Old pressure sensor - Page 1
Forum section: electronics / projects

author: mazurov
author_rank: Frequent Contributor
timestamp: January 11, 2022, 12:46:43 am
content: Hello all,I'm trying to determine the pinout of an old (~1990) pressure sensor. As usual, the manufacturer info is not available. If anyone recognizes it or have experience with similar devices, please help me guess what a bias voltage of this thing could be.Thank you,/felis

author: CaptDon
author_rank: Super Contributor
timestamp: January 11, 2022, 04:08:23 pm
content: You need to determine if it was 5vdc excitation, 10vdc excitation, or a 4 to 20 milliamp sensor. I have worked with all of those types and can't begin to guess what yours is. Can you put a scope on the unit that sensor connects to?? We also used LVDT sensors for positioning and they had A.C. excitation.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2022, 04:02:23 am
content: Measure the resistance between each pin. A lot of pressure sensors use a 4-wire Wheatstone bridge very much like a strain gauge load cell.

author: DBecker
author_rank: Frequent Contributor
timestamp: January 12, 2022, 04:54:48 am
content: Pressure senders of that era were sometimes simple rheostats (3 pins) packaged with a separate N.C. low pressure switch (2 pins) for the warning light.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 12, 2022, 04:56:42 am
content: I traced around the sensor connector and made a couple pictures to get resistor values. The connector is not keyed so the sensor side could be flipped on the schematic. Unfortunately, V- can't be determined with certainty. I'll try to power it tomorrow with +-8V and see if it starts.

author: moffy
author_rank: Super Contributor
timestamp: January 12, 2022, 05:46:39 am
content: Looks a bit like it might be a Wheastone bridge. U6a and U6b look like the input buffers/amplifiers to a differential amplifier, that and the use of precision resistors on the board(the brown ones). You should measure the various resistance values between the pins of the pressure sensor.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 12, 2022, 07:24:10 pm
content: This is the resistance measurements. When I compress the bellows the resistance decreases by ~50 ohm except for white-yellow pair where it's close to 20.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2022, 08:06:26 pm
content: Those values indicate a semiconductor strain gauge bridge; 3 kilohm element resistance is typical and they are more common in pressure transducers. They have higher sensitivity but worse accuracy compared to a metal film strain gauge bridge.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 12, 2022, 08:59:56 pm
content: Quote from: David Hess on January 12, 2022, 08:06:26 pmSeems to fit the original application as well. It was a medical infusion pump with this sensor feeling the line pressure. Last time I checked the accuracy requirements for this function is +-10%.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 13, 2022, 02:22:32 pm
content: I think this was a Danfoss part for medical gas monitoring.This makes me think this was for very high pressure ball park monitoring.Say on an anaesthetic workstation or ventilator. Probably 10- 100bar or more for O2. (Not at the patient end)It is a bridge. A 4-20mA sender wouldn't have so many connections. IT wouldnt have the cal resistor.Industry standard: RED is EX+, BLACK is EX-, GREEN is signal +, and WHITE is signal +.K3 and K4 connect to a regular intrumentation amp. But it's a crap low precision design. In the 90's precise amps were very affordable.Still, if its medical, you design a pcb and mark it up by >1000% plus just becasue you can!As the opamps run on 8V its very likely that the bridge exitation voltage is also 8V.The 407R resitor probably limits the bridge current to redcuce self heating.U6c and R11 etc look like a mistake in the reverse engineering. If you are going to reuse the sensor, update to a propper inst amp like an INA827.

author: David Hess
author_rank: Super Contributor
timestamp: January 13, 2022, 05:37:37 pm
content: Quote from: Terry Bites on January 13, 2022, 02:22:32 pmSemiconductor bridges are not accurate enough to require a better instrumentation amplifier, so they used a 324 which works because the signal level is so much higher than with a thin film resistive bridge. When I say they are not accurate, I mean they have poor linearity, hysteresis, and creep, and a lot of gain and zero drift.

author: jbb
author_rank: Super Contributor
timestamp: January 13, 2022, 10:00:38 pm
content: What do you want to do with the signal? There are a number of sigma-delta ADCs available these days with a Programmable Gain Amplifier (PGA) built in. So you could use a regulated +5V (or +3.3V) supply for the ADC and strain gauge and have minimal parts. Although you might incur some noise penalty.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 14, 2022, 04:23:37 am
content: Thanks everyone for your input! It looks like the pins are:YELLOW Ex-RED, GREEN outputsWHITE Ex+I found that the only other linear regulator in this device is 79L05 suggesting that the V- is -5V.