duncan - more useful info. I got around it by making a two-stage driver, but shall look at these devices.

Robin.