DIY Voltage Reference Done - but a question! - Page 1
Forum section: electronics / beginners

author: stevedc
author_rank: Contributor
timestamp: May 05, 2015, 09:48:08 pm
content: Hi everybody,I had a lot of fun with a first project, building a DIY 10V voltage reference on a breadboard using an LT1236 -- built with the intention of a) learning and b) seeing how accurate my DMM is.I've attached the schematic that represents the circuit currently (you'll see I was playing around with diode protection).But I do have a question: some designs I've seen online also show a series input resistor for the circuit... so I'm wondering -1) why add an input resistor to this circuit? does it provide some kind of protection?2) if it is needed for protection, how to determine the resistor value?3) should it go before or after D1?

author: stevedc
author_rank: Contributor
timestamp: May 06, 2015, 02:59:11 pm
content: Adding inline image of my 10V reference so folks don't have to download the pdf...

author: ivan747
author_rank: Super Contributor
timestamp: May 06, 2015, 05:39:09 pm
content: Can you add a link to the circuit you saw with that resistor? It could be one of at least two things:*To form a low pass filter together with the decoupling cap*To provide ESD protection*To protect the circuit, somehowIt all depends on where it's placed and how large is the resistance.

author: stevedc
author_rank: Contributor
timestamp: May 06, 2015, 05:53:56 pm
content: Ivan - sure - here it is. It's from the September 2007 issue of Nuts and Volts-Steve

author: RR
author_rank: Contributor
timestamp: May 06, 2015, 07:02:23 pm
content: That R1 is for current limit. With TL431 it makes simple voltage regulator.

author: Rudane
author_rank: Regular Contributor
timestamp: May 06, 2015, 07:04:46 pm
content: Those are two different circuits. I think the resistor in the second is all about the Zener diode. Look up the TL431 and you'll see you need to keep the input and the settable output voltage of the TL431 separate or you would be shorting the input to the output. The resistor separates the input of the circuit and the output of the Zener.

author: Andreas
author_rank: Super Contributor
timestamp: May 06, 2015, 07:15:17 pm
content: for optimum performance (line regulation) you should use a voltage regulator of 15V after the reverse protection diode.With best regardsAndreas

author: stevedc
author_rank: Contributor
timestamp: May 06, 2015, 07:37:54 pm
content: RR, Rudane - that makes sense. So on my own circuit (see original post), there would be no need for me to have a resistor on the input, then, right?

author: tszaboo
author_rank: Super Contributor
timestamp: May 06, 2015, 07:50:54 pm
content: That LT chip can source and sink 10mA. If you put there a 100Ohm resistor, worst case 1V is dropping on it, and with a resonably sized (not 100 nano) capacitor you have a low pass filter for input noise. Like 100 Ohm 10u will start cutting at 160 Hz, PSRR start dropping at 200Hz, you get the idea?This is a very basic circuit you made, there are a tons of improvement which can be done on any reference circuit. Usually the ones are not straight from the datasheet, and the support circuit end up being more expensive than the IC itself.

author: stevedc
author_rank: Contributor
timestamp: May 06, 2015, 08:04:56 pm
content: NANDBlog,How important is filtering the input into the LT1236? I only ask because the LT1236 is billed as an ultra-low noise output IC. So does it make a big difference on the output?

author: Rudane
author_rank: Regular Contributor
timestamp: May 06, 2015, 08:17:27 pm
content: Quote from: stevedc on May 06, 2015, 07:37:54 pmI wouldn't jump to that conclusion. For the circuit I think the main purpose is to isolate the input and output, but that resistor can also provide additional functionality like current limiting.

author: stevedc
author_rank: Contributor
timestamp: May 06, 2015, 10:28:30 pm
content: So if I only use my voltage reference circuit to check calibration of multimeters, is there any reason to use the resistor, given the capabilities of the LT1236?Oh, and this was another place I saw a information about adding a current limiting resistor on the input. I just wasn't sure if it applied to my circuit.http://www.digikey.com/en/articles/techzone/2012/apr/protecting-inputs-in-digital-electronics

author: radioFlash
author_rank: Regular Contributor
timestamp: May 07, 2015, 04:57:32 pm
content: According to the datasheet, the LT1236 can handle a short-circuit indefinitely for VIN <= 20V. If you stay in this range, it doesn't seem like you need a current limiting resistor for protection.From the datasheet (http://cds.linear.com/docs/en/datasheet/lt1236.pdf)Output Short-Circuit DurationVIN = 35V...10 secVIN ? 20V ...Indefinite