H11F1 Opto-Isolated FET is back!

Posted: 10/9/2010 8:30:41 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]"I'll wait until I find something similar in a classical DIL package. I'm too old to handle SMDs" - Thierry[/i]

Hi Thierry,

Yeah - I know what you mean! Alas though, it seems that new IC's are rarely being produced in DIL.. There are many components (comparators, op-amps etc) with wonderful spec's but only obtainable in SMD now.

SMD does have an advantage of reduced board area - and my designs tend to have a lot more components than conventional Theremin products.. But the problems dont stop with assembly - testing / repairing SMD boards is much more difficult.

I am being 'forced' to look at a mixed SMD / TH approach - Building (or more accurately, having them built) moderately simple circuit 'blocks' which connect (using standard soldered 0.1" right angle connectors) onto a small 'motherboard' - this way I can place TH components, trimmers etc on the 'motherboard' - this makes testing / debugging easier, and allows any faulty module to be replaced quickly without having to re-work a large SMD board.

I dont like it - And I dont think technically minded Theremin builders / modifiers will like it as much as having TH components - but this method is starting to look like the best compromise.

The other option is to have SMD IC's fitted onto small adaptor boards bringing them out to a DIL footprint - I looked into this, but at about £3 each to get the PCB's and have the Ic's fitted (if one does a 100+ run) it is only possibly viable for expensive IC's like the multipliers - and one loses the space-saving cost reduction that SMD gives.. So I only do this when prototyping (and solder the IC's to the adaptor board myself - an absolute nightmare job)

For most applications, and for small volumes, I would agree that SMD is not the way to go - the LM633 is an excellent part, costs about £7, even if one needed four the £28 would still probably be a better option than buying a single >£10 SMD part and having all the complications.

There is a bigger issue which really bothers me though - this is the issue of future part availability.. Back at the beginning of this crazy persuit of Theremin 'perfection' I was determined to stay with well established components which should be available (in production) for the forseeable future (>10 years).. Alas, it now appears that there are no components one can be sure of for even 5 years.. I do not understand what is going on.. The H11F1 was a classic example - suddenly dropped by everyone for years - No good designer will design using components which are only available on the 'obsolete' market - so we were forced to find alternative ways to do what we needed to do .. Then, years later, they re-introduce the damn parts! .. WTF?? When these parts were discontinued, I phoned EVERY manufacturer who had made them, asking if there were any plans for a replacement part - I was told by them all that "analogue opto FETS were extinct and would never return" That "We have disposed of the production facilities required to make these parts" that "The masks for this part are not compatible with the new geometries required" ..etc..

The only comforting thing is the knowledge that, with a carefully designed and adequately soak-tested circuit, most semiconductor parts rarely fail.. they have (unlike tubes) nothing which suffers from 'rapid' ageing.. But, unlike the remaining RCA's which, although difficult, can still be reconditioned 80 years after they were built, I do not think there is much chance that parts would be available in 20 years time if one of my Theremins needed repair.. and that's a bit upsetting.


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