On Thursday, July 13, 2017 at 11:22:37 PM UTC-4, John Larkin wrote:
> On Thu, 13 Jul 2017 18:42:06 -0700 (PDT), dagmargoodboat@yahoo.com
> wrote:
>
> >On Thursday, July 13, 2017 at 6:16:34 PM UTC-4, Phil Hobbs wrote:
> >> On 07/13/2017 12:21 PM, dagmargoodboat@yahoo.com wrote:
> >> > On Thursday, July 13, 2017 at 12:13:23 PM UTC-4, John Larkin wrote:
> >
> >> >> Some opamps, especially RRO types, are c-load stable.
> >> >>
> >> >> Almost any opamp is c-load stable with enough C!
> >> >
> >> > I once put a 100,000uF low e.s.r. capacitor on the output of a SMPS.
> >> > It cleaned up the load-step response nicely.
> >> >
> >> >
> >> BFC superpowers to the rescue.
> >>
> >> Cheers
> >>
> >> Phil "use a bigger hammer" Hobbs
> >
> >Yep. I had to drive through a 300A load step without losing more than
> >about a volt, IIRC.
> >
> >Hey, Wikipedia's disambiguation page for "BFC" lists one possible
> >meaning as "Engineering slang for a supercapacitor." :-)
>
> It must be French. Beaucoup Farads.
Wouldn't that have to be Beau Farads Coup?
It was a monster computer-grade electrolytic, an absolute beast.
My task was helping a COTS Mean Well 24V supply weather a 300A peak impulse
without losing voltage, and without modifying the Mean Well. That didn't
leave many options, so out came the BFC.
I tested the impulse response and monitored the SMPS' switching--the Mean Well
seemed to like it just fine.
Cheers,
James Arthur
Reply by John Larkin●July 14, 20172017-07-14
On Thu, 13 Jul 2017 18:42:06 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:
>On Thursday, July 13, 2017 at 6:16:34 PM UTC-4, Phil Hobbs wrote:
>> On 07/13/2017 12:21 PM, dagmargoodboat@yahoo.com wrote:
>> > On Thursday, July 13, 2017 at 12:13:23 PM UTC-4, John Larkin wrote:
>
>> >> Some opamps, especially RRO types, are c-load stable.
>> >>
>> >> Almost any opamp is c-load stable with enough C!
>> >
>> > I once put a 100,000uF low e.s.r. capacitor on the output of a SMPS.
>> > It cleaned up the load-step response nicely.
>> >
>> >
>> BFC superpowers to the rescue.
>>
>> Cheers
>>
>> Phil "use a bigger hammer" Hobbs
>
>Yep. I had to drive through a 300A load step without losing more than
>about a volt, IIRC.
>
>Hey, Wikipedia's disambiguation page for "BFC" lists one possible
>meaning as "Engineering slang for a supercapacitor." :-)
>
>Cheers,
>James Arthur
It must be French. Beaucoup Farads.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by John Larkin●July 14, 20172017-07-14
On Thu, 13 Jul 2017 19:09:12 -0700 (PDT), pcdhobbs@gmail.com wrote:
>>Really low-noise amps need low-value feedback resistors to keep
>>Johnson noise down, which implies a lot of opamp current even with no
>>load. So sometimes one adds a buffer just to drive the feedback
>>resistors.
>
>Yup. 300-kelvin resistors are a serious inconvenience when your active devices have 0.01-K noise temperatures. (BF862s are about that good in the flatband.)
>
>Cheers
>
>Phil Hobbs
The NMR amps needed PPM-flat current pulses, so we didn't want thermal
tails in the current shunt amp. Hence the buffer inside the loop.
We made our own current shunts, too. That's a whole nother story.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Reply by ●July 13, 20172017-07-13
On Thu, 13 Jul 2017 06:10:12 -0700 (PDT), dagmargoodboat@yahoo.com
wrote:
>On Thursday, July 13, 2017 at 3:32:23 AM UTC-4, Marke wrote:
>> Hi All,
>>
>> I have a need to generate +100mV DC to drive a load which is capable of sourcing up to ~50mA but will typically see high impedance. Easily available I have a clean 24V source and 208VAC. My intent is to drop from 24V to ~1.8V using a buck converter then follow it up with a 1.0V precision shunt (ADR510 or other) followed by a 9:1 precision resistor divider. Buffer the 100mV output by a low noise single supply op-amp such as ltc1014. Although it looks like ltc1014 won't be able to source enough current so possibly followed with a BJT?
>>
>> Voltage drift is more important than precision for this application and +/- 3mV should be good enough. The environment the board will reside in is ~25C +/- 5deg. The output voltage must not drift above ~130mV to prevent causing issues in the test chamber.
>>
>> The voltage across the load is buffered, low-pass filtered (2-pole active = 1kHz) and multiplied by 10x before being sampled. The low pass filter frequency is chosen to preserve rise times in the ~500uS range.
>>
>> I expect that the largest source of noise will not be from the shunt (70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from the 5 foot run of cable to the test chamber and back and am interested in best practice shielding techniques to mitigate induced noise.
>>
>> I'm not much of an electrical designer and haven't designed anything similar to this before so I'm interested in hearing about other possible topologies that can meet the requirements or how others would approach the problem. The solution will be put onto a pcb as part of a larger circuit so i cannot use a benchtop supply.
>>
>> To summarize:
>>
>> - Vin = +24V and/or 208VAC
>> - Vout= 100mV DC +/- 2mV
>> - Iout = up to 50mA into short circuit load
>>
>> Thank you,
>>
>> Mark
>
>Once you have +1.8V, why not something like this?
>
>
> (view in Courier font)
> +1.8V
>Vref >---. -+-
> | |
> [R1] [R4] 20
> | 100mV |\ |
> +----------|+\ |/ Q1
> | | >---[R3]----|
> [R2] .---|-/ |>.
> | | |/ |
> === | |
> '--------------------+-----> Vout = +100mV
> |
> [1k] R5
> |
> ===
>
>Vout is a precision, low-impedance output. R4 provides
>short-circuit protection. R5 provides an optional
>minimum load.
I need a .4V supply and was going to do exactly this but the opamp is
kinda expensive. I'm used to paying 1/3 of the cost of an opam for
the entire regulator (above 1.1V).
Reply by ●July 13, 20172017-07-13
>Really low-noise amps need low-value feedback resistors to keep
>Johnson noise down, which implies a lot of opamp current even with no
>load. So sometimes one adds a buffer just to drive the feedback
>resistors.
Yup. 300-kelvin resistors are a serious inconvenience when your active devices have 0.01-K noise temperatures. (BF862s are about that good in the flatband.)
Cheers
Phil Hobbs
Reply by ●July 13, 20172017-07-13
On Thursday, July 13, 2017 at 6:16:34 PM UTC-4, Phil Hobbs wrote:
> On 07/13/2017 12:21 PM, dagmargoodboat@yahoo.com wrote:
> > On Thursday, July 13, 2017 at 12:13:23 PM UTC-4, John Larkin wrote:
> >> Some opamps, especially RRO types, are c-load stable.
> >>
> >> Almost any opamp is c-load stable with enough C!
> >
> > I once put a 100,000uF low e.s.r. capacitor on the output of a SMPS.
> > It cleaned up the load-step response nicely.
> >
> >
> BFC superpowers to the rescue.
>
> Cheers
>
> Phil "use a bigger hammer" Hobbs
Yep. I had to drive through a 300A load step without losing more than
about a volt, IIRC.
Hey, Wikipedia's disambiguation page for "BFC" lists one possible
meaning as "Engineering slang for a supercapacitor." :-)
Cheers,
James Arthur
Reply by Phil Hobbs●July 13, 20172017-07-13
On 07/13/2017 12:33 PM, Marke wrote:
> Not to completely derail all the helpful discussion but google groups
> seems like a horrible way to view these topics. I assume most people
> use a usenet reader of some kind?
>
Thunderbird. Other folks like knode, Forte Agent, or one or two others.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Reply by Phil Hobbs●July 13, 20172017-07-13
On 07/13/2017 12:21 PM, dagmargoodboat@yahoo.com wrote:
> On Thursday, July 13, 2017 at 12:13:23 PM UTC-4, John Larkin wrote:
>> On Thu, 13 Jul 2017 09:09:09 -0700 (PDT), George Herold
>> <gherold@teachspin.com> wrote:
>>
>>> On Thursday, July 13, 2017 at 10:56:48 AM UTC-4, dagmarg...@yahoo.com wrote:
>>>> On Thursday, July 13, 2017 at 10:13:13 AM UTC-4, George Herold wrote:
>>>>> On Thursday, July 13, 2017 at 4:00:57 AM UTC-4, Phil Allison wrote:
>>>>>> Marke wrote:
>>>>>>
>>>>>> -----------------
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> I have a need to generate +100mV DC to drive a load which is capable of sourcing up to ~50mA but will typically see high impedance. Easily available I have a clean 24V source and 208VAC. My intent is to drop from 24V to ~1.8V using a buck converter then follow it up with a 1.0V precision shunt (ADR510 or other) followed by a 9:1 precision resistor divider. Buffer the 100mV output by a low noise single supply op-amp such as ltc1014. Although it looks like ltc1014 won't be able to source enough current so possibly followed with a BJT?
>>>>>>>
>>>>>>> Voltage drift is more important than precision for this application and +/- 3mV should be good enough. The environment the board will reside in is ~25C +/- 5deg. The output voltage must not drift above ~130mV to prevent causing issues in the test chamber.
>>>>>>>
>>>>>>> The voltage across the load is buffered, low-pass filtered (2-pole active = 1kHz) and multiplied by 10x before being sampled. The low pass filter frequency is chosen to preserve rise times in the ~500uS range.
>>>>>>>
>>>>>>> I expect that the largest source of noise will not be from the shunt (70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from the 5 foot run of cable to the test chamber and back and am interested in best practice shielding techniques to mitigate induced noise.
>>>>>>>
>>>>>>> I'm not much of an electrical designer and haven't designed anything similar to this before so I'm interested in hearing about other possible topologies that can meet the requirements or how others would approach the problem. The solution will be put onto a pcb as part of a larger circuit so i cannot use a benchtop supply.
>>>>>>>
>>>>>>> To summarize:
>>>>>>>
>>>>>>> - Vin = +24V and/or 208VAC
>>>>>>> - Vout= 100mV DC +/- 2mV
>>>>>>> - Iout = up to 50mA into short circuit load
>>>>>>>
>>>>>>> Thank you,
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> ** Using the KISS principle, all you need is a 5V, 1Amp TO220 reg IC and 3 resistors. The resistors are 120ohm, 82ohm and 1ohm - all 1% types..
>>>>>>
>>>>>> The 120ohm & 82ohm go in parallel to make 48.7ohms, then 1ohm in series across 5V.
>>>>>>
>>>>>> This makes a 1:49.7 divider so you get 100mV with a 1 ohm source impedance and a SCC of 100mA.
>>>>>>
>>>>>> A small cap is also needed across the output of the reg IC for stability.
>>>>>>
>>>>>>
>>>>>>
>>>>>> .... Phil
>>>>>
>>>>> Right, that's what I'd try. If you need a lower source impedance
>>>>> maybe buffer with an opamp. (Is there some dip opamp that does ~100mA
>>>>> besides the TCA0372?.. too lazy to troll Digikey.)
>>>>>
>>>>> George H.
>>>>
>>>> I interpret the OP as needing 100mV and holding +/-3mV for currents of
>>>> 0 <= i.out <= 50mA. Most op-amps would need a buffer for that, yes..
>>>>
>>>> If efficiency doesn't matter, just use an LM317 as the preregulator
>>>> with the Vadj terminal grounded. That gets you to 1.2V with one part.
>>>> An LMV431-1.2 with a 50mA-limiting resistor to +24V might even be
>>>> simpler (& more accurate) for getting to 1.2V.
>>>>
>>>> Cheers,
>>>> James Arthur
>>>
>>> Sure... you know my opamp idea is probably not so good.
>>> I used an opamp as a voltage reference in a circuit and then
>>> found when I hung a bunch of bypass caps on the output...
>>> It sang for me. :^)
>>>
>>> Your circuit with transistor would be better.
>>>
>>> George H.
>>
>> Some opamps, especially RRO types, are c-load stable.
>>
>> Almost any opamp is c-load stable with enough C!
>
> I once put a 100,000uF low e.s.r. capacitor on the output of a SMPS.
> It cleaned up the load-step response nicely.
>
> Cheers,
> James Arthur
>
BFC superpowers to the rescue.
Cheers
Phil "use a bigger hammer" Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Reply by ●July 13, 20172017-07-13
On Thursday, July 13, 2017 at 6:33:51 PM UTC+2, Marke wrote:
> Not to completely derail all the helpful discussion but google groups seems like a horrible way to view these topics. I assume most people use a usenet reader of some kind?
Thunderbird works as a traditional usenet reader. Google groups does rather more for you, so I tend use it, rather than Thunderbird.
--
Bill Sloman, Sydney
Reply by George Herold●July 13, 20172017-07-13
On Thursday, July 13, 2017 at 2:52:29 PM UTC-4, Jon Elson wrote:
> Marke wrote:
>
>
> > I expect that the largest source of noise will not be from the shunt
> > (70ppm/C), the resistor divider (25ppm/C), or the Op-Amp ~160uV but from
> > the 5 foot run of cable to the test chamber and back and am interested in
> > best practice shielding techniques to mitigate induced noise.
> >
> I think you want a 4 wire connection into the chamber, so there is no
> current flowing through the voltage sense wires. Put modest resistors
> outside between +sense and +supply, and -sense and -supply, so if any
> connection in the chamber is broken, it will not cause overvoltage.
> Something like 100 Ohms would be fine. When the 4 wire circuit is hooked up
> right, the resistors will become insignificant. This is the classic remote-
> sense power supply scheme.
>
> For noise, you can use 4 wire shielded cable.
Yeah and get well shielded cable, copper mesh and Aluminum foil.
And make it well bonded to ground everywhere you can.
What's the impedance of the load?
George H.