Volt nuttery: calibrator on a (USB bus-powered) stick - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 10:21:10 pm
content: Heh heh, just a plain old LM399, in a USB flash drive-like form factor...The circuitry around IC1, from the USB plug to capacitor C3, is a 1.2MHz boost converter that emits a voltage somewhere over 15V. IC3 (TL431), IC4 (BCV62) and resistors R4, R5 forms a current mirror to drive the LM399 with a constant current. Trimpot R6, resistors R7, R8 and resistor network RN2 forms the fine adjustment network that emits 5V. Finally the output op amp IC5 and resistor network RN2 emits a 10V reference voltage, filtered by capacitor C4.The circuit is adapted from Linear Technology's application note regarding LM399, and instead of expensive precision resistors, resistor networks are used as a cheaper alternative of a fixed ratio voltage divider because resistors on the same chip tend to drift in the same way, cancelling out each other's temperature coefficient.

author: ralphd
author_rank: Frequent Contributor
timestamp: August 23, 2015, 05:29:46 am
content: Quote from: technix on August 22, 2015, 10:21:10 pmAn explanation of the circuit would help.

author: Andreas
author_rank: Super Contributor
timestamp: August 23, 2015, 07:05:07 am
content: Quote from: technix on August 22, 2015, 10:21:10 pmKeep on dreaming.Standard thick film resistors will never cancel out in the range of a LM399.by the way: the LM399 in the cirquit diagram has the wrong polarity.And a USB-port is the worst power supply you can give to a EMI-sensitive device like a LM399.With best regardsAndreas

author: plesa
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:18:34 am
content: Throw it away and use some design from LM399 thread. Like multiple LM399 with PWW resistors or thin film with low tempco.In you case I suppose that better will work LT1027 ( one or multiple) with external decent trimmer to adjust voltage (Vishay 1280 or Bourns 3250) also add better power supply than USB ( battery or linear regulator).

author: technix
author_rank: Super Contributor
timestamp: August 23, 2015, 07:22:46 am
content: Quote from: plesa on August 23, 2015, 07:18:34 amTry fit them in the size I have here. Also whatever you are suggesting they are EXPENSIVE! On my board there are only three precision component: LM399, OP07 and the trim pot. Everything else are not - including the divider resistor network - since all I need is the ratio to stick and resistor networks tend to drift together.

author: plesa
author_rank: Frequent Contributor
timestamp: August 23, 2015, 07:32:35 am
content: LT1027 is cheap (5-6 USD) annd you will needs only few external components ( if you do not needs to adjust the exact 5/10V on output you can forget the relativelly expensive trimmer).Check datasheet.

author: TiN
author_rank: Super Contributor
timestamp: August 23, 2015, 07:35:43 am
content: Mostly whole reason why references are stable like they are listed in appnotes are _because_ of those expensive stable resistors.There is no way around it, unless you don't care of stability. But then much cheaper integral VREF chip such as AD58x would show way better performance, than poorly powered LM399 with discrete opamp ratio.

author: jd
author_rank: Contributor
timestamp: August 23, 2015, 07:55:11 am
content: QuoteSounds like a challenge!...A programmable PWM arrangement should work, without needing precision resistors. But I agree a USB stick is about the worst possible thing to build a reference in.

author: tszaboo
author_rank: Super Contributor
timestamp: August 23, 2015, 08:08:26 am
content: Quote from: technix on August 23, 2015, 07:22:46 amProbably the reason no-one does it this way is because it doesn't work. Noname Chinese boost converter boost converter powering a LM399 with a random divider... No thank you. Even the OP07 is a bad choice for this.Even if you would measure the output of several selected units with a 3458A I wouldnt trust this further than I can throw it.

author: ralphd
author_rank: Frequent Contributor
timestamp: August 23, 2015, 01:31:02 pm
content: Quote from: plesa on August 23, 2015, 07:32:35 amIf you're not going to trim, then would the REF5050 be a cheaper overall choice? I ordered 2 0.05% REF5050s for $3.5 (175c ea) of aliexpress.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: August 23, 2015, 03:31:07 pm
content: maybe a usb flail with a 1lb inductor and a big capacitor and linear regulator heat sink to provide a power supply for the LM399