Brymen BM235:  Help identifying smd  Q3 (calibration temperature) - Page 1
Forum section: electronics / repair

author: sacimoto
author_rank: Newbie
timestamp: March 17, 2022, 10:17:41 pm
content: Help identifying smd Q3 .When disassembling, demolished from the board Q3,see smd involved in temperature measurement,but even without it, the multimeter measures the temperatureWhat is this smd responsible for?

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 10:51:13 pm
content: The transistor is connected as a diode. Could it be involved in cold junction compensation for the thermocouple? If you short the temperature input, does the DMM display ambient temperature? If so, and if you heat Q3 (with a hair dryer?), does the meter reading increase?https://lygte-info.dk/info/DMMThermocouplers%20UK.html

author: sacimoto
author_rank: Newbie
timestamp: March 17, 2022, 11:00:38 pm
content: I can't heat d3, it's lost, I'm looking for something to put

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 11:05:33 pm
content: Quote from: sacimoto on March 17, 2022, 11:00:38 pmWhat temperature do you measure when you short the temperature inputs?What temperature do you measure if you replace Q3 with a diode (eg 1N4148) while shorting the inputs of the meter?

author: sacimoto
author_rank: Newbie
timestamp: March 17, 2022, 11:13:50 pm
content: if you short-circuit the input, it shows the temperature 2 degrees lower than it should be. on contacts Q3 1.74v when I connect the diode, the voltage drops to 0.4v and the measurement threshold shifts strongly shows 460 degrees

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 11:55:38 pm
content: The temperature coefficient of a PN junction is -2.4 mV per deg C. So if the voltage falls from 1.74V to 0.40V, then that corresponds to a temperature difference of (1740 - 400) / 2.4 = 558C.

author: sacimoto
author_rank: Newbie
timestamp: March 18, 2022, 12:18:04 am
content: when connecting the diode, the multimeter shows (---C) because it cannot show 558. Then what should I put there? the photo shows the marking (fz) perhaps 2sd2657k, then it will work like a diode, but then when connected 558 degrees is not the norm.

author: fzabkar
author_rank: Super Contributor
timestamp: March 18, 2022, 12:40:43 am
content: Maybe I'm misunderstanding the circuit. Try an NPN transistor and see what happens.

author: sacimoto
author_rank: Newbie
timestamp: March 18, 2022, 12:53:08 am
content: the same happens as with the diode - a drop to 0.4v and overestimated performance --- C

author: sacimoto
author_rank: Newbie
timestamp: March 26, 2022, 09:13:40 pm
content: It turns out that I didn’t remove Q3, the transistor was badly soldered back at the Bryman factory, the transistor fell off there and they calibrated the multimeter without it, because when I installed the 2sd2657k transistor, there was such an offset in temperature. It was necessary to calibrate the multimeter for temperature again. Trial was able to do it.How did i do it--Short J1 inside multimeter (otherwise you will not enter the adjustment mode)-While holding the (Hz) button, turn the switch from the (OFF) position to the (Ohm.buzzer) position, hold the (Hz) button until the display turns off, release the ss button-300.0 will appear on the display, then with the rotary switch we select what needs to be calibrated, I chose (mV/Temperature)-Button (range) select modes and ranges, chose (--0C)-Then I connected the thermocouple, lowered the thermocouple into a glass with melted ice (you need to cool the sensor to 0 degrees Celsius)-Then press the button (select) the calibration values are displayed on the display, wait for the values ​​to calm down, then press the button (hold) the values saved-Rotary switch switch to (OFF)mode

author: modz786
author_rank: Regular Contributor
timestamp: October 31, 2022, 12:43:05 pm
content: Hi did you look at calibrating/checking the DC Volts?I tried but couldn't adjust the voltage displayed. Is there a different key combo?Thanks!