Designing a "discrete" pot
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8th July 2011 12:54 AM Thread Starter #11ADWSystems
I'm picturing a string of 8 diodes with each switch position. There would be a switch position, a diode cathode, a diode anode, and FET gate all connected together at each but the first and last (they would be missing a diode cathode or anode connection). Here's my question, when selecting minimum current, the last voltage of the last FET gate will be 7 diode drops less than the first diode. When selecting maximum current the FET will see the most voltage, will the FET gate handle that or work under both extremes. I'll sketch something shortly.
P.S. I thought of a similiar design using relays but was fairly sure there wouldn't be enough voltage to trigger the last relay after 7 diode drops.0
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8th July 2011 01:18 AM Thread Starter #12ADWSystems
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8th July 2011 02:05 AM #13carbonzit
Yes, exactamente.
If the last few diode drops are a problem (don't think they will be, just set things so that the gate with the most diode drops still gets sufficient gate voltage), you could use Schottky diodes with smaller drops (or germanium: 7 x 0.3 = 2.1V).0Last edited by carbonzit; 8th July 2011 at 11:54 PM. Reason: typo
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8th July 2011 09:37 PM #14crutschow
You would want to use P-MOSFET or PNP bipolar transistors for you switches if you are driving a grounded load.
What is the supply voltage that these switches will be connected to?0Zapper
Curmudgeon Elektroniker
--Inside every little problem is a big problem trying to get out.--
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9th July 2011 12:05 AM Thread Starter #15ADWSystems
I definately don't want to use PNP transistors. Why P MOSFET for grounded rather than N-MOSFETs?
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9th July 2011 12:45 AM #16ronv
I'm still a little confused.
If the input is a rotary switch like you have shown why not just switch in the resistor values like 100, 50, 33, 25 etc.0
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9th July 2011 01:07 AM Thread Starter #17ADWSystems
Because some of the values required aren't (readily) available such as 16.3 and 14.7 ohms.
0Last edited by ADWSystems; 9th July 2011 at 01:09 AM.
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9th July 2011 03:22 AM #18Jaguarjoe
People like Caddock make custom value low ohm resistors that can be very accurate:
Originally Posted by ADWSystems
http://www.electro-tech-online.com/c.../07/TypeMV.pdf
Seeing as they are low ohms, hand winding these wouldn't be too big of a task if the wire was small.
Or just use a handful of multiturn trimpots. Pots would allow you to dial out errors from switch contact resistance or leadwire resistance or jfet resistance which will be significant at low ohms and high precision.0Last edited by Jaguarjoe; 9th July 2011 at 03:34 AM. Reason: Added Caddock link
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9th July 2011 03:43 AM #19ronv
How about 10 + 6.8 and 10 +4.3.
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9th July 2011 03:59 AM #20crutschow
Because to drive a grounded load an N-MOSFET has to operate in a source-follower mode, thus the gate of the N-MOSFET must be 10V above the drain (supply voltage) for a typical FET or 5V above the supply for a logic-level type MOSFET. Originally Posted by ADWSystems
So what's your beef against PNPs?0Zapper
Curmudgeon Elektroniker
--Inside every little problem is a big problem trying to get out.--
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