Help, please, selecting some Electronic Components

White LEDs are usually constructed using a blue/UV led covered with a white fluorescent material and the shade of white depends on the materials recipe. I suspect that the material may have a degree of persistence that would moderate 10Mhz operating.

I used a similar circuit to make an LED strobe light to visually check the operation and timing of watch escapements but these operate at relatively low frequency.

CS

I looked at the circuit last night, but didn't comment because it was past my bedtime. Just as well perhaps.

I assumed the circuit to be rather simple, seeing a fast chip on the input, which I guessed was to sharpen up incoming pulses, a fast FET connecting an LED to ground, and powered by a small fast electrolytic, backed up by a big slow one.

The blurb explains the circuit can be used at 5, 10 or 20V, so I took R1 as intended to be protective rather than a RC filter with R1/C1+C2. It also says the input pulses are about 1 microsecond.

In my limited experience the C1+C2 arrangement is used to speed up decoupling. It makes use of the fact that small electrolytics charge and discharge much faster than big ones, and of course all capacitors are much faster than a battery or DC power supply.

Is it possible in this circuit that flashing the LED with wide spaced 1uS pulses doesn't fully discharge C1 + C2? Instead the pulse empties C1 whilst C2 is still slowly waking up, and finishes before C2 has had time to discharge much energy. Tjhen, after the pulse stops, C2 quickly recharges C1, whilst both are slowly topped up by the supply. I note the duty cycle is very low, 0.00005:1 at 50Hz. In this scenario the R1/C1+C2 filter maths would have to be modified because 1 microsecond pulses mean the C2 to C1 time constant dominates. In comparison to C1/C2, I think R1/C2 is dozing in the slow lane. Putting a scope on the circuit would reveal all.

Anyway, C3 are ceramics - dreadful tolerances, but being cheap and wonderfully fast makes them excellent decouplers. I think the diode is belt and braces protection and almost anything would do, provided it's faster than the LED. How about a 1N4148?

Curiously I was thinking of trying a circuit like this after a pendulum discussion about determining amplitude with a camera. I was going to set a pendulum swinging in a blacked out room and photograph it with my camera on the 'B' setting (shutter always open), whilst strobing the bob with a bright white LED. The photo should show the position of the bob at each accurately timed flash, with a scale behind graduated in degrees.

Too many ideas, too little time! It's past 11am and I haven't started any of today's jobs.

Dave

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So it would appear, Martin … and it wouldn’t be the first time that a modern schematic had been mis-interpreted

I really need to check with the authors in case it’s a typo in their Fig. 4(a)

… I mourn the passing of those helpful little ‘hump-back bridges’ that we had on old drawings !

MichaelG.

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That was my understanding too, Dave, and pretty-much the extent of it.
… which is why I came here asking advice about which specific components to buy.

Unfortunately I seem to opened the proverbial ‘can of worms’ instead.

Regarding the Diode … I still don’t understand whether it needs to be something ultra-fast

I have a bag-full of 1N4006 available, but am concerned that might throttle the performance of the transistor.

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If we can’t write a shopping-list between us all, then I will just have to build it with what I can find, and have a look on the oscilloscope … a potentially long-winded series of iterations.

MichaelG.

The diode is totally unnecessary since it is always reverse biased! I suspect that the "designer" adapted a circuit originally intended to drive an inductive load which would need a diode to catch the inductive kick on switch off. Not understanding why it was there they left it in just in case. From other artefacts of the design understanding was thin on the ground.

The MOSFET seems to be a fairly generic power device and other types would be suitable.  I don't know where your pulses are coming from but the threshold voltage of the device specified can be as high as 4v which is a bit marginal for a 5v CMOS drive.  

Edited By John Haine on 17/03/2023 11:52:12

Goodness Gracious !!

I thought RS was a little pricey, but:

**LINK** https://www.ebay.co.uk/itm/171948255344

MichaelG.

Yes I know. Muddled thinking for a bit. Engage brain before opening mouth and all that. It really wants a switching device across the 100R.
Not the most brilliant cct I’ve seen there are loads of High Power led drivers that would be more suitable.

regards Martin

I'm late to this thread,

Clives description of a white LED is correct and thoughts on persistence reasonable. Indeed I made a similar assumption when looking for flash lamp for a day job instrument design.

I was surprised to find the LED was very fast. It actually had faster turn of and less afterglow than a xenon tube. Even a short arc tube with quench circuit was slower. The limiting factor on measurement was the photodiode I was using.
So don't dismiss the white LED.

Robert.

Edited By Robert Atkinson 2 on 17/03/2023 12:36:37

It’s obviously of no great consequence, but …

I have to admit that in several posts I have mentioned Fig. 4(a)

… this should, of course read Fig. 3(a)

MichaelG.

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Don’t worry, Robert … That is most unlikely in my case !!

MichaelG

That and the four-way wire junctions made me suspicious from first reading of the circuit. That style of drawing was superseded by straight crossings and 'blobs' at junctions in... what?.. the 1960s?

George

Hi, Why not get the components and make the circuit and try it.,then you will know if it’s suitable.

Maurice

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Because some of the components are not adequately specified

I asked for advice here in the hope of avoiding problems caused by sub-optimal choices.

… Type and brand-name of capacitors with the lowest available ESR, and that sort of thing.

Hoping to benefit from the experience of others.

MichaelG.

[ UPDATE ]

I have made contact with Susan Parker [who designed the Xenon Flash part of the system] and although this little circuit is not her design she has kindly commented regarding the Diode:

I suspect the D1 is there as the LED is on a cable and it snubs cable reflections. There LEDs can get quite expensive, so it’s a just in case belt and braces part. Whether it ever actually does anything is of course another matter.

I hope to have further discussion sometime over the weekend.

MichaelG.

I tried a simple discrete mosfet driver out of curiosity. If I understand correctly the goal is to drive the LED with pulses of 1-2µs? This is my schematic. I just used components that happened to have around:

I decreased the value of the drain resistor. I think it make sense to have more current through LED. The circuit is supplied at 5V. This are the waveforms for 1µs pulse:

Yellow is in the drain of T3 and the blue trace is from the base of T1 and T2. This is with a 200ns pulse:

I didn't switch the normal ground wires of the oscilloscope probe with the low inductance springs. I guess that might cause that ringing. This is the circuit:

The rising/falling edges looks good. Increasing supply voltage doesn't improves them. But increasing the driving voltage amplitude improves the edges significantly.
     T3 turns off very fast almost completely. But then it need a lot to fully switch off. Since the LED emission vs voltage is non-linear this long tail might not be noticed.  

Edited By Sonic Escape on 17/03/2023 17:09:42

Edited By Sonic Escape on 17/03/2023 17:10:17

OK ,

I've desigined and built exactly this type of circuit.

1/ The 0.1uF on the input of the driver chip is incorrect. Probably a misdraw of the datasheet test circuit which has two power input bypass capacitors.

2/ The two 0.1uF capacitors on the driver chip should be connected directly across the IC pins. One between 1 and 4 the other between 5 and 8. Use ceramic capacitors.

3/ The diode is required. At these switching speeds the parasitic inductances can cause enough bacj EMF to damage the LED. Connect the diode directly across the LED pins.

4/ I would not worry too much about the inductance of the 100 ohm resistor, just don't use a wirewound. A film type is good choice.

5/ Any modern low ESR electrolytics will be OK.

You need a neat layout with short lead lengths and a good earth. I'd do it "dead bug" style on a bit of plain copper board.

Robert.

Thanks, both

Definite progress in my comprehension

MichaelG.

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Still a little bewildered by the specified component for R1 … I wonder if they were just lucky enough to have one in the store-cupboard

MichaelG.

That seller is just listing RS aand Farnell parts at huge markup. They hold no stock, just order as required. Ther are a number of similar sellers. Bizzarly they actually have lots of sales.

If you want a low inductance 100R thick film power resistor PM me and I'll send you one. It might be used depends what I find first....

Robert.

Robert.

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Done !

Thank You

MichaelG.

The self inductance of R1 doesn't matter here. It will work exactly the same way even a wire resistor

Edited By Sonic Escape on 17/03/2023 19:26:07

Edited By Sonic Escape on 17/03/2023 19:26:18