StompLFO Troubles

[moosapotamus]

Has anyone else killed one of Electric Druid's StompLFO chips?

I have no idea what I am doing wrong. No smoke. The chip did not get hot. But I seem to have fried two of them, so far.

The first time, I set up the StompLFO to modulate a filter on the breadboard. I moved a couple of wires (I think I might have been trying some different filter caps), my LED blinked once, and then no more LFO.

The second time, I was comparing the feel of a rotary switch to the feel of a pot for the wave selector. I swapped the wipers, and the same thing happened. The LED blinked once, and that was all she wrote.

In both cases, everything was working great for well over an hour. In both cases, I did not disconnect power when I swapped wires on the breadboard, but was careful not to short anything (although, maybe I did?).

I love these chips. I think they are awesome. But, I would really love to figure out how to stop frying them. What would have to happen to kill one of these chips?

Thanks for any ideas.

[ElectricDruid]

I haven't actually managed to kill a StompLFO, but I have fried a few PICs over the years. Although the outputs are pretty robust, they can only supply 20mA (25mA on some models, I think) and shorting the output to ground will kill them.
Alternatively, I've fried inputs by connecting them to the wrong power supply, usually when working with a +/-15V/+5V supply (a pretty common combination for synth circuits). The poor 5V chip can't handle 15V inputs without protection! Who'd 'ave thunk it?!?
I'm not sure whether inputs left open when the chip is powered up could fry anything. I'm sure I've done that several times and I've got away with it, so the answer is definitely "not always", but it might also be "maybe sometimes", I don't know. It's certainly true that the chips behaviour goes completely weird when inputs are left floating, and it's not always anything related to the open input (e.g. if you leave the Freq CV unconnected, it's not necessarily the frequency that goes funny - it might be the waveform).

I'm surprised you don't know what happened though.That is a bit odd. Usually it's pretty obvious: first you swear, then you hope you got away with it, and then you realise that you didn't!

[moosapotamus]

Thanks for the reply, Tom. For the record, I do have one successful build under my belt with the StompLFO. So, I know that I must not be completely inept.

But yeah, I really don't know what I did wrong. I've killed three chips so far, 2 StompLFOs and a OneShot. Most recently with the StompLFO, I had set up a pot and also a rotary switch (with a string of resistors) for the waveform so I could easily swap one for the other by swapping the wiper connection to pin 6. So, that's all I was doing when the chip died. And, this was after everything was working great and I had been playing through it for at least an hour. But, I did not disconnect power when I swapped the wiper connection to pin 6.

None of the inputs are open. I'm using them all except offset, so there's a 10k from pin 2 to ground. I'm using a +9V supply, and the 5V regulator is doing its thing... +5V on pin 1.

I suppose I might have inadvertently bumped something with a stray finger and shorted the output to ground while swapping connections/wires with power still connected. But, to have done that exact same thing three different times with three different chips would make me a total super klutz. Not that that isn't a very real possibility. But, from now on, I guess I will never leave power connected while changing anything on the breadboard with one of these chips. And, I still have a couple more chips left, so... fingers crossed.

[Kipper4]

I've had a few close calls but nothing terminal, IIRC i was unsure if id blown them up but a later retrial has shown that no its not dead.
Cant remember why or how.
Rich

[moosapotamus]

Thanks, Rich. I'm pretty sure mine are done for.
I know you have some experience with these chips. You have posted a number of cool designs with the StompLFO where you did not use the cap/resistor on each of the controls, as shown in the datasheet. What is the purpose of that cap/resistor, and what are the risks of leaving them out? Are they there to protect the chip in some way?

[ElectricDruid]

The resistor/cap are just a basic RC lowpass to help filter noise from any scratchy pots. I got in the habit of including this from my earliest PIC designs. However, starting with the StompLFO, I also included digital filtering of the control inputs, so the external filtering is no longer essential from a noise point of view.

In terms of protection, yes, 1K in series with the input does provide considerable protection since it limits the current that can flow through the internal protection diodes if the input exceeds the 0-5V rails.

[moosapotamus]

Good to know, thanks. Don't know if that's what's been my problem, or not. But, I'll be going with that 1k series resistor on each of the control inputs from now on, just to be safe.

[garcho]

For the record, I have messed with and abused stompLFO chips to hell and back and never fried one. Not trying to rub it in, Charlie, just sayin' 
Also for the record I've always used the 1K resistors to control wiper (along with the cap).

[moosapotamus]

Thanks, Gary. 
I guess I've just been lazy about it, since I've seen others go without. But, I guess I shouldn't try to cut corners.
<fingers crossed/>

[ElectricDruid]

Not trying to rub it in, Charlie, just sayin' 

Lol!

Charlie, maybe cut the corner on the final version once you've done the development stage, and keep the extra bits on the breadboard for safety?

[garcho]

And it's nice to see you back around the forum the last couple years Charlie, your website hipped me to a lot of valuable info, cool stuff, and hours of joy, thank you, cheers!

[moosapotamus]

Right, Tom. Actually, I think I'll play it safe across the board unless I really need pcb space.

Thanks, Gary. I'm glad to have helped to inspire. But yeah, my career was interfering with my personal interests for a while there. Glad to be getting back to it. Cheers!

[ElectricDruid]

But yeah, my career was interfering with my personal interests for a while there.

Oh dear! That sounds dangerously like working too hard! Glad to hear you're back with us!