Post by thread:[EE] analog / current loop

Visiting a client this weekend, he was asking me about doing a simple board (they always think its simple dont they?) that can take either 0-10VDC and change the range to 0-3.3V, and also allow for a 4-20mA input (either...not both at the same time of course) and again push it to 0-3.3VDC By themselves, both are accomplished by an opamp to change the range, but I was looking at some of the 4-20mA receiver chips made by TI and ADI thinking that was the better way of doing it? But to keep the board simple, wouldn't a single opamp work, where using a jumper to select if the incoming signal could either run thru a shunt resistor to get the voltage, or just feed into the negative gain stage? Its an industrial control solution, so better performance trumps simplicity. I still need the answer for the 0-10V if its floating or if its referenced to the board ground.

On Mon, Jan 18, 2010 at 9:34 PM, alan smith <spam_OUTmicro_eng2TakeThisOuTyahoo.com> wrote: > But to keep the board simple, wouldn't a single opamp work, where using a > jumper to select if the incoming signal could either run thru a shunt resistor to > get the voltage, or just feed into the negative gain stage? Its an industrial > control solution, so better performance trumps simplicity. I still need the > answer for the 0-10V if its floating or if its referenced to the board ground. > If there are not many channels, it is ok to use a switch to select current or voltage. The tricky part is often the protection. Some 4-20mA transmitters can start at quite high voltage and you need to protect your current shunt. If the input voltage is not referenced to the board ground, then you will need more expensive circuits depending on the required common mode voltage. -- Xiaofan http://mcuee.blogspot.com

What do you suggest using for protection? Just a zener to clamp the voltage across the sense resistor? --- On Mon, 1/18/10, Xiaofan Chen <.....xiaofancKILLspam@spam@gmail.com> wrote: {Quote hidden}> From: Xiaofan Chen <xiaofancKILLspamgmail.com> > Subject: Re: [EE] analog / current loop > To: "Microcontroller discussion list - Public." <.....piclistKILLspam.....mit.edu> > Date: Monday, January 18, 2010, 5:58 AM > On Mon, Jan 18, 2010 at 9:34 PM, alan > smith <EraseMEmicro_eng2spam_OUTTakeThisOuTyahoo.com> > wrote: > > But to keep the board simple, wouldn't a single opamp > work, where using a > > jumper to select if the incoming signal could either > run thru a shunt resistor to > > get the voltage, or just feed into the negative gain > stage? Its an industrial > > control solution, so better performance trumps > simplicity. I still need the > > answer for the 0-10V if its floating or if its > referenced to the board ground. > > > > If there are not many channels, it is ok to use a switch to > select current > or voltage. The tricky part is often the protection. Some > 4-20mA transmitters > can start at quite high voltage and you need to protect > your current shunt. > > If the input voltage is not referenced to the board ground, > then you will > need more expensive circuits depending on the required > common mode > voltage. > > > > -- > Xiaofan http://mcuee.blogspot.com > > -

On Mon, Jan 18, 2010 at 10:21 PM, alan smith <micro_eng2spam_OUTyahoo.com> wrote: > What do you suggest using for protection? Just a zener to > clamp the voltage across the sense resistor? If you do that, make sure your zener is protected. There might be some IC based protection scheme (from Maxim?). Or you can use a PTC together with some over-voltage protection circuits. -- Xiaofan http://mcuee.blogspot.com

OK, didn't know if you had any example circuits to share, its been about 5 years since I've done a 4-20mA interface, and that one was a controlled environment where I didnt have to worry about the source, and I just used a shunt across the opamp. --- On Mon, 1/18/10, Xiaofan Chen <@spam@xiaofancKILLspamgmail.com> wrote: {Quote hidden}> From: Xiaofan Chen <KILLspamxiaofancKILLspamgmail.com> > Subject: Re: [EE] analog / current loop > To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTmit.edu> > Date: Monday, January 18, 2010, 7:02 AM > On Mon, Jan 18, 2010 at 10:21 PM, > alan smith <spamBeGonemicro_eng2spamBeGoneyahoo.com> > wrote: > > What do you suggest using for protection? Just a zener > to > > clamp the voltage across the sense resistor? > > If you do that, make sure your zener is protected. There > might be > some IC based protection scheme (from Maxim?). Or you can > use > a PTC together with some over-voltage protection circuits. > > > -- > Xiaofan http://mcuee.blogspot.com > --

> Visiting a client this weekend, he was asking me about doing a simple board (they always think its simple dont they?) ... Mini Glossary I built up along the years: Simple circuit - must cost less than the Chinese equivalent they reject due to too low performance Definitive design - at least 3 major changes will be requested in the last week before the project must be delivered. We know exactly what this should do - the previous consultant who left in despair left his beer coaster skizzo with them, and they are going to show it to you. You must sign our NDA but we can't tell you what it is about - they have no clue how to solve the problem, but hope to pick your mind for it in endless mind-numbing 'brainstorming' talks which are not billable. We already have a working prototype in <insert BASIC based interpreted language here>, we want you to clean it up, it's a 2 week job at most - a worker with half an arts degree wrote spaghetti code in his spare time in the only language he 'knows' for a time-sharing real-time application. I could go on for a while like this, but I stop here. I assure you that the above are based on real life occurrences. I am not sure if it is unprofessional to talk about these things or whether it is unprofessional *not* to talk about them. Peter

> I am not sure if it is unprofessional to talk about these things or whether it > is unprofessional *not* to talk about them. It is unprofessional to talk to such clients any longer... The list is all too familiar. I tend to go into alarm mode on the mentioning of any percentage in the 80 .. 100 range, with 95 and 99 triggering berserk mode. No need to check what the percentage refers to. -- Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

alan smith wrote: > Visiting a client this weekend, he was asking > me about doing a simple board Better connect to the client directly. By the way, who was that misterious "he" who was visiting a client this weekend and at the time was asking you about doing a simple board? Is he EE? > (they always think its simple dont they?) that > can take either 0-10VDC and change the range > to 0-3.3V, and also allow for a 4-20mA input > (either...not both at the same time of course) > and again push it to 0-3.3VDC You may wish to tell that "he" to use 2 optocouples: one to be fed by 4-20mA circuit and the other - by 0-10VDC circuit (some Op Amp needed to feed the second optocouple to shift the voltage a bit) Both optocouples are to be connected to a resistor. The second leg of the resistor will be grounded. Amplify the voltage on the resistor to fit the needed range. The circuit that is connected to the inputs would produce the input signal.

On Mon, Jan 18, 2010 at 11:29 PM, alan smith <TakeThisOuTmicro_eng2EraseMEspam_OUTyahoo.com> wrote: > OK, didn't know if you had any example circuits to share, its > been about 5 years since I've done a 4-20mA interface, and > that one was a controlled environment where I didnt have to > worry about the source, and I just used a shunt across the opamp. For a start, try this. The protection may be good for your client. This is quite a good article. We work closely with ADI (as well as Maxim and TI) in the analog front. http://www.analog.com/library/analogDialogue/archives/43-04/process_control.pdf We are doing a bit more but our analog I/O modules typically cost more. -- Xiaofan http://mcuee.blogspot.com

Good reference...thanks for that! --- On Tue, 1/19/10, Xiaofan Chen <RemoveMExiaofancTakeThisOuTgmail.com> wrote: {Quote hidden}> From: Xiaofan Chen <xiaofancEraseME.....gmail.com> > Subject: Re: [EE] analog / current loop > To: "Microcontroller discussion list - Public." <EraseMEpiclistmit.edu> > Date: Tuesday, January 19, 2010, 4:16 AM > On Mon, Jan 18, 2010 at 11:29 PM, > alan smith <RemoveMEmicro_eng2EraseMEEraseMEyahoo.com> > wrote: > > OK, didn't know if you had any example circuits to > share, its > > been about 5 years since I've done a 4-20mA interface, > and > > that one was a controlled environment where I didnt > have to > > worry about the source, and I just used a shunt across > the opamp. > > For a start, try this. The protection may be good for your > client. This is quite > a good article. We work closely with ADI (as well as Maxim > and TI) > in the analog front. > www.analog.com/library/analogDialogue/archives/43-04/process_control.pdf > > We are doing a bit more but our analog I/O modules > typically cost more. > > -- > Xiaofan http://mcuee.blogspot.com > --