Unexpected Results from Custom Ohm Meter - Page 1
Forum section: electronics / projects

author: IronDruid
author_rank: Contributor
timestamp: July 06, 2021, 08:00:42 pm
content: I thought I would create a simple multi-range Ohm meter.Something like this:https://youtu.be/W9J9J3lIZ7c?t=290and not wanting to get too deep in the wood by more complex designs like:https://youtu.be/anE0jDeBuxo?t=576I came up with a simple voltage divider Ohm meter design that I could select ranges and read voltages from a micro.After loading of LTspice for some basic modeling and conformation I sent the design off to JCLPCB.I used a dual n-channel fet with low Rds (FS8205) to isolate the selected channel, and trim pots for calibration.When I got the boards back I selected a few test resistors and I can't explain the Voltage reading from J2, with 330Ohm range selected.I have included images of the schematic, board and data.FS8205 DS:https://datasheet.lcsc.com/lcsc/1810110910_Fortune-Semicon-FS8205_C32254.pdf

author: TiN
author_rank: Super Contributor
timestamp: July 06, 2021, 08:15:42 pm
content: Hmm, your MOSFETs don't have drains connection to anything. So they do nothing. Suggest to use conventional transistor symbol in schematics too, instead of obscure boxes. Helps with less confusion in future and good habit.

author: aiq25
author_rank: Regular Contributor
timestamp: July 06, 2021, 08:19:13 pm
content: Yeah, user TiN is correct. I was going to post the same thing. His suggestion of putting in the actual electrical symbol is what I do. We had an issue on a production design at work where a similar thing happened and after that I always put the symbols inside the IC boxes. I like to have the IC boxes since it helps in layout (for me at least) and also if I'm prototyping something. Having the electrical symbol helps to avoid confusion as what the part is and also helps to recognize mistakes easily.

author: IronDruid
author_rank: Contributor
timestamp: July 06, 2021, 08:22:07 pm
content: Does not the block diagram of the FS8205 suggest the drains are internally connected?

author: aiq25
author_rank: Regular Contributor
timestamp: July 06, 2021, 08:28:17 pm
content: Quote from: IronDruid on July 06, 2021, 08:22:07 pmThe drains are internally connected yes but for the FET to act as a switch, it needs to be connected. I have attached a crude drawing of what you want. As you can see in this drawing, you have an N-Channel FET, so it's a low side switch. The source needs to be connected to ground and then the drain needs to be connected to the node you are trying to switch.Edit: Looking at your schematic, you don't seem to have the FET connected as a low side switch. I would recommend connecting it as a low side switch. If you have the FET in between the resistors then you will run into issues where the FET will not have enough VGS to fully saturate (didn't do the calculation in your circuit but I have had this issue at work and that's why I'm really against using N-Channels FET's as anything other than low side switching, with few exceptions of course).

author: IronDruid
author_rank: Contributor
timestamp: July 22, 2021, 07:05:35 pm
content: At this point I guess I am documenting this journey.I wrongfully thought the diodes across the source and drain of the FETS would prevent current from flowing back through the channels that are not enabled. I did not stop to consider the order (direction) in which the diodes were operating.I also thought the internally connected drains (feature) of the FS8205 simplified the design. After a bit of research I believed I had a better understanding.I revised my design, this time making sure the diodes were correctly orientated (across the source and drain) to prevent current flow when the N-Channels are disabled.I ordered a new round of boards and too my surprise ... The design fails again.I can correct the reading in software, but I'd still like to know what I am missing.NChannel Datasheet:https://www.mouser.com/datasheet/2/308/1/NTGD3148N_D-2318534.pdf

author: aiq25
author_rank: Regular Contributor
timestamp: July 23, 2021, 02:11:57 am
content: Can you please post the voltages are you seeing at the load resistor? Also, are you using the FET's in a common drain or source configuration?

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 23, 2021, 03:25:25 am
content: The mosfet sources should all be grounded and the unknown resistance isn’t shown so it’s impossible to understand how this is supposed to function. Putting aside the uncertainties of how you’re actually hooking things up, one error is obvious that will cause non-linearity.Lift the ground connections on the three trim pots. You don’t want a voltage divider like that, you just want to trim the parallel resistors.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2021, 03:32:40 am
content: I highly suggest using a breadboard, with adapters if need be, rather than sending out untested designs for boards. Also, LTSpice is nice for basic experiments on systems you aren't completely familiar with.Also you should review the gate drive voltages needed to make this work. If you are just using +3.3V from your MCU, I'm guessing your upper FETs are not turning on. Perhaps the voltage difference is because there is a drop across the body diode of the upper FET.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2021, 03:35:07 am
content: Quote from: TiN on July 06, 2021, 08:15:42 pmIt's a dual series N-MOSFET with common drains. Yeah, the correct symbols would have made that a lot easier to spot!

author: EPAIII
author_rank: Super Contributor
timestamp: July 23, 2021, 08:14:36 am
content: Breadboards are your friend! Faster, cheaper, and easier. Get one or several.And they also make adapter boards for components that do not have the wire leads for through-hole installation. Look for places like Schmartboard. And others. You solder the surface mount part on the adapter board and then some pins on it will plug into a standard, 1/10" spacing breadboard. Then you can play with those parts in real time instead of waiting days for PCBs to arrive.

author: aiq25
author_rank: Regular Contributor
timestamp: July 23, 2021, 01:23:04 pm
content: Quote from: bdunham7 on July 23, 2021, 03:35:07 amYeah having the component symbol in the schematic symbol would be super helpful and was suggested couple of times already. They changed the MOSFET so now it's two independent FET's. I just didn't feel like figuring it out by looking at the datasheet lol.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 01:24:21 pm
content: Quote from: aiq25 on July 23, 2021, 02:11:57 am<< Can you please post the voltages are you seeing at the load resistor?I only took resistance measurements. I can re-run data if need be.<< Also, are you using the FET's in a common drain or source configuration?The first Dual NChannel FET Arrary was common drain, the second was not.

author: aiq25
author_rank: Regular Contributor
timestamp: July 23, 2021, 01:24:58 pm
content: Quote from: bdunham7 on July 23, 2021, 03:32:40 amI second during an LTspice simulation. That would help a lot to figure out simple mistakes.

author: aiq25
author_rank: Regular Contributor
timestamp: July 23, 2021, 01:27:02 pm
content: Just some final thoughts, you don't need two FET's. Just use one low side N FET. Low side meaning connect the source to ground. These two things will help you resolve a lot of your issues.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 02:26:23 pm
content: << The mosfet sources should all be groundedThat would defeat the purpose. Hopefully the attached simplified schematic better illustrates the purpose.<< unknown resistance isn’t shown so it’s impossible to understand how this is supposed to functionThe unknown resistance is the point. Given 3.3VDC, and known R1, R2, R3 the value of Rv can be calculated by measuring the voltage drop across Rv. Only 1 path through R1, R2, or R3 should be active at any time to set the range.<< Lift the ground connections on the three trim pots.Yes, 100% correct. My mistake. However this should only affect the range max of the trimming.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 02:31:14 pm
content: Quote from: aiq25 on July 23, 2021, 01:27:02 pmThis has been mentioned a few times. Please see my reply "« Reply #15 on: Today at 02:26:23 pm »".I don't see how using a low side N FET would work. What do I not understand? Or did I not communicate what I was trying to accomplish well enough?

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 02:46:24 pm
content: Quote from: EPAIII on July 23, 2021, 08:14:36 amI agree about prototyping.I had never designed and ordered a PCB through JLCPCB, or PCBWAY. Going through the process of designing, drawing, and using JLCPCB is part of this project.After the first failed design, I was going to get all the components and prototype using through hole parts on a breadboard.Problem is, like a fool I was so confident I had identified the problem and I just KNEW the revision was going to work.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 02:54:49 pm
content: Quote from: bdunham7 on July 23, 2021, 03:32:40 amThe gates are driven by +5VDC through a 10k current limiting resistor. Bottom left corner of the schematic.In my initial post I stated "After loading of LTspice for some basic modeling and conformation I sent the design off to JCLPCB."Perhaps I made a modeling mistake in LTspice (I didn't save the file), but the model worked as I expected.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 03:04:12 pm
content: Quote from: aiq25 on July 23, 2021, 01:23:04 pm<< Yeah having the component symbol in the schematic symbol would be super helpful and was suggested couple of times already.Noted.

author: IronDruid
author_rank: Contributor
timestamp: July 23, 2021, 03:27:53 pm
content: Quote from: IronDruid on July 23, 2021, 02:26:23 pmIs the attached the low-side design in which you are referring?

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2021, 03:40:31 pm
content: Quote from: IronDruid on July 23, 2021, 02:54:49 pmIf you try 8 volts instead of 5, your circuit may work as intended. Also, I did miss that separate block, but I, like many people, loathe schematics that are broken up into tiny functional blocks with the interconnections represented by flags. I'm not advocating a return to huge fold-out hand drawn schematics that you hang on a wall, but if it all fits comfortably on sheet of paper, it should all be one schematic. When I see a flag, I assume that it goes to another sheet.QuoteShort on hard drive space?It would be interesting to see the file, but I'm thinking you have the wrong VGSdata. Without seeing it we can't know, but the first thing that comes to mind is thatthreshold ≠ on.

author: aiq25
author_rank: Regular Contributor
timestamp: July 23, 2021, 05:27:14 pm
content: Quote from: IronDruid on July 23, 2021, 03:27:53 pmAh, now your application makes more sense. I also didn't realize you mentioned you had done a SPICE simulation before getting the boards made.You are correct, this is how a low side switch works. If you want to place the switch on the power side, you can use a P-Channel FET, although your logic then would be reserved but you can use another N-Channel FET to invert it.Please see two examples (one of which you have already posted).An N-Channel FET is usually easy to control if the Source is connected to GND (thus you don't have to worry about the Source voltage for the Vgs threshold).The inverse is true for an P-Channel FET, if the Source is connected to the power input, it's easy to control. But this has an inverse logic but most people use an N-Channel FET or NPN BJT to flip the logic (very commonly used in measuring voltage rails, have been using it myself in a lot of production designs). Attached is an example.This is a different topic but getting a FET to block the body diode of another FET is usually tricky. This is where you would typically use a gate driver or charge pump. Alternately you could just use a load switch IC, which has everything built in (including reverse current protection).EDIT:Looking at the P-Channel FET being controlled by an NPN BJT example I posted, they have the Source and the Drain connections reversed. The Source should connect to the power source and thus the body diode should point that way, otherwise the body diode will always conduct and you will not be able to disconnect the load.