Idea for fast variable resistive load (0.2 - 1MHz) - Page 1
Forum section: electronics / projects

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 13, 2017, 12:30:34 am
content: I have circuit I've designed which I need to test, but in order to do so I need a variable resistive load I can control with my signal generator.This way I can vary the resistance as function of frequency.I was considering just using a simple transistor. The load would need to vary from around 10k - 1M Ohms at a periodic frequency up 1MHz.Is it standard practice to use a transistor for an application such as this or is there a better method?Thanks!

author: Rerouter
author_rank: Super Contributor
timestamp: April 13, 2017, 02:18:17 am
content: the hard part is your periodic 1Mhz requirement, as this generally means the control loop needs to have a response a fair bit faster than that to keep up, if its a sine or triangle, it stands a chance, but square would be very difficult to not ring longer than the waveform,Might i ask what resistance vs frequency pattern / curve you need to match? if its a log curve you can likely just use a RLC circuit to match it.

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 13, 2017, 02:44:52 am
content: Quote from: Rerouter on April 13, 2017, 02:18:17 amThank you for the reply.I would apply a sine wave at a particular frequency to oscillate a restive load's value.Or I could apply a triangle as well. A square wave would not work.Resistance would likely be logarithmic, with resistance starting out at around 10kOhm then quickly going to "infinity" over a very short time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2017, 06:05:52 am
content: Willing to take your idea at face value but let's take a quick X-Y problem examination:What are the characteristics of your signal source?How important is it, that the load actually be resistive, and variable?How much reactance is tolerable?If you don't understand these questions, don't hesitate to ask... but also, don't be surprised if their answers lead to a very different solution!Tim

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: April 13, 2017, 06:28:11 am
content: what I, V? I assume gnded from the word "load" - floating 2 terminal programmable is R a bigger problemsmall signal long channel fets are used as variable R, some parasitic C, only ma currentsanother approach would be multiplier controlled current sink, multiplying a V from a ref R and your control signal

author: bson
author_rank: Unknown
timestamp: April 13, 2017, 10:05:41 am
content: An optocoupler might work for this.https://www.digikey.com/product-detail/en/luna-optoelectronics/NSL-32SR2/NSL-32SR2-ND/5039808Edit: nvm, didn't notice you want to change the resistance at up to 1MHz, not pass 1MHz through it. These things are very slow, on the order of ms to change.

author: danmcb
author_rank: Regular Contributor
timestamp: April 13, 2017, 10:12:37 am
content: Quote from: Plasmateur on April 13, 2017, 12:30:34 amIs this what you actually mean?Do you want the resistance to vary with frequency (e.g. 10kohm at 100kHz, 20kohm at 1MHz or whatever)?Or do you want a resistor whose resistance can waggle about as fast as 1MHz?

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 13, 2017, 03:03:41 pm
content: Quote from: T3sl4co1l on April 13, 2017, 06:05:52 amI am attempting to simulate what is attached in the picture. I need to know the current across a resistor. And that current will depend on another resistor which will be the variable resistor.

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 13, 2017, 03:05:02 pm
content: Quote from: danmcb on April 13, 2017, 10:12:37 amSorry. I just face palmed myself. Yes I need the resistance to oscillate or waggle or ramp over a short time.

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 13, 2017, 03:06:58 pm
content: Quote from: f5r5e5d on April 13, 2017, 06:28:11 amIt appears the fets might be the best option for me. I only need a few mA of current

author: PointyOintment
author_rank: Frequent Contributor
timestamp: April 14, 2017, 04:46:06 am
content: Quote from: Plasmateur on April 13, 2017, 03:06:58 pmHow much voltage, though? Plasma suggests HV… MOSFETs don't go higher than hundreds of volts.

author: Plasmateur
author_rank: Regular Contributor
timestamp: April 15, 2017, 12:58:39 am
content: Quote from: PointyOintment on April 14, 2017, 04:46:06 amIt's not high voltage in the plasma for this particular regime.The plasma's space potential is only a few volts at most.