> (My goal is to add digital voltage and current monitoring to some > older bench power supplies.) Measuring voltage is simple. If the input range is too big, you need a few resistors. If it's small, you lose a few bits from the top of the A/D. Add an op-amp if you need them. Measuring current gets tricky. There are several chips designed for measuring current on 48V busses. I think they are just an instrumentation amplifier with big enough resistors on the front end so that the common range goes up to 48V. Alternatively, if the supply is floating, you can put the sense resistor in the ground lead, but you still need a good amplifier to make the signal big enough for an ADC. -- These are my opinions, not necessarily my employer's. I hate spam.

In message <20080118095515.8B26BBE34@ip-64-139-1-69.sjc.megapath.net>, Hal Murr ay writes: > >> (My goal is to add digital voltage and current monitoring to some >> older bench power supplies.) > >Measuring voltage is simple. If the input range is too big, you need a few >resistors. If it's small, you lose a few bits from the top of the A/D. Add >an op-amp if you need them. > >Measuring current gets tricky. Actually not, there are several good chips like the INA197. Or the very simple way, buy a suitable hall-effect current monitor like LTSR from Lem.com. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.

Be careful that if you do not isolate the A/D converter analog circuits from the USB ground, you may create all sorts of nasty ground loops with the computer. When dealing with power supplies, it's always best to be isolated. I do not know enough about USB to know if it can be simply isolated with opto couplers, but I know that RS-232 is very easy to isolate. So if your data rate is not great, a serial solution would be easier to implement (even if you go through a serial-USB adapter for convenience on the PC side) Linear Technology, Analog Devices and Maxim have hot side current sense chips that are accurate and cheap. They are specialized diff amps, not regular IA. LT for instance has one that has less than 50uV offset (LT1787, see Design Note 227) Didier KO4BB > -----Original Message----- > From: time-nuts-bounces@febo.com > [mailto:time-nuts-bounces@febo.com] On Behalf Of Hal Murray > Sent: Friday, January 18, 2008 3:55 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Slightly OT: inexpensive USB > analog-digital converter? > > > > (My goal is to add digital voltage and current monitoring to some > > older bench power supplies.) > > Measuring voltage is simple. If the input range is too big, > you need a few resistors. If it's small, you lose a few bits > from the top of the A/D. Add an op-amp if you need them. > > Measuring current gets tricky. > > There are several chips designed for measuring current on 48V > busses. I think they are just an instrumentation amplifier > with big enough resistors on the front end so that the common > range goes up to 48V. > > Alternatively, if the supply is floating, you can put the > sense resistor in the ground lead, but you still need a good > amplifier to make the signal big enough for an ADC. > > > -- > These are my opinions, not necessarily my employer's. I hate spam. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release > Date: 1/13/2008 8:23 PM > > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release Date: 1/13/2008 8:23 PM

I forgot the link: http://www.linear.com/ad/current_sense.jsp Didier KO4BB > -----Original Message----- > From: time-nuts-bounces@febo.com > [mailto:time-nuts-bounces@febo.com] On Behalf Of Hal Murray > Sent: Friday, January 18, 2008 3:55 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Slightly OT: inexpensive USB > analog-digital converter? > > > > (My goal is to add digital voltage and current monitoring to some > > older bench power supplies.) > > Measuring voltage is simple. If the input range is too big, > you need a few resistors. If it's small, you lose a few bits > from the top of the A/D. Add an op-amp if you need them. > > Measuring current gets tricky. > > There are several chips designed for measuring current on 48V > busses. I think they are just an instrumentation amplifier > with big enough resistors on the front end so that the common > range goes up to 48V. > > Alternatively, if the supply is floating, you can put the > sense resistor in the ground lead, but you still need a good > amplifier to make the signal big enough for an ADC. > > > -- > These are my opinions, not necessarily my employer's. I hate spam. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release > Date: 1/13/2008 8:23 PM > > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release Date: 1/13/2008 8:23 PM

The LTC6102 is even better (did not notice it before) with 10uV max offset. That lets you use a very small series resistor (in the milli-ohm range) and still have milliamp resolution and several amps of full scale dynamic range. I am in the process of doing exactly what you are talking about (power supply remote monitoring and control). I currently have a Maxim current sense chip in my power supply (MAX471, no longer manufactured) which I will replace with probably the LT6102, the processor is a TI MSC1210 which, for all its quirks, has a 24 bit A/D converter built in. The voltage reference is an AD580, not the best but sufficient and I have several of those. I have used that chip before to plot the current draw versus temperature of an HP 10811's oven. The temperature was measured using a 10k NTC thermistor and the A/D converter of the same TI chip: http://www.ko4bb.com/Test_Equipment/HP10811-Current.png Here is the link to the Linear Tech page on high side current sense (again): http://www.linear.com/ad/current_sense.jsp Didier KO4BB > -----Original Message----- > From: time-nuts-bounces@febo.com > [mailto:time-nuts-bounces@febo.com] On Behalf Of Didier Juges > Sent: Friday, January 18, 2008 5:26 AM > To: 'Discussion of precise time and frequency measurement' > Subject: Re: [time-nuts] Slightly OT: inexpensive USB > analog-digitalconverter? > > Be careful that if you do not isolate the A/D converter > analog circuits from the USB ground, you may create all sorts > of nasty ground loops with the computer. When dealing with > power supplies, it's always best to be isolated. > I do not know enough about USB to know if it can be simply > isolated with opto couplers, but I know that RS-232 is very > easy to isolate. So if your data rate is not great, a serial > solution would be easier to implement (even if you go through > a serial-USB adapter for convenience on the PC side) > > Linear Technology, Analog Devices and Maxim have hot side > current sense chips that are accurate and cheap. They are > specialized diff amps, not regular IA. LT for instance has > one that has less than 50uV offset (LT1787, see Design Note 227) > > Didier KO4BB > > > > -----Original Message----- > > From: time-nuts-bounces@febo.com > > [mailto:time-nuts-bounces@febo.com] On Behalf Of Hal Murray > > Sent: Friday, January 18, 2008 3:55 AM > > To: Discussion of precise time and frequency measurement > > Subject: Re: [time-nuts] Slightly OT: inexpensive USB > analog-digital > > converter? > > > > > > > (My goal is to add digital voltage and current monitoring to some > > > older bench power supplies.) > > > > Measuring voltage is simple. If the input range is too > big, you need > > a few resistors. If it's small, you lose a few bits from > the top of > > the A/D. Add an op-amp if you need them. > > > > Measuring current gets tricky. > > > > There are several chips designed for measuring current on > 48V busses. > > I think they are just an instrumentation amplifier with big enough > > resistors on the front end so that the common range goes up to 48V. > > > > Alternatively, if the supply is floating, you can put the sense > > resistor in the ground lead, but you still need a good amplifier to > > make the signal big enough for an ADC. > > > > > > -- > > These are my opinions, not necessarily my employer's. I hate spam. > > > > > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > No virus found in this incoming message. > > Checked by AVG Free Edition. > > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release > > Date: 1/13/2008 8:23 PM > > > > > > No virus found in this outgoing message. > Checked by AVG Free Edition. > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release > Date: 1/13/2008 > 8:23 PM > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, > go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release > Date: 1/13/2008 8:23 PM > > No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.516 / Virus Database: 269.19.2/1223 - Release Date: 1/13/2008 8:23 PM

This has been a really interesting thread. I think I am going to get a labjack too.They have a 24 bit A/D model but it is expensive and has features I don't need. I am looking for something with 20 bit+ resolution but a little cheaper. The sampling rate can be very slow. Thanks in advance-Patrick

Hal Murray wrote: >> (My goal is to add digital voltage and current monitoring to some >> older bench power supplies.) > > Measuring voltage is simple. If the input range is too big, you need a few > resistors. If it's small, you lose a few bits from the top of the A/D. Add > an op-amp if you need them. > > Measuring current gets tricky. Fortunately the power supplies in question already have analog voltage and current metering, so I should be able to just tap into the existing shunt resistors, though I may need to do some calibration and scaling. John

Hal Murray wrote: > Alternatively, if the supply is floating, you can put the sense resistor in > the ground lead, but you still need a good amplifier to make the signal big > enough for an ADC. Sorry... my earlier response blew past the issue of whether you have enough voltage developed for the ADC to use. Good point, and I'll keep in mind using an amp in front of the ADC. John

Didier Juges wrote: > Be careful that if you do not isolate the A/D converter analog circuits from > the USB ground, you may create all sorts of nasty ground loops with the > computer. When dealing with power supplies, it's always best to be isolated. > I do not know enough about USB to know if it can be simply isolated with > opto couplers, but I know that RS-232 is very easy to isolate. So if your > data rate is not great, a serial solution would be easier to implement (even > if you go through a serial-USB adapter for convenience on the PC side) > > Linear Technology, Analog Devices and Maxim have hot side current sense > chips that are accurate and cheap. They are specialized diff amps, not > regular IA. LT for instance has one that has less than 50uV offset (LT1787, > see Design Note 227) That's a really good point. Thanks for reminding me of the potential ground loop problems. John

Hi John, One other possible problem is that the existing shunts may be in the hot side of the supply. This could exceed the common mode range of the ADC or even the amplifier (if used). Robert G8RPI. John Ackermann N8UR <jra@febo.com> wrote: Didier Juges wrote: > Be careful that if you do not isolate the A/D converter analog circuits from > the USB ground, you may create all sorts of nasty ground loops with the > computer. When dealing with power supplies, it's always best to be isolated. > I do not know enough about USB to know if it can be simply isolated with > opto couplers, but I know that RS-232 is very easy to isolate. So if your > data rate is not great, a serial solution would be easier to implement (even > if you go through a serial-USB adapter for convenience on the PC side) > > Linear Technology, Analog Devices and Maxim have hot side current sense > chips that are accurate and cheap. They are specialized diff amps, not > regular IA. LT for instance has one that has less than 50uV offset (LT1787, > see Design Note 227) That's a really good point. Thanks for reminding me of the potential ground loop problems. John _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. --------------------------------- Yahoo! Answers - Get better answers from someone who knows. Tryit now.