Does this ground plane look ok?

Thread Starter

Canobi

Joined Mar 4, 2015
29
Hi guys :)

I'm having a first proper go at designing a two sided PCB and would appreciate someone having a look at the ground plane to see if it will be sufficient, though it should be noted that I'm still tinkering with it so the pic below doesn't represent it's finished state.

The system is mainly analogue, with digital only playing a part for ISP purposes, at which point the analogue circuits won't be active.

I'm still getting to grips with proper component placement on the top layer though, so the majority of analogue circuits have ended far left, with a 3w amp plonked far right, so my idea was to create an island for the analogue return currents to play in that was separate from power ground and then connect both at a single point to the battery's negative terminal port in the hope that this would prove a sufficient.

 

Thread Starter

Canobi

Joined Mar 4, 2015
29
I've attached a PDF of my schematic as all my attempts at resizing a pic for the forum results in major loss of important detail but below is a pic of the top layer, which is still WIP:


I inverted the colours as it's a lot easier on the eyes than the original.
 

Attachments

TeeKay6

Joined Apr 20, 2019
572
I've attached a PDF of my schematic as all my attempts at resizing a pic for the forum results in major loss of important detail but below is a pic of the top layer, which is still WIP:


I inverted the colours as it's a lot easier on the eyes than the original.
@Canobi
Before you worry about the ground plane, I suggest you do a thorough review of your circuit. There are numerous errors that would prevent correct operation. Additionally, you show many zener diodes scattered throughout the schematic; is that what you intended or did you perhaps mean for those to be Schottky diodes? You might try breaking your schematic into simple blocks and use a circuit simulator (e.g. a Spice simulator) to evaluate correctness.

Regarding your current ground plane (pls heed schematic warning above), there seem to be two ground planes shown in the "green" view and they are joined via a single "thermal" pad connection (location is about 35% to the right of the left edge and 10% up from bottom edge). Is that what you intended? The "outer" ground plane encircles the board with no break in the copper, forming a large conductive loop. That loop may (or may not) lead to induced noise from any nearby varying electromagnetic field. It is best to avoid such large conductive loops.
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
Thank you for taking the time to look, would you mind pointing out the worst offenders, my understand is a little shaky having had no formal education on the subject.

As to the diodes, the plan was to use the analogue circuits to pull the modules "key" pins to ground in order to initiate a playback cycle. However, I wasnt sure if connecting the modules key pins to floating signal lines would cause a reverse voltage scenario as I couldnt find any info in the datasheets for the modules (or their chip datasheets) regarding the matter, so I added the diodes as a precautionary measure. The choice was based on their very low voltage drop rather than type but can they can be changed or removed in not required and/or cause problems.
 

TeeKay6

Joined Apr 20, 2019
572
Thank you for taking the time to look, would you mind pointing out the worst offenders, my understand is a little shaky having had no formal education on the subject.

As to the diodes, the plan was to use the analogue circuits to pull the modules "key" pins to ground in order to initiate a playback cycle. However, I wasnt sure if connecting the modules key pins to floating signal lines would cause a reverse voltage scenario as I couldnt find any info in the datasheets for the modules (or their chip datasheets) regarding the matter, so I added the diodes as a precautionary measure. The choice was based on their very low voltage drop rather than type but can they can be changed or removed in not required and/or cause problems.
@Canobi
My comment re: diodes was simply referring to the symbol used; it is commonly used to represent zener diodes, and having so many zeners in a circuit is highly unusual. I do suspect that you intend them to be Schottky diodes (a.k.a. hot-carrier diodes).

Your circuit is quite complex. That is why I suggest breaking it into smaller pieces for analysis. Once the small pieces are correct, joining them together is more likely to work as expected. If you have not already done so, I highly recommend that you learn to use a circuit simulator; there are several free versions that are quite competent (search the Wev and AAC for "simulator" or "simulation" for ideas). One free version that is popular among users of this site is LTSpice (www.analog.com). Unfortunately, learning a simulator is a task in itself, but ultimately very worthwhile if you intend to continue circuit design. You can use AAC as you work on each "small piece" of your circuit; specific questions about simple circuits get better answers and more quickly, than questions about complex circuits. You have started correctly by posting schematics and layout images, and by asking specific questions. The best way to get no answer is to be vague about what you are doing and why.

I will not attempt a complete review of your schematic. However, I will point out something to get you started. View at the upper left of your schematic the circuitry around the TS5A3357. Note that the gate terminals of Q3, Q4, Q5 are "floating" when the two unnamed switches are open. This is not allowed; the gate must always be driven to an acceptable voltage. When used as switches, as in this case, the gate must always have a voltage applied that will either turn the MOSFET "on" or "off". Although I do not know what logic you intend via Q3, Q4, and Q5, I can tell you that pins 5 & 6 of the TSSA3357 will always be near 3.3V due to the "intrinsic" diodes of those MOSFETs; turning the devices "on" or "off" will make no difference to the pin 5 & 6 voltages. Even with the two mechanical switches (just to the right of Q5) closed there is a problem with gate drive due to Q6. When Q6 is driven "on", it will pull the switches to 3.3V; when Q6 is off, the gates of Q3,4,5 will all be floating. There are additional problems with the use Q3-Q6. To turn an N-channel MOSFET (e.g. Q3) "on", its gate must be driven more positive than its source (terminal) by several volts (more than the "gate threshold voltage" shown in the datasheet for the device). If this Vgs drops below the threshold, or goes negative, the MOSFET will be "off". The voltage of a floating gate is unpredictable. In a similar manner, to turn a P-channel MOSFET "on", its Vgs must be negative by several volts (i.e. more than the gate threshold voltage given in the datasheet); restating, for an "on" condition, the source must be several volts more positive than the gate (all voltages measured relative to circuit ground/common). Thus, none of Q3-Q6 will behave as you expect. Note that a device terminal--if it is an "input"--must be driven to a correct voltage. If an input must be at 0.0V, then there must either be a direct connection to ground or a connection via a "pull-down" resistor to ground. So...your first task might be to learn how MOSFETs "work" and are used and attempt a correction to this "small piece" of your overall schematic. Once you believe you have that part of the schematic correct, you can post an image of that part of the schematic (although someone will almost certainly ask you to post the full schematic--just ignore comments about parts you are not questioning about) along with a description of how you want it to behave (i.e. which input signals will cause which responses). (If you post the full schematic, also post separately the image of the limited part of the schematic about which you are asking for help.) Schematics should show all component designators (Q1, Q2, R1, etc) to make discussion easier. Also show values (usually placed near the designators).

Note that my comments regarding MOSFET usage also apply to Q1a, Q1b, and Q2a. I have not viewed the data sheets for the TS5A3357 or NCW7P00 so I offer no comments as to the correctness of the connections you have shown.

Both Web searches and searches of this AAC site can be very informative. There is an immense store of knowledge available at AAC if you choose to find it.

Be prepared for direct, not necessarily tactful, comments. ;) With good luck no one will accuse you of developing a weapon of mass destruction. :rolleyes:
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
Thank you teekay6, I had totally forgotten about floating gates being a no no (doh!). The idea behind the trio of fets was to change the analogue switch's input logic such that a Low/Low at the inputs is circumvented. The reason for this is that the three channels are NO by default and I had designated the blade on and off to be controlled by the analogue SPST.

I will take your advice and have a close look at simulators, breaking the system into chunks will be easy as the system was built up using borrowed circuits anyway and I had planned on making the system modular to start with so I could tackle each part separately, as you suggested.
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
I do, I've also decided to start again with the first part of the RGB control system. With floating gates as an issue, I figured I could rope in an SPST for each fet. I've added pull up and pull down resistors on each channel, though I'm not entirely sure they're needed (apart from the 3R3 which the LEDs current limiting resistor).

 

Attachments

TeeKay6

Joined Apr 20, 2019
572
I do, I've also decided to start again with the first part of the RGB control system. With floating gates as an issue, I figured I could rope in an SPST for each fet. I've added pull up and pull down resistors on each channel, though I'm not entirely sure they're needed (apart from the 3R3 which the LEDs current limiting resistor).

@Canobi
OOPS! See update below...
Hmm, as I understand your circuit, you intend to use a 0-5V logic signal (S) to set the gate voltage of an FDN327N MOSFET to either 0V (via a 10K R) or 5V. That would control current through the LED, D1.
If the Y0 & Y1 signals you intend to use are indeed ground and 5V, then the analog switch 74lvc1g3157 is entirely superfluous. Connect the S signal directly to the gate of the MOSFET. The gate threshold voltage (for 250uA drain current) is only 1.5V (max), so 5V-3.3V=1.7V is just sufficient to turn the MOSFET "on". Depending on how much current you intend to pass through the LED, that may or may not be enough "on", worst case; in a typical case (Vthreshold=0.7V), you would be safe. There is another problem however, the symbol you have used is for a P-channel MOSFET; the FDN327 is an N-channel device; so...correct the symbol. When you looked at your circuit, you should immediately have noticed that in using that wrong symbol you were connecting a diode directly between 3.3V and the 3.3ohm resistor...clearly not the right way to use a MOSFET. Try again and post your updated schematic for controlling D1.
Also, consider placing D1 in the drain path of the MOSFET, not the source; then when the gate=0 (i.e. S=0V), the MOSFET would be off & the LED off. When S switches to 5V, that would very easily turn the MOSFET fully "on" (Vgs=5V). Questions? By all means, ask!
OOPS! See update below.

Update: I just noticed that you are powering the 74lvc1g3157 with 3.3V, not 5V as I previously, mistakenly, assumed. That is a problem if you continue to use this analog switch as its terminals must remain in the 0 to 3.3V range. You cannot select a terminal to be 5V. Fortunately, there is a solution as I suggested. First, eliminate the analog switch. Then tie the S signal directly to the MOSFET gate. Connect the MOSFET source directly to ground. Connect the anode of the LED to +3.3V, connect the cathode to the 3.3ohm current limiting resistor. Connect the other end of that resistor to the drain of the MOSFET. The 0 to 3.3V level of the S signal is sufficient to fully turn the MOSFET "on" and "off".
 
Last edited:

Sensacell

Joined Jun 19, 2012
2,919
I recommend you take a pause and re-think your strategy here.

The PCB design here is putting the cart in the next city, while the horse is still in the barn, waking up.
Trying to debug a system built from blocks that themselves may be flawed makes the process 100X more difficult.
Not to mention all the money and time wasted building stuff that will not work!


There are many issues with the hardware design, address this first, step by step.

1) Outline the function blocks, what they need to do, what inputs and outputs are needed.
2) Take a swing at each function block design, post them here for feedback.
3) Once you have a collection of workable blocks - assemble this into a system diagram.
4) Build a breadboard prototype - build it by hand so you can easily change the stuff that is wrong.
I often build each function block on its own little board, so I can test and evaluate it in isolation, then add it to the master system board.
5) Design a PCB based on the WORKING hardware.

Hope this helps.
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
@Canobi
OOPS! See update below...
Hmm, as I understand your circuit, you intend to use a 0-5V logic signal (S) to set the gate voltage of an FDN327N MOSFET to either 0V (via a 10K R) or 5V. That would control current through the LED, D1.
If the Y0 & Y1 signals you intend to use are indeed ground and 5V, then the analog switch 74lvc1g3157 is entirely superfluous. Connect the S signal directly to the gate of the MOSFET. The gate threshold voltage (for 250uA drain current) is only 1.5V (max), so 5V-3.3V=1.7V is just sufficient to turn the MOSFET "on". Depending on how much current you intend to pass through the LED, that may or may not be enough "on", worst case; in a typical case (Vthreshold=0.7V), you would be safe. There is another problem however, the symbol you have used is for a P-channel MOSFET; the FDN327 is an N-channel device; so...correct the symbol. When you looked at your circuit, you should immediately have noticed that in using that wrong symbol you were connecting a diode directly between 3.3V and the 3.3ohm resistor...clearly not the right way to use a MOSFET. Try again and post your updated schematic for controlling D1.
Also, consider placing D1 in the drain path of the MOSFET, not the source; then when the gate=0 (i.e. S=0V), the MOSFET would be off & the LED off. When S switches to 5V, that would very easily turn the MOSFET fully "on" (Vgs=5V). Questions? By all means, ask!
OOPS! See update below.

Update: I just noticed that you are powering the 74lvc1g3157 with 3.3V, not 5V as I previously, mistakenly, assumed. That is a problem if you continue to use this analog switch as its terminals must remain in the 0 to 3.3V range. You cannot select a terminal to be 5V. Fortunately, there is a solution as I suggested. First, eliminate the analog switch. Then tie the S signal directly to the MOSFET gate. Connect the MOSFET source directly to ground. Connect the anode of the LED to +3.3V, connect the cathode to the 3.3ohm current limiting resistor. Connect the other end of that resistor to the drain of the MOSFET. The 0 to 3.3V level of the S signal is sufficient to fully turn the MOSFET "on" and "off".
Thank you so much for all the invaluable info and help, it's very much appreciated :D

Ok, I'm going to take this one step at a time.

Change PNP symbol to NPN _ check.
Move LED and resistor to source pin _ check



I am a little confused regarding the removal of the SPST though as the SP3T will be driving S but as all three channels are normally open when not selected, wouldn't that leave the inactive gates floating?

Thank you for the heads up regarding mixed voltage levels, this has also given me food for thought. So, is it good practice then to choose a voltage level that the whole system will operate at, or are mixed voltages ok within certain limitations (ie, device and travelling voltage levels being at the same potential)?
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
I recommend you take a pause and re-think your strategy here.

The PCB design here is putting the cart in the next city, while the horse is still in the barn, waking up.
Trying to debug a system built from blocks that themselves may be flawed makes the process 100X more difficult.
Not to mention all the money and time wasted building stuff that will not work!


There are many issues with the hardware design, address this first, step by step.

1) Outline the function blocks, what they need to do, what inputs and outputs are needed.
2) Take a swing at each function block design, post them here for feedback.
3) Once you have a collection of workable blocks - assemble this into a system diagram.
4) Build a breadboard prototype - build it by hand so you can easily change the stuff that is wrong.
I often build each function block on its own little board, so I can test and evaluate it in isolation, then add it to the master system board.
5) Design a PCB based on the WORKING hardware.

Hope this helps.
Thank you Sensacell, I've never approached a project using a function block system of thinking, this is due to the fact I've never attempted something quite so complex and involved before. To date, all my projects have been quite small circuits that are modular in nature, much like the function block breadboarding approch you described in point 4 and it is the same approach I was planning on taking.

I had a hunch that posting my PCB design would open a can of worms but that's ok, even I do have to start from scratch. The way I see it is any time spend making schematics and using PCB design software is experience gained and getting my hands dirty is the way I learn the best.

The only limitation I have is that I primarily work with surface mount componants. This is due to the fact that I am quite colourblind and resistor colour bands are an absolute nightmare for me to try and work out.

As such, it has always been my plan to take the modulelar approach even further by designing a set of breakout boards for each the SMT semiconductor and component package types to make a miniaturised version of those "connect and play" electronics kits for kids but for breadboarding as it would save me having to buy through hole equivalents and will hopefully teach proper component layout and placement techniques which will be applicable during the final PCB design stage.
 

TeeKay6

Joined Apr 20, 2019
572
Thank you so much for all the invaluable info and help, it's very much appreciated :D

Ok, I'm going to take this one step at a time.

Change PNP symbol to NPN _ check.
Move LED and resistor to source pin _ check



I am a little confused regarding the removal of the SPST though as the SP3T will be driving S but as all three channels are normally open when not selected, wouldn't that leave the inactive gates floating?

Thank you for the heads up regarding mixed voltage levels, this has also given me food for thought. So, is it good practice then to choose a voltage level that the whole system will operate at, or are mixed voltages ok within certain limitations (ie, device and travelling voltage levels being at the same potential)?
@Canobi
Symbols & devices: Transistors come in many varieties. Those you are currently using are bipolar junction transistors (BJT's) and MOSFETs with a further subdivision into NPN, PNP for BJTs and N-channel, P-channel for MOSFETs. There is a different symbol for each so that just a glance at a schematic tells the viewer exactly which type is shown. Here are some links to help:
https://www.allaboutcircuits.com/technical-articles/schematic-symbols-for-electronic-components-transistors/
https://www.allaboutcircuits.com/te...nic-components-passives-resistors-capacitors/

In your simple schematic of the LED driver, the symbol is incorrect; the symbol shown is for a P-channel MOSFET and is drawn upside down (i.e. the source and drain terminals are interchanged). The FDN327 is an N-channel MOSFET; we know this by viewing the datasheet for the device. HINT: Datasheets are of utmost importance; you should be very familiar with the datasheet for every component you use.

The LED and resistor are shown acceptably. However, a QUESTION: how much current do you intend to flow through the LED when it is lighted?

Regarding a SPST, SP3T switch not shown in this simple schematic...before you refer to something, make clear what it is. In this case it would help to say (if it were true): "Referring to my original schematic of post#1..." As I have previously stated, no MOSFET gate (nor BJT base) can be left floating. The S signal must never be undefined; that is, its state must always be either ground (0V) or high (3.3V); if no other device will correctly define the state, then a pull-up or pull-down resistor must be used to define the state.

Regarding power supply voltages, they must be what they must be. There is no rule that says specific voltage power supplies must be used in a design. Simplicity compels you to limit the number of different voltages as providing each voltage requires additional circuitry (i.e. design time and component costs & sizes). However, it is very common for designs to require multiple power supply voltages. In some cases the voltage will be defined by availability--If your design must operate from the AC power mains, then that voltage is defined; if your design must operate from a common lead-acid auto battery, then that voltage is defined. If your design uses a component that has a maximum or minimum power supply rating, then you must provide a voltage that meets those limits. Power supply values are defined by component ratings and by what you logically intend for the power supply voltage to do. That introduces another subject, noted by @Sensacell: Before you design a circuit, you must know clearly what it is that you want the circuit to do. That is, a design begins with a logical map (mental for very simple circuits; drawn for more complex circuits), not a schematic. You must have clearly in mind the function of each block--and have reviewed that logical map to ensure it does what you expect--before you draw a schematic. If your thinking at this stage is not clear, your design will not do what you expect and you will waste time and energy...and become frustrated. So, for the tiny block you just showed, think something along this line: I need to indicate the state of some net (signal) and I choose to do so by lighting or not-lighting a red LED; thus I will need a "red LED driver" block. I do not yet know what voltages will be available so my schematic for this simple block cannot yet be completed. After you complete the design of other blocks, the available power supply voltages will become clear; you can then revisit this simple "red LED driver" block to add the necessary component and power supply values. This is an iterative process. Your first attempt may use a power supply of 5V, only to discover later that a preferable power supply would be 3.3V...and you must revisit the schematic of the red LED driver to provide new values. As your sketch of blocks becomes more complete (as you add more information to it) you will eventually reach a point where drawing a schematic of certain blocks is reasonable. As @Sensacell has noted, your goal at this stage is to gain confidence that your design will logically provide the function you intend; that is, by connecting the blocks you have created, the overall design goal is met.
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
Hopefull the mosfet symbol is correct now and the LED has a max Vf of 4v @1w.



I may have also figured out how change the SP3T's input logic as I wanted using two SPSTs instead of the three mosfets as per my original schematic in post #9:
 

TeeKay6

Joined Apr 20, 2019
572
Hopefull the mosfet symbol is correct now and the LED has a max Vf of 4v @1w.



I may have also figured out how change the SP3T's input logic as I wanted using two SPSTs instead of the three mosfets as per my original schematic in post #9:
@Canobi
I am limiting my first reply to the LED driver issues. First, yes, the schematic now looks correct except that the red color of the LED symbol indicates a "red" LED to me. From the Vf=4V data you state, I infer that the LED is "white". Now assume that you apply 3.3V to the gate of the FDN327, turning it "on". According to the FDN327 datasheet, the "on" resistance could be about 0.075 to 0.08 Ω (ignoring any resistance outside the MOSFET, e.g. thin PCB copper traces). With a Vf=4V and a power dissipation of 1W, the LED would have to pass a current of 0.25A. Look at the voltage stack-up from ground to 3.3V via the LED and resistor: 0V(ground) + 0.02V (MOSFET) + ???V (resistor) + 4.0V (LED) = 3.3V. This means that the resistor voltage must be negative, = -0.72V (3.3-4.02), and that is impossible. Unless you use a voltage greater than 3.3V, you cannot cause current to flow through an LED with Vf=4. The situation is actually a bit worse due to tolerances in the 5V (it might well be 4.9V or 5.1V), the Vf of the LED (it might well be 3.8V or 4.1V), the tolerance of the resistor (it might well be 0.242Ω or 0.248Ω--assuming you used a ±1% resistor)...and all of those will vary somewhat with temperature. So, powering a 1W white LED from a nominal 5V power supply is only marginally possible. A better option would be to use a higher voltage power supply (6V, 9V, 12V?). So that you don't have to recompute everything in the future, I would mark your schematic to show current through the LED is to be 0.25A. You must perform an analysis similar to this for every LED you wish to drive. Note that different color LEDs produce different Vf values (best to see datasheets).
 

Thread Starter

Canobi

Joined Mar 4, 2015
29
My humble apologies, it is indeed supposed to be red with a Vf of 2.2v @1w (hence the 3R3 current limiting resistor).
 

MisterBill2

Joined Jan 23, 2018
9,882
What you do have is indeed a grounded plane chopped into 2 islands. My suggestion is to avoid that by running whatever traces must be on the GP side much closer to each other so that the ground plane is not divided like that. I have not looked at the actual circuit although others have, and so I make no comment on the actual circuit, only that a split ground plane sort of defeats the purpose.
 

TeeKay6

Joined Apr 20, 2019
572
Hopefull the mosfet symbol is correct now and the LED has a max Vf of 4v @1w.



I may have also figured out how change the SP3T's input logic as I wanted using two SPSTs instead of the three mosfets as per my original schematic in post #9:
@Canobi
Okay, let's look at your new logic circuit using 74LLVC1G3157 and TS5A3357 devices. I fear you are headed in a wrong direction. You are using devices meant to interface/control analog signals with logic signals. While that can be done, it is not the correct approach. You seem unaware of a whole universe of "logic devices" intended to handle purely logic (i.e. either true or false) signals; these devices are simple, low cost, and there are a huge variety of devices available. The first IC's were called Transistor-Transistor-Logic (TTL) devices and most of those device types are still available although there are many, many newer replacements also available. These devices include logic gates, inverters, buffers, multiplexers, counters, timers, latches, flip-flops, etc, etc--hundreds of different functions. When your intent is to control logic signals with other logic signals, use digital logic devices, not devices intended to control analog (i.e. subject to variation) signals with logic signals. So that we get started correctly, please describe in words exactly what you are trying to accomplish in your new logic circuit. That is, what is the functional description of this logic block? What are the input/control signals (and their possible values, e.g. 0V or 3.3V) you wish to use; what are the output signals that you need (e.g. signals to drive red, green, and blue LEDs); how are the input signals related logically to the output signals? For example, if SW1 (spst switch) is closed, you want only a blue LED lighted; if SW2 is closed, you want the blue LED plus a red LED lighted; if SW1 & SW2 are both closed, you want only a green LED lighted. It will be clearer if you give switches and LEDs names that are meaningful to you (e.g. SW1 might be called "faster", SW2 might be "top speed", the red LED might be "Stop", etc. Likewise, give this overall logic block a meaningful-to-you name (e.g. "speed selection"). If your circuit must operate with a certain power supply voltage, then state that voltage; otherwise, just call that voltage Vsupply for now. We will work from your logical description toward a circuit and devices that will implement that logic. One goal will be to minimize the number of components used and to use only commonly available components.
 
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