Schematic Review Request - Page 1
Forum section: electronics / general

author: daisizhou
author_rank: Super Contributor
timestamp: August 21, 2025, 10:28:04 pm
content: This is a resistance measurement circuit.I don't know if there are any errors in the schematic I drew and whether it complies with the conventions.H1 is connected to a 0-220 ohm adjustable resistor,H2 is the external connection port.The normal working mode is that H2 is connected to the external test interface, H1 is adjusted to determine the resistance value and remains unchanged.Press the "READ" button on the host, K1 and K2 relays will work,RTEST-IN returns the voltage value corresponding to the measured resistance value to the host.The initial circuit comes from herehttps://www.eevblog.com/forum/projects/help-with-circuit-analysis/I chose the PMOS tube solution,And replace LM358 with OP200A

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 22, 2025, 05:50:18 am
content: You could start by drawing an opamp as a triangle, like any normal opamp. I'm not going to put in extra effort to decipher the components around a square box with pins in the order of the IC instead of some logical order.Same for the relays.You can also make it a bit easier for the people who want to dive deeper to add a direct link to the OP200A datasheet.

author: daisizhou
author_rank: Super Contributor
timestamp: August 22, 2025, 06:22:45 am
content: Thanks.Is that what you mean?OP200--https://www.alldatasheet.com/datasheet-pdf/pdf/549471/AD/OP200ARC.html

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2025, 08:53:17 am
content: Quote from: daisizhou on August 22, 2025, 06:22:45 amBut it is not the same schematic.U2 at first is connected to GND, while U1 at second (looking to be the same as U2 at first) is not connected to GND.Which one is to be reviewed?D1 and D2 (at first), (= ZD1 and D1 at second) are connected paralel. It makes no sense.

author: daisizhou
author_rank: Super Contributor
timestamp: August 22, 2025, 10:21:24 am
content: Quote from: PGPG on August 22, 2025, 08:53:17 amThanks.I have corrected the incorrect connection.K1 and K2 are 5v relays, so I added a 5.8v voltage regulator and in4148 to stabilize the voltage and prevent inductance impactI also don't know if I shielded the second part of the OP200AZ op amp. Is this operation in compliance with the correct specifications?

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2025, 12:56:19 pm
content: Quote from: daisizhou on August 22, 2025, 10:21:24 amHaving 12VDC in their type symbol?Quote from: daisizhou on August 22, 2025, 10:21:24 amWhat do you think about: (15-5,8)^2 / 150 = 0.56 ?Quote from: daisizhou on August 22, 2025, 10:21:24 amZener diode in forward direction works like standard diode. If K1 and K2 are close to each other then 1N4148 does really nothing.

author: daisizhou
author_rank: Super Contributor
timestamp: August 22, 2025, 01:26:44 pm
content: Quote from: PGPG on August 22, 2025, 12:56:19 pmNo.Quote from: daisizhou on August 22, 2025, 10:21:24 amIs the maximum current 0.56A? The maximum current of a 5V relay is 200-300mA.The specific model of the relay is SRC-05VDC-SHin4148 is to protect the relay from reverse shock, I don't know how to install it accuratelyThanks.

author: tggzzz
author_rank: Super Contributor
timestamp: August 22, 2025, 01:41:26 pm
content: Quote from: daisizhou on August 21, 2025, 10:28:04 pm01.jpg is an annotated PCB layout, not a schematic. 02.jpg is much closer to being a useful schematic.A schematic indicates the intendedfunctionof a circuit, not itsphysicallayout. Wherever a subcircuit implements a standard function (e.g. an op-amp inverting amplifier, or a common-emitter buffer) then the schematic should drawn in the traditional way. That is equivalent to using "design patterns" when writing software.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 22, 2025, 02:25:53 pm
content: Quote from: daisizhou on August 22, 2025, 01:26:44 pmHow not, when so.Is it your schematic where you used SRC-12VDC-SH or we are speaking about different circuits?Quote from: daisizhou on August 22, 2025, 01:26:44 pmIf you know P=V*I and V=I*R you would not ask this question.Quote from: daisizhou on August 22, 2025, 01:26:44 pmAccording to datasheet its coil resistance is 125Ω (5V/40mA).So I have for you more equations:125/2 = 62.515 * 62.5/(150+62.5) = 4.41Do you think your Zener diode regulates 5.8V?And from it also the previous value (0.56) changes to:(15 * 150/(150+62.5))^2 / 150 = 0.75Isn't it too much for R5 (SMD1206-150).

author: daisizhou
author_rank: Super Contributor
timestamp: August 22, 2025, 08:31:16 pm
content: Sorry, when I was drawing the schematic, I did not find the schematic library file corresponding to SRC-05VDC-SH in the PCB library, but SRC-12VDC-SH happened to have the schematic library file corresponding to the PCB library. Since their pin definitions are the same, I chose SRC-12VDC-SH.Do you mean that this 5.8V voltage regulator is redundant?Does this mean that R5 (SMD1206-150) needs to be replaced with a high-power resistor?Usually K1 and K2 are powered on at the same time, and the duration is about 5-10 seconds.(That is, after the "READ" button is pressed, the host measures the time)Is there any other better solution? Thank you

author: PGPG
author_rank: Frequent Contributor
timestamp: August 23, 2025, 12:04:59 am
content: Quote from: daisizhou on August 22, 2025, 08:31:16 pmYou have to know that no one is sitting in your head. The only information about relay you provided in your first post was its type at schematic and it tells 12V.Quote from: daisizhou on August 22, 2025, 08:31:16 pmYes.Because of R5 + relay coils forming voltage divider you will have at coils only 4.4V and Zener will have nothing to do here.Quote from: daisizhou on August 22, 2025, 08:31:16 pmExactly.I see typical 1206 resistors power being specified as 0.3W while you dissipate in it 0.75W.May be it will withstand 5..10s but I would not assume this.It is wrong idea to use 5V relays supplied from 15V. You should use 12V relays. They easily tolerate 15V.Quote from: daisizhou on August 22, 2025, 08:31:16 pmI have only pointed you the errors you have done.I will not be thinking about your circuit functioning.Better solution is to not design ohmmeter but simply use multimeter.There is no idea to build things you can easily buy.In 1982 I have designed and build oscilloscope, but those time and in place I live it was the only way for me to have a scope and I really needed to have one.Now doing it once more I would consider as wasting my time.

author: Bud
author_rank: Super Contributor
timestamp: August 23, 2025, 12:38:41 am
content: I have not seen resistor symbol with a line through it. In some conventions this is the symbol for fuse.

author: daisizhou
author_rank: Super Contributor
timestamp: August 23, 2025, 07:49:36 am
content: The signal at the other end comes from ULN2803A, which controls the operation of the 5V relay.I tried replacing the 150 ohm resistor with a 125 ohm resistor, and the 5V relay control signal could reach 5.0v, but the current flowing through the 125 ohm resistor was only 0.8.Is the only solution to this problem to replace the 125 ohm resistor with a higher power resistor? Are there any other solutions?Thanks

author: daisizhou
author_rank: Super Contributor
timestamp: August 24, 2025, 06:19:10 am
content: Please see pages 97-99 of the manual for details on the REM/ARM module and the internal parts BOM.Since the schematic was missing, I drew the schematic myself.Please see the BOM table, which also uses two 5V relay. I don't know how the specific circuit schematic should be designed.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 25, 2025, 09:02:54 am
content: At your schematic both relays work synchronously (their coils are connected together).If you connect their coils in serie they will also work synchronously but you will have to dissipate in resistor 4 timers less power (voltage at R 5V instead of 10V and current 40mA instead of 80mA).I suppose that 5V relay can be driven from up to may be 9V so you even can drive them directly from 15V (7.5V per relay).

author: daisizhou
author_rank: Super Contributor
timestamp: August 25, 2025, 09:53:03 am
content: Quote from: PGPG on August 25, 2025, 09:02:54 amDo you mean connect as shown below?I calculated the current.I=15/(150+150+150)≈33ma. K1、K2： 0.033*150=4.95v R5-SMD1206(150R):0.033*0.033*150≈0.163WThe current in the circuit is 33 mA,The voltage obtained on a single relay is 4.95V.I removed D1 and D2. Do I need to add them back to protect them from shock?

author: PGPG
author_rank: Frequent Contributor
timestamp: August 25, 2025, 01:31:56 pm
content: Quote from: daisizhou on August 25, 2025, 09:53:03 amYes.Quote from: daisizhou on August 25, 2025, 09:53:03 amRounding errors.If you have 3 identical resistors than voltage is divided between them equally - you should get 5V.But as I have written few posts before in that relay datasheet I have found I sow coil resistance being 125Ω. I don't check it a second time.Quote from: daisizhou on August 25, 2025, 09:53:03 amYou should left diodes as they were (but be careful about current direction in coils) or you can have one diode through both coils.

author: daisizhou
author_rank: Super Contributor
timestamp: August 25, 2025, 10:10:05 pm
content: ThanksIs this the way to connect?

author: PGPG
author_rank: Frequent Contributor
timestamp: August 26, 2025, 11:22:50 am
content: No.You connected diode in serie with coils while in your previous schematics diodes were connected (correctly) in parallel to coils.Think of these two coils as being one coil and connect diode parallel to it.It looks for me that you are trying to do something too complicated for you.

author: daisizhou
author_rank: Super Contributor
timestamp: August 26, 2025, 11:56:03 am
content: Thanks.Is this the way to connect?I am a beginner in drawing PCB circuit diagrams, and this is my first amateur electronic production project

author: PGPG
author_rank: Frequent Contributor
timestamp: August 26, 2025, 06:23:17 pm
content: Yes. Driving the relay coils is ok (if you want them be switched synchronously).Some relays specify at which coil pin you should provide (+) and at which (-). It is because their magnetic circuit has intentionally some initial magnetization.If your relay specifies it you have to update your schematic (you drive one relay coil in opposite direction than the other), if not then everything is ok (in relay coils driving aspect).I believe you understood the problem with power dissipation in resistor.Don't be afraid to give some more voltage to relay coils. They are not delicate semiconductor but solid mechanical devices. They will not be burned by even 10x higher short voltage pulses as they can generate them themselves. But the driving voltage can't be so high as the source of limit is the heat dissipated in coil while relay is activated.Datasheet should specify maximum. The higher voltage you will give to the coils the less voltage you will have to drop at series resistor (down to may be 0) and you will have less power at resistor.I will not look into rest of your circuit.

author: daisizhou
author_rank: Super Contributor
timestamp: August 28, 2025, 10:40:51 pm
content: Hallo everyoneThis is my first PCB that I drew myself,Based on the schematic above, I have drawn the PCB.I don't know if these wiring conforms to the standard settings.If someone helps me check, I would be extremely grateful