Rotary switches for calibration changes, type question

[jalai]

What is a pullup resistance with this diode "chain"? -> I try understant how this work .
And what is the type of diodes?

[TimH]

IN4148 diodes, and I would think something like a 10k pullup, but I don't know for sure.

[jalai]

It can't be work with 10k pullup.
I try 330ohm pullup to 5V and this is not good (with 6x 1N4148).

Syvecs people wakeup! What is this diode circuit and it's pullup?
And... can You tell what is goin wrong with resistors?

[TimH]

6xIN4148 diodes would be a 6*.65=3.9V drop (approx). 1.1V across 330R would be about 3mA, so, yes, that would work fine.

A 10k would be 0.1mA which would also work.

So, what didn't work in your test?

And, sorry for the dumb question, but you did connect the diodes the right way round I presume?

[Jasper K]

Tim,

I think it's better to use diodes instead of resistors for the simple fact that the body drop on a diode is ALWAYS 0.6~0.7V provided the voltage of the source, in this case a pull-up 5V source, is sufficient to drive all the diodes into conduction.

If you would use resistors then the voltage drop on the resistors would depend on the voltage coming from the pull-up 5V source which would depend on the amount of current that is drawn. It's just a lot more complicated to get to proper resistor values when more or lesser cal. positions are used. To me it just doesn't make any sense to use resistors. You do not have these problems with diodes as the voltage drop is FIXED and almost doesn't vary with the current drawn out of the 5V supply.

People saying you cannot make a 10 pos. CAL-switch with diodes on a 5V powersupply are both right and wrong

When using normal Silicon diodes with a voltage drop of roughly 0.6~0.7V it is indeed not possible.
However there are such things as Germanium diodes which have a voltage drop of 0.3V as well as there are Schottky diodes with a 0.25V ~ 0.35V Voltage drop depending on the selected model. Therefor it is certainly no problem to do so.

Cheers,

Jasper Kopinga
CS Racing
Taiwan.

[Jasper K]

It can't be work with 10k pullup.
I try 330ohm pullup to 5V and this is not good (with 6x 1N4148).

Syvecs people wakeup! What is this diode circuit and it's pullup?
And... can You tell what is goin wrong with resistors?

Jalai,

You shouldn't add an external pull-up. You should use the Resistive Inputs on the Syvecs preferably (AR1~AR4) which have an INTERNAL pull-up. Then you connect the Rotary switch to this input. The way the rotary switch works is that every time you switch to a higher position it will ADD one more diode in series increasing it's output voltage by the voltage drop on the diode itself.

Cheers,

Jasper.

[Benkku]

I think it's better to use diodes instead of resistors for the simple fact that the body drop on a diode is ALWAYS 0.6~0.7V provided the voltage of the source, in this case a pull-up 5V source, is sufficient to drive all the diodes into conduction.

If you would use resistors then the voltage drop on the resistors would depend on the voltage coming from the pull-up 5V source which would depend on the amount of current that is drawn. It's just a lot more complicated to get to proper resistor values when more or lesser cal. positions are used. To me it just doesn't make any sense to use resistors. You do not have these problems with diodes as the voltage drop is FIXED and almost doesn't vary with the current drawn out of the 5V supply.

I don't agree. Diodes has significant voltage drop vs. temperature slope. Source voltage do not drift nor change much. So resistor network is more accurate, than any diode.

[Chris Wilson]

Whilst this battle rages I am happily twiddling my RS Components rotary switch with Ebay resistors and all is well with the world. I really had no idea my initial post would turn into this technofest marathon

[jalai]

And more...

I do a small test today.

5, 6 or 7 1N4148 and 2,2k pullup resistor to 5V (I don't know what is correct pull up?).

Temp__5D____6D___7D
-20C__3,42V__4,00__4,49
20C___3,07___3,59__4,07
50C___2,79___3,27__3,72

Results in the picture


With Schottky this "problem" is bigger, because temperature coefficient is about same, but voltage step is much smaller.

[TimH]

For me, that's the nail in the coffin for diodes. I spoke with Pat the other day and I'm pretty sure he said the pullup was 3k3, by the way.

The only argument for using diodes is if the voltage at the top of the chain is unknown. So for a universal selector switch there is a case for diodes but where the voltage is known (as it is for the Syvecs) I really cannot come up with an argument in favour of diodes.

FYI Toucan has a voltage output to drive the Syvecs input so won't suffer this problem