Resistor for pots on Profi 3010

[Epitaph]

Hello people!

I was at home reading whilst wondering what to do last night when I got an idea. I haven't been able to put the sliders on my MPX radio yet because they cost a fortune and are hard to get hold of. But then I remembered I still had the 2 pots from the Losi car radio that I used the guts for to make my homemade 2 channel radio for my little DLG. So off I started hacking away at the MPX to add a pot to each corner for channels E and F, and I can always add an internal switch to change from slider to knob when I get the sliders and use whatever is best for that particular model. I just love the way tray radios are designed to be adaptable and upgradable!

Anyway, I added the pots to their position, and put the Losi trim knobs on them as they happen to be the same colour, and I am very pleased with the final aesthetics of the upgrade! Now I can have 2 proportional channels for things like proportional flaps, proportional butterfly, proportional spoilers, gyro gain, etc.



I've already done some upgrading on this radio, as the 3010 comes as basic 4 channel radio that is upgradable to 9 channels plus 6 multifunctional switches (for things like dual rates, expo, channel inhibitor, etc.), plus digiknob for inflight adjustments and plus a memory switch to change between 3 models inflight, which is the radios version of flight modes. I've already added 3 of the multifunction switches and a 3 position switch for channel G... all the switches were taken from a broken Futaba T6EXA, the 3 position switch being originally the trainer switch which I opened up and renoved the spring mechanism off of with an exacto knife.

Now comes the connecting part of it all... and here is the issue. The pots were designed to be able to utilize the entire movement of the servo with only a fraction of the pot movement, as with the case of gimble pots for example. But now the pots will be using their entire range of movement because of the application. But I think because of this, the resistance may be too great. Now you can reduce the resistance value by adding a fixed resistor in parallel, but I'm not sure in this case if it would be a single resistor per pot between the two outer pads of the pot (positive and negative), or if it would be 2 resistors per pot, one from one outer pad to the middle one (positive and signal), and the other between the middle pad and the other outer pad (signal and negative).



Any ideas anyone?

 

[SteevyT]

Here is the basic wiring of a pot. http://www.maximintegrated.com/images/appnotes/234/1902Fig01.gif I think it would be two resistors in parallel, one from each side, going to the output. But without doing the actual math on it, I'm not entirely sure.

 

[Epitaph]

Yeah, I remember these diagrams, that the pot is tecnically one big resistor from end to end, at the same time as it is 2 smaller resistors in series with the middle pad in account. I just can't seem to figure out which of these 2 speculations would be pertinent in this case, although a part of me is inclining towards the one parallel resistor case, not sure why, maybe because this is of fixed value, and the variable one is the middle pad. Also, I get to thisby how you would add dual rates on a radio, putting a resistor in series on either end of the pot' rather than one in the middle, because the idea is to change the value of the pot, not to change the value of the input....

Need to think about it some more, but I think it's going to come down to trial and error...

 

[xuzme720]

What do you need the output value to do? Up or down in resistance, compared to what it is now?

 

[Epitaph]

The output value would have to be less resistance to what they are now. It's to get the movement of the servo on the full 85° the pot turns, and not just the middle 30 or so degrees that it usually uses as a part of a gimble. Basically, turn the value of those 85° to that of the 30 or so degrees it normally uses.

Actually, might not need it... I was just investigating, and someone somewhere on a dark and forgotten page says the sliders are 5K ohms, which happens to be the value of these pots. I'll solder them up tomorrow if I get a chance and let you know... might be all a false alarm!

 

[xuzme720]

That's what had me thinking.
If you add resistors to the pot in parallel it will raise the value. Or maybe I'm looking at it backwards...

The cold/flu thing I have is making me a bit fuzzy right now...

 

[Epitaph]

Adding in series raises resistance, and in parallel reduces it.

 

[xuzme720]

Yup, backwards. I gotta get to feeling better. I hate being all loopy like this...

 

[Epitaph]

Finally got around to soldering it, and yes, there is a movement problem. I only get movement of the servo on tbe centre third of it's movement, covering the entire servo throw...

Now here's the thing: I've been thinking about this completely backwards! I don't need to lower the resistence, I need to raise it! Think about it... there is a way to add dual rates to a radio that doesn't have them by adding a resister in series on either extreme of the pot to raise the pot value. Well this is exactly what I need! If you do this and activate the dual rates, say set for 50%, then at the gimbal limit the servo would have only moved half its limit. But lets say I can push that gimbal past that limit, carry on through the plastic, then the servo would carry on moving until it reaches its limit, wouldn't it? So with this in mind, if I'm only getting movement on the centre third, and I tripple the resistence value of the pot, that would get it to the end (in theory)! So if the pot is 5K ohm, and I add a5K on each side, that would turn it into a 15K pot with the central 5k free... or... one sec...

Let me think it over a sec... if it's stopping at one third more or less... then that means each half of the pot is 2.5K value, and is stopping about 1/6th of the way per side, which is around 400 ohms... this means that it needs about 2k ohms to stop the servo at it's end... if I need to move the flagpole and thats it, then all I need is a 2K on each side...

Think it's time to get the multimeter out.

 

[Epitaph]

Ok, found the problem, and it's time for me to get out the big pointed hat with a "D" on it and sit in the corner!

Turns out the value is correct, 5K ohm, but these pots are no good as they have a huge deadzone. At the end of that one third turn the resistence is 5K, and the rest is direct with no resistence. So now I have to change them, no way around it!

Me... need... chocolate!