HOW to add ameter RIGHT? - Page 1
Forum section: electronics / beginners

author: 001
author_rank: Super Contributor
timestamp: October 24, 2018, 06:11:53 pm
content: Hi!I have homebuilt power supplySimple basic 7812 regulatorNow I want to add cheap ebay ammeterBUT! If I put meter in series with the load (simply after regulator) some voltage will drop across inner ameter resistance isnt it?But I need for clear 12.00V output at all curents!How to add ameter RIGHT?

author: gigabyte091
author_rank: Contributor
timestamp: October 24, 2018, 06:33:19 pm
content: This is the right way to install the ammeter, your shunt will drop like 50 mV or maybe 75 mV at full output current, and i highly doubt this will affect your circuit in any way...You can put ammeter before regulator, but then it will measure output current plus some quiescent current draw from regulator (about 6 to 8 mA for 7812)

author: 001
author_rank: Super Contributor
timestamp: October 24, 2018, 10:04:30 pm
content: 75mV is a BIG difference in some mc casesThe question is how to eliminate output voltage drop

author: rstofer
author_rank: Super Contributor
timestamp: October 24, 2018, 10:39:07 pm
content: Accept the quiescent current error and put the meter shunt ahead of the regulator. You can account for the error.You could use a Hall Effect current sensor, the internal drop will be a lot less. The resistance is in the single digits of mV - like 1.2 mV for one I just looked at.

author: Momchilo
author_rank: Regular Contributor
timestamp: October 24, 2018, 10:43:03 pm
content: If you really want (almost) no voltage drop at all, I would suggest a Hall Effect current sensor.Something like this:https://www.mouser.de/datasheet/2/397/S22PXXXS05-31126.pdfBut normally you would use a shunt to measure a power supply output current.

author: digsys
author_rank: Unknown
timestamp: October 25, 2018, 12:25:37 am
content: ... or ... make up a tiny PCB with a dedicated ISense amp ie INA286 etc there are literally 100s. Use a fixed, say x100 gain, and a shunt to give 1-10mV MAXThat will get you 100mV > 1V DC op, where you use a Voltmeter. If you already have a current meter, many / most can be converted to Volts by removingthe internal shunt. Worst case, it's not a huge cost. In higher current scenarios, I've simply picked 2 points on the output leads, and calibrated to that.The temp accuracy is a bit drifty at high currents, but you're only doing 1A max.Make up your own escutcheon with your own symbols etc .. always liked doing those.

author: gcewing
author_rank: Regular Contributor
timestamp: October 25, 2018, 12:28:12 am
content: I really don't think you need to worry about the voltage drop. 75mV out of 12V is about half a percent of error. No practical circuit is going to be anywhere near that fussy about its supply voltage, and your 7812 probably isn't going to regulate that accurately anyway when you put a load on it.Some circuits do require a precision voltage reference, but they use a specialised component to produce it, they don't rely on the supply voltage being super accurate.

author: spec
author_rank: Frequent Contributor
timestamp: October 25, 2018, 01:54:45 am
content: + 001You can do what you want with a three-terminal regulator, an opamp and a voltage reference (zener diode). But for a stabilized voltage of 12V it would be better to use a three-terminal regulator with an output voltage less than 12V. An LM317 would be ideal. If this approach is of any interest just say and I will post an outline schematic. By the way, the circuit would produce a stabilized 12V.

author: KaneTW
author_rank: Frequent Contributor
timestamp: October 25, 2018, 04:39:15 am
content: Which MCU requires 12V+-75mV? I just can't imagine that being an issue, especially considering that accuracy adds significant cost to the PDN design.

author: perieanuo
author_rank: Frequent Contributor
timestamp: October 25, 2018, 05:35:55 am
content: Quote from: 001 on October 24, 2018, 10:04:30 pmNo,it.s not.Sometimes 75 mV it can be noise, this will stop your TV to work?Every microcontroller have Vcc larger margins.75 mV out of 12V it's negligeable, total acceptable error margin.A good board should accept maybe 200mV or more variation in Vcc.No comment...Envoyé de mon iPad en utilisant Tapatalk