Barnstorming in my underwear, a DIY story

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Re: Barnstorming in my underwear, a DIY story

Post by FAP »

Haven't had the chance to try this out yet, but I should say thanks before I forget to.
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Re: Barnstorming in my underwear, a DIY story

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Okay, here's one that might actually come up sooner for me:
Series battery voltage sag.gif
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What is the purpose of the black wire bridging 1 and 2 on the pot? What's the benefit of doing that as opposed to leaving 1 alone?
I ask because, in my experience, anything above 200 ohms simply shuts off whatever I'm trying to sag... granted I've thusfar only done voltage sag with things under 9v, so that may explain it. But perhaps referencing 1 to 2 improves the "range" of sag, so to speak?
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Re: Barnstorming in my underwear, a DIY story

Post by crochambeau »

FAP wrote: Thu Jan 23, 2020 10:49 pm Okay, here's one that might actually come up sooner for me:

Image

What is the purpose of the black wire bridging 1 and 2 on the pot? What's the benefit of doing that as opposed to leaving 1 alone?
I ask because, in my experience, anything above 200 ohms simply shuts off whatever I'm trying to sag... granted I've thusfar only done voltage sag with things under 9v, so that may explain it. But perhaps referencing 1 to 2 improves the "range" of sag, so to speak?
That little black jumper is just finalizing the conversion of a potentiometer (three leg with variable division) to a rheostat (two leg variable resistance). Tying the outer leg to the wiper is not critical in assuring a variable resistance function. It is, however, good building practice.

If your unused leg is tied to the wiper (in a rheostat application) then instances where the wiper leaves the substrate (either through wear, misalignment, or contaminants) the part as a whole is far less likely to become an open circuit, it'll just be a higher resistance for that span of time. There is also the fact that tying that leg to the other results in a more finished appearance, which can eliminate a non-necessary focal point in the assembly process.

I prefer using a pass transistor if I want to chew on the rails of a following circuit, as then you have an almost "built in" path by which automatic modulation (be it CV or something like a timer source) can be configured to play with your power as well.
When in doubt, add resistance.

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Re: Barnstorming in my underwear, a DIY story

Post by FAP »

Okay, so what is a pass transistor?
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Re: Barnstorming in my underwear, a DIY story

Post by crochambeau »

FAP wrote: Fri Jan 24, 2020 8:36 am Okay, so what is a pass transistor?
It's "where the rubber meets the road" with respect to a linear regulator. Numerous examples of this on line, but here's a simple one. You'll have to forgive me for not including the CV interface on this image, I had drafted a tested example in an earlier revision of KiCad and the file on my machine did not render in a meaningful way...
pass_transistor_regulator.png
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Apologies if anyone is color blind.

The magenta section is just garden variety power inlet with polarity protection and some low pass filtering to clean the power up, standard stuff.

The cyan section is a really low brow voltage reference that will lock the transistor into a state of conduction when the switch is 2-3 connected.

The yellow is the pass element, selected for whatever load is further on down the line. I get away with using 2N3904 in most fuzz/noise maker circuits, but it's advisable to know beforehand what ultimate load your build will be hanging on that power line, and scale the transistor up to fit (if need be). The caps in the yellow are a localized AC feedback network aimed at cleaning the DC rail up as much as possible (they may come in handy if you've got a zener diode pushing noise onto the rail), they're probably not required here...

With the switch in its present condition, 2-1, supplying a voltage onto the base on the transistor will turn it on, and NOT supplying a voltage at all will see that pass transistor turned off.

Once you supply enough voltage to the base to bias the transistor (typically around 0.6 volts), it will slightly turn on. There is a more or less linear region in which the more voltage you supply at the base, the more the transistor turns on (it effectively is becoming a smaller resistance as far as the DC path from power inlet to power rail in your build is concerned).

One note of caution: simply connecting a CV source directly to the base of the transistor will reflect a somewhat low impedance on whatever you are using as a source. That is to say, the power consumption of the build that is being "starved" by this will draw current from the CV source. I resolve this by placing a mosfet buffer on any CV inlet.

Working from memory (hahaha) you want to tie the "drain" of the mosfet to *unmolested* power, you could probably tap off the "3" on the switch, the "source" of the mosfet would connect directly to the base of the transistor, and the "gate" can then safely interface with whatever voltage reference you're feeding it.

At least that's how I recall implementing CV modulated power rails.. YMMV
When in doubt, add resistance.

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Re: Barnstorming in my underwear, a DIY story

Post by FAP »

:shock:

...I think I'll just stick with the previous method.
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Re: Barnstorming in my underwear, a DIY story

Post by crochambeau »

FAP wrote: Fri Jan 24, 2020 4:48 pm :shock:

...I think I'll just stick with the previous method.
:lol:

If it works, it works!
When in doubt, add resistance.

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Re: Barnstorming in my underwear, a DIY story

Post by FAP »

I'm circuit bending something that has, among other things, a sort of feedback tone that can be adjusted with a pot. However, with the pot I'm currently using, the tone really only kicks in during the last 10-20% of a full rotation, which leads me to believe it's linear. I'm considering getting some audio taper pots, but I'm hesitant because it seems to me like they're only really useful for smooth volume swells. Would they work the same way (i.e. a smoother rotation) for something that isn't strictly volume-based?
Also, not all audio taper pots are made equal (at least that's what I'm told). Some are just flat out lies. Which ones are worth my time? There's probably datasheets for some of these that I can imagine would show the taper in a graph: is that all I can really go on?
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Re: Barnstorming in my underwear, a DIY story

Post by crochambeau »

Is it safe for me to assume you own a multimeter?

You can test taper of a pot by rotating it physically to center and measuring resistance from the wiper (center) to each side.

If it's linear taper (or an S, but those are not common), those measurements will more or less match. If they don't match you're probably looking at a log or anti log (audio and inverse audio respectively).

I prefer to work with linear pots whenever I am able, and sort of look upon a log taper as last resort. If I find that the desired effect only occurs in say 20% of of a pots rotation, I will measure the range I want to isolate and replace the unwanted portion of the range with a fixed resistance, like this:
pot_ranging.png
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So say I really like the last 20% CW end of a 100K pot, I could pair an 82K resistor and a 20K linear pot and essentially "zoom in" on the sweet spot.

If you want to get fancy, that resistor can be a trim pot. That resistor can also be from "top" down, or both sides.

Naturally, none of what I just said is helpful if you want to preserve the effect that occurs at full CCW and full CW rotation of the pot, then yeah, you'd be looking at a taper tweak.
When in doubt, add resistance.

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Re: Barnstorming in my underwear, a DIY story

Post by FAP »

Oh yeah, Ixe2x80x99ve already had to do that trick with the additional resistor; the problem is, that desired 20% is usually toward the less resistance end, not more. Sure I could just use a lower value pot, but the rub is I also lose whatever I couldxe2x80x99ve got out of the higher value pot.
I guess ultimately the only way to know is to do a comparison for myself.
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