Was CdS opto's

Hello Martin,

I did but that was way back when. Probably I still have the schematic somewhere but it would require archaeological digging.

The oscillator ran very nicely. Clean as a whistle.

Regards, Joerg

Just wondering if anyone has used a CdS opto as a gain control element for a low distortion oscillator? No, I havent googled, it was just a thought after reading the Watch crystal thread and the CdS thread

martin

Ahhh... The Great Jim Williams, I'd forgotten about AN-43, thanks for the reminder

martin

Well, so does any resistor, despite Ohm's legislative attempts :-).

I'm pretty sure that with reasonable amplitudes that a CdS photocell will beat the pants off of any solid-state gain-control part. My results in testing on the bench were comparable with Jim Williams' in AN-43 (although my test equipment and methodology are shoddy in comparison.)

Silonex and others make CdS photocells that have guaranteed limits on distortion.

Tim.

That's a "long O" if you want to start getting phonetic.

Purely an amateur. But 100% on the money.

Are you gonna call the NIST police (they're up the road from me) and turn me in for violating Ohm's law?

Tim.

"martin griffith"

** It can be done:

CdS elements have voltage dependant resistance so generate 3rd harmonics on a sinewave.

The tempco is an issues if level stability maters.

.......... Phil

I retrofitted an LDR into a commercial Wein Bridge audio generator.

To reduce the effects of capacitance, I suggest you keep the LDR in its 1K ohm region, not 10s of K ohm. Many LDRs will go down to 200 ohm.

Some gotchas on LDRs :

1. Drift over lifespan. You won't get the same ohms for the same illumination over time. If the illuminator is a LED, it too will age with time. LDR drift by a factor of 10 is quite on the cards. I have seen severely drifted parts after only one year operation.

1. Life. I have measured LDRs as open circuit after 10 years of operation. I think life is decreases with equipment temperature. Life may depend on encapsulation quality, etc.

2. Distortion will depend on the voltage across the LDR. If the LDR is only fine tuning another series or parallel resistance, the distortion is a lesser fraction of the total signal. LDRs are good enough to be used a Wein Bridge or other notch filter fine tuning circuit in distortion analysers from HP, etc.

Have a look at :

orapplicationnotes.pdf

i.e.

to see discussion of distortion "Keep voltage across LDR to 1.0V or less"

Roger Lascelles

I've seen this in spades actually when testing an audio power amplifier into a test load with serious tempco.

My 'tech' was puzzled. So was I initially - surprised to be more accurate.

He couldn't fathom why the THD was different at the amplifier terminals compared to the load.

I reckon he still isn't happy with measuring THD at the amplifier terminals.

Graham

No - a classic ceramic tubular wirewound test resistor.

And apparently you don't know that this happens.

No surprise. You're not exactly the brightest bulb on the planet.

Graham

Shame you're apparently not capable of reading a spec sheet.

Btw - it surpised me too.

The difference is that I can read the spec and understand the issue wheras you're simply a denialist.

Graham

"Tim Shoppa

** Tell me Mr Schlopper

Do you specialise in posting smartares s**te - or are you merely an amateur ?

** No you are not.

............ Phil

** Like what - 1000 tiny light bulbs driven at 20 Hz ???

You really are a COLOSSAL f****ng wanker - Graham Stevenson.

Ex bloody DJ & cretin.

.......... Phil

"Tim Shoppa"

** Yawn .....

** I like to see a man who enjoys his work.

Even if it is shovelling his own shit.

........ Phil

"Pooh Bear" Phil Allison

** Shame how they are made with near zero tempco alloy.

Shame how their thermal inertia eliminates tempco at audio frequencies.

** Au contraire Mr Pommy DJ - I am painfully aware of the phenomenon.

........ Phil