what is the purpose of "don't float away" resistors? - Page 1
Forum section: electronics / beginners

author: wergor
author_rank: Contributor
timestamp: November 03, 2021, 01:13:54 pm
content: We have a Stanford Research Lock-In amp in out lab, I was browsing through its schematic and came across "don't float away" resistors, weak resistors tying the grounds of isolated power domains to the chassis ground:What is the purpose of these resistors? Is it problematic if different power domains are floating with respect to each other? Won't tying the grounds to chassis ground risk injecting noise into the circuit?

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2021, 04:26:13 pm
content: If the separate assemblies are galvanically isolated from each other, which could be the case in a low noise design, then a relatively high value resistor tied to the chassis ground will sink any leakage currents and keep the potential between assemblies low. This must also be done for safety if high voltage are involved to prevent isolated assemblies from charging to a dangerous voltage through leakage.

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 03, 2021, 04:40:50 pm
content: Any thoughts on having capacitors in parallell with the resistors for HF coupling to ground?

author: David Hess
author_rank: Super Contributor
timestamp: November 03, 2021, 06:36:05 pm
content: Quote from: AndersJ on November 03, 2021, 04:40:50 pmI have added parallel capacitors for exactly that reason before.

author: AndersJ
author_rank: Frequent Contributor
timestamp: November 03, 2021, 06:57:50 pm
content: Me too.I have now and then needed to defend those design decisions when told that”everything” should be galvanically connected to ”ground”, i.e. protective ground.This is unfortunately how a PC is designed,and causes isolation issues when my systems are connected to PC’s.My reason for floating my products is to prevent ground loops,especially in industrial environments where ground potentials oftendiffer between various parts of the plant.Comment and insights much appreciated, and appologies for hijacking this thread.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2021, 07:16:30 pm
content: For protection groundind (to protective earth) is sometimes needed.One can have an inductor in the path to PE, to at least breake a possible ground loop for the RF range.I have seen connections with back to back diodes - not totally by the norm, but better than nothing.The resistors against floating parts are relatively low. I would normally expect more like 100 K to 1 M. Just highly isolated can be a problem as static charge can build up under rare conditions and than cause damages from relatively powerfull discharge, especially if the is a capacitor to keep it grounded in the RF range.

author: David Hess
author_rank: Super Contributor
timestamp: November 04, 2021, 05:33:39 pm
content: Quote from: AndersJ on November 03, 2021, 06:57:50 pmYou have to be very careful when dealing with external connections which connect additional grounds. If you use a hard ground internally, then it sets up for a massive ground loop which can destroy things. If you use resistor/capacitor isolation, then the external ground can pull the ground of the module all over the place which can also destroy things.If you really need to protect against a hostile ground connection, then galvanic isolation is the best option.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 04, 2021, 09:33:32 pm
content: Quote from: David Hess on November 03, 2021, 04:26:13 pmEven if the voltages aren't dangerous they can damage the equipment itself. If one instrument is at 50 V relative to another and you plug a cable between them the first point to contact will see an effective ESD event. If that is a chassis ground maybe that is safe but if it is an sensitive input maybe not. I've blown out the JFET inputs in SRS preamps doing exactly that.

author: David Hess
author_rank: Super Contributor
timestamp: November 05, 2021, 04:37:00 am
content: Quote from: ejeffrey on November 04, 2021, 09:33:32 pmI have blown out a few logic drivers and receivers that way, and removed some ground traces which can be interesting because the equipment may be left in a condition where it only works when something else is connected and providing ground to part of the circuit instead of the reverse.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: November 05, 2021, 04:47:18 pm
content: Quote from: AndersJ on November 03, 2021, 06:57:50 pmTry this thread:https://www.eevblog.com/forum/metrology/metrology-related-bookshelf-recommended-read-about-science-of-measurements/In particular a good starting point is the books by Ralph Morrison.