does anyone know how these electronically controlled resistance boxes work? - Page 1
Forum section: electronics / general

author: croyleje
author_rank: Regular Contributor
timestamp: June 20, 2012, 07:00:50 pm
content: hi everyone,i saw this on line and in next months EPS the cover story is for a 16 bit digital pot just wondering how these work looks like it would be a fun project. would be nice to be able to just type in needed resistance and get it out.http://www.tequipment.net/IETOhmSource.htmlalso might make a cool teardown.is it just a precision voltage reference?found a DIY one on line but it just used lots and lots or relays kinda doggy.Jason

author: chrome
author_rank: Regular Contributor
timestamp: June 20, 2012, 07:20:21 pm
content: I also wonder how this is done

author: kripton2035
author_rank: Super Contributor
timestamp: June 20, 2012, 07:27:22 pm
content: +1 I saw this device some times ago and also wonder how ...?Dave, get one and open it !!!I thought of controling precisely the gate of a fet transistor, you can get the current between source and drainand then act as a resistance ?

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: June 20, 2012, 07:31:57 pm
content: I have an older but similar unit manufactured by ProStat. It has a bunch of super-mini mercury wetted reed relays that shunt precision resistors.

author: SeanB
author_rank: Super Contributor
timestamp: June 20, 2012, 07:40:06 pm
content: Probably uses a mosfet as a resistor, controlling the gate voltage to synthesise the required resistance by measuring the voltage and current through it.

author: croyleje
author_rank: Regular Contributor
timestamp: June 20, 2012, 08:08:01 pm
content: Quote from: JohnS_AZ on June 20, 2012, 07:31:57 pmyes i have found plans online of this type but i don't think you can get the precision these unit claim using that method. the idea of using the mosfet and controlling the gate voltage sounds promising but lets see if i got this right. the mosfet would be basically an open with no voltage then has you increase voltage resistance would drop. but what about saturation of the fet and also if resistance of the load circuit changes then wouldn't the fet be effected also?

author: croyleje
author_rank: Regular Contributor
timestamp: June 20, 2012, 08:09:04 pm
content: i think this would make a great teardown or atleast would like to see what Dave thinks about how they work.

author: Scarionn
author_rank: Contributor
timestamp: June 20, 2012, 08:19:23 pm
content: My guess is, A op amp with a variable gain controlling a mosfet.

author: amspire
author_rank: Super Contributor
timestamp: June 21, 2012, 02:27:20 am
content: The fact they have a power specification but no voltage limits makes it sound like it is switched resistors. The limit of 24 MOhms for the 0.1 ohm resolution model sounds like it is 32 binary resistors in series, each with a latching relay across the resistor. 0.1-0.2-0.4-0.8-1.6-3.2-6.4-12.8-25.6-51.2-102.4 etc up to 13.4 MOhms.The data sheet does say "Uses Real Resistors" so that is another reason to think resistors and relays are used. They say that the minumum resistance is about 1 ohm. A typical relay has a contact resistance when new of about 20 milliohms, so 32 in series would be 0.64 ohms so it fits. The software calibration compensates for the relay resistance.The OS250 model look like it is the OS-260 model without 3 relays and without the 0.1, 0.2 and 0.4 ohm resistors.For a product from a company like IET, the specifications seem to be very meagre. Makes me suspicious. There should be temp coefficient specs, long term drift specs, maximum voltage ratings (I do not believe it can take 1W at 24MOhms as the specs imply - that would be almost 5,000 VDC).Richard.

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 21, 2012, 02:56:43 am
content: An important feature is the current limiter. I picked up a used IET manual resistance substitution box cheap on eBay with a cracked case. Inside, four of the resistors controlling the second decade were burned out. It looks easy to fix (I have resistors on order) but even easier to blow up again!