I'm studying a design I found online and the author uses a resistor of 0 ohms.

I searched online as to why you would do this but the results I got back wanted me to turn off my ad blocker which I refuse to do.

Wouldn't a 0 ohm resistor pretty much be just a wire?

Thanks.

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Cat; the other white meat.

I think it would. Is it used in effect as a jumper, to select an option?

Edit: see also
https://electronics.stackexchange.com/q ... ire/385941
and
https://electronics.stackexchange.com/q ... m-resistor
where we learn that it's easy for machine placement of components, compared to a wire link, and also that it can be useful if there are design variations, in some of which it might be a non-zero resistor.

Not that I can see. It's used in a video circuit.

Let me see if I can take a screen shot...

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Cat; the other white meat.

Sorry, the scan is pretty low-res.

Thanks

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Cat; the other white meat.

Likely a few other uses:

1- Current limiting
2- Make it easier to route the PCB
3- Provide for a future circuit change, i.e., a voltage divider

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Regards, KM
https://github.com/floobydust

So, that chip's COMVID output happens to have a suitable impedance already, perhaps. Maybe some other chip or other variation would need a series resistor?

The chip is the TMS9918.

There's actually two things I'm trying to figure out on that design, why the 0 ohm resistor and what value of ferrite bead they use on L1.

Perhaps the resistor was just so other values could be added in later for adjustments. Or maybe a variable resistor could be added?

For those that are curious, this design comes from a PDF I have that shows how to replace the dynamic RAM in a TMS9918 design with static RAM.

https://raw.githubusercontent.com/cbmee ... 2BVRAM.pdf

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Cat; the other white meat.

Or the 9928A could be an alternate board option? that has "Y" on that pin.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

That's an emitter follower, so no resistor required.
But there is also the 470 ohm resistor to ground, which makes me think this is perhaps a voltage divider, or at least provides an option for one. Finding the value requires either thought and calculation, or trial and error. It's quite possible that it's a "work out the right value later" placeholder.

TMS9918 datasheet, PDF page 80:
Nominal video output voltage levels specified for a 470 Ohm load:
Vwhite=3.0V, Vblack=2.3V, Vsync=2.0V.

Since monitors tend to have AC coupled video inputs, it isn't supposed to matter if there is a little DC bias voltage on the video signal.

The 2N4401 transistor works as an impedance converter for driving a 75 Ohm coaxial cable.
When a smaller video signal amplitude is needed, the Zero Ohm resistor can be replaced with another resistor for building a voltage divider.

From the datasheet, 2N4401 can drive up to 500mA.
Assuming a 0.65V voltage drop in the transistor, and a 75 Ohm termination resistor at the monitor input, the collector current would be
(3.0V - 0.65V) / (75Ohm || 75Ohm) = 63mA for white level,
(5.0V - 0.1V) / (75Ohm || 75Ohm) = 131mA for a short circuit between transistor base and VCC if the transistor goes into saturation.

2N4401: 500mA, 250MHz.
BC547: 100mA, 300MHz.
BC337: 800mA, 100MHz.

If you don't have a 2N4401 at hands, try using a BC337 instead, it's supposed to be fast enough.
If you don't have a 75 Ohm resistor at hands, use two 150 Ohm resistors in parallel instead.

;---

Composite video signal levels at wikipedia:



Hope, this helps.

Thanks everyone for the information.
And thanks ttlworks for that awesome deep dive!

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Cat; the other white meat.