Digital potentiometer with low value of resistance (0-100 Ohm) - Page 1
Forum section: electronics / projects

author: Javier
author_rank: Contributor
timestamp: January 06, 2015, 07:58:17 pm
content: Hello everybody,I need to implement a digitally controlled potentiometer to test the response of a circuit (I need to vary the gate resistance of a MOSFET, so I need this value of resistance, not the value of the ratio of two resistances like in a voltage divider). The value of the resistance must vary between 0-100 Ohm. A digital potentiometer like the AD8400 has 256 steps from 0 to 1kOhm, so in theory I could just use the first 25 steps, but I would be "wasting" a great percentage of the steps. So my question is: Why there is no digital potentiometer with a value of resistance in a lower range (0-100 Ohm or so)? Is there some clever way to achieve what I'm looking for?Cheers

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2015, 08:03:41 pm
content: It's not going to happen. The wiper resistance of the AD8400 is between 50R and 100R.You won't get a low resistance because of the tiny CMOS transistors used inside the IC will have a channel resistance of more than 50R.

author: dannyf
author_rank: Super Contributor
timestamp: January 06, 2015, 08:18:36 pm
content: Before you get too carried on with this digital pot, think about what you are dealing with and if the digital pot idea is really good.Having said that, there are pots with low(er) full scale resistance. If that doesn't work for you, you can always parallel your digital pot: two gets you down to 500ohm. Four to 250ohm, etc.

author: vargoal
author_rank: Regular Contributor
timestamp: January 07, 2015, 04:39:39 am
content: Wouldn't putting the digital potentiometer in parallel with a 10k ohm resistor allow you to utilize a better range because the overall resistance will be decreased.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2015, 04:59:38 am
content: Given gate drive currents, I don't think I would trust any electronic thingamagiggy simulating a resistor.

author: AndyC_772
author_rank: Super Contributor
timestamp: January 07, 2015, 08:57:42 am
content: The effect of any gate resistor is to change the transient behaviour of the MOSFET, so it's not just the resistance you need to control, but the inductance too. Anything involving strings of components, switches, or more than a few millimetres of PCB trace, is likely to skew your results to the point of uselessness.I'd do the experiment by physically swapping resistors according to a successive approximation algorithm. Start with 50R, and measure the figure of merit that you're looking for (eg. conversion efficiency, EMI, or whatever). Then change it for 75R and see if it's better or worse. If worse, try 25R instead.You may well find you don't need many swaps at all, to get a value that's close enough to optimal, or to show that changing gate resistors isn't going to give the performance you need.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 07, 2015, 01:47:46 pm
content: Drop the digital control, buy some assortment of different resistors and glue them to ice sticks (I call it Resistor-on-a-stick, trademark almost pending)!

author: Kalvin
author_rank: Super Contributor
timestamp: January 07, 2015, 02:06:17 pm
content: A servo or a stepper-motor turning a 100ohm potentiometer?

author: babysitter
author_rank: Frequent Contributor
timestamp: January 07, 2015, 02:36:29 pm
content: Stepper motor for a resistor revolver and a pull-magnet to press it against the pcb !

author: zapta
author_rank: Super Contributor
timestamp: January 07, 2015, 05:35:34 pm
content: Quote from: Seekonk on January 07, 2015, 04:59:38 amAny programable electronic load can simulate constant resistance at high currents.

author: zapta
author_rank: Super Contributor
timestamp: January 07, 2015, 05:38:35 pm
content: Quote from: vargoal on January 07, 2015, 04:39:39 amIt will reduce the range but will also make the steps non linear. Nothing is free ;-)

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2015, 06:12:54 pm
content: We obviously share a different belief system.

author: c4757p
author_rank: Super Contributor
timestamp: January 07, 2015, 06:36:01 pm
content: Quote from: zapta on January 07, 2015, 05:35:34 pmIt's gate drive - you're not just concerned with I, but also dI/dt. Good luck making an electronic load that can handle the very high dI/dt of a gate driver.This is definitely something I'd test by actually swapping physical resistors, and they'd likely be small, surface-mount (lower inductance) types, regardless of what a PITA they might be to swap.

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2015, 08:04:12 pm
content: As mentioned, any gate drive layout that includes enough "real estate", to borrow a phrase from our American friends, to include some form of automatic resistance control is going to have such a large loop area (inductance) that the end results will be totally meaningless. IE, you built it, sweep your gate resistances, find the optimum, rebuild a normal layout with the same resistor and bingo, you're miles away from optimum again!It's not a long process to "half step" to converge on a realistic resistance value for any given layout and switching frequency to do it with just changing decrete values.

author: Javier
author_rank: Contributor
timestamp: February 20, 2015, 05:16:06 pm
content: Thanks to all ! (Sorry that I answer so late but better late than never). I'm fully aware, that this automatic set-up will have more parasitic inductance than a fixed system, but we're building a demo-system for educational purposes, so we can live with that.@ Hero999 : Thanks a lot! Your comment about wiper resistance really enlightened me.@ DiligentMinds.com : Thanks a lot! Thats a really clever idea.Cheers!