PCB design. Voltage drop. - Page 1
Forum section: electronics / beginners

author: tommys
author_rank: Newbie
timestamp: March 18, 2023, 10:07:15 pm
content: Hello everyone!I'm designing a PCB to control fluids using brush motors with valves.The motors I'm using are 385 DC 12V low noise, working current is 0.5-0.7A combined with liquid suction pump:https://www.amazon.com/Suction-Electric-Diaphragm-Machine-Dispenser/dp/B07PHSWYM9Also I have connected 12v 1.5A valves in parallel to these motors:https://www.electricsolenoidvalves.com/1-2-12v-dc-electric-brass-solenoid-valve/I control these motors with STM32 microcontroller through IRLZ34N transistors, The schematic fragment looks like this:https://postimg.cc/06Yb5Z9HAnd the PCB that I managed to design:https://postimg.cc/bG7HWm3C/da1c8a03The problem I'm facing is that when starting one engine after another a large voltage drop occurs in the motors. For example, if one motor receives 12v voltage, the next one will receive 10v when activated, the third motor will receive 7v when activated and so, until eventually neither engine starts. By disconnecting one by one back, the remaining motors start working. Also, the mosfet transistors begin to heat up strongly.I'm fairly new here and this problem seems quite strange. Maybe you have faced a similar problem and where should I start looking for the problem? Maybe PCB tracks?Thanks for the answers!

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 18, 2023, 10:38:36 pm
content: Post a schematic. That'd be much more useful than just a PCB layout... Sounds like your power source is dipping under load. What does the power supply voltage do as each additional motor is activated?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 19, 2023, 07:04:01 am
content: Post full schematic.It sounds like your power supply is dropping its output voltage due to too much current. Did you try connecting the same loads to the same power supply manually, bypassing your whole PCB?

author: tommys
author_rank: Newbie
timestamp: March 19, 2023, 12:01:09 pm
content: thanks for the answers,Power supply voltage remains stable, almost unchanged, when the engines are started.And yes, I tried connecting everything directly to the power supply, bypassing the PCB. Everything worked without any problems.I am attaching a schematic of the MCU part:https://postimg.cc/K38P9m26Connectors part:https://i.postimg.cc/tJHzwjCQ/connectors.pngPower part:https://i.postimg.cc/VkSbqhmj/power.pngEngines control part:https://i.postimg.cc/fW2L8vH5/pump-control.png

author: Siwastaja
author_rank: Super Contributor
timestamp: March 19, 2023, 01:34:34 pm
content: Make sure the LD1117 is not oscillating due to wrong type of output capacitor, maybe there is even a DC shift in output voltage causing the MOSFETs to not fully conduct. Use an oscillosscope to check the stability of 3.3V.3.3V gate drive for IRLZ34N is marginal. It's possibly not turning fully on with your Id (current through the MOSFET) (see the curve set Vds vs. Id with different gate drive voltages).

author: jonpaul
author_rank: Super Contributor
timestamp: March 19, 2023, 01:55:06 pm
content: OP should directly upload images,of schematic so readers see the schematic directly in the post.Why bother with four separate lu,KS to,some unknow and potential risky server?j

author: Terry Bites
author_rank: Super Contributor
timestamp: March 19, 2023, 05:21:45 pm
content: True

author: tommys
author_rank: Newbie
timestamp: March 19, 2023, 07:01:19 pm
content: Images are now uploaded directly. Sorry about that.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 19, 2023, 08:10:51 pm
content: Sounds like you're getting too much current draw to actually power the motors as you add each one in. The way to debug is to check what voltages you have as you put two or three motors in there. Start by checking your power supply, then check each motor high side and low side, then the sources of all of your FETs that you are using as load switches. At some point, you're going to find that you have several volts drop between a couple of nodes and that will be where the issue lies.Likelihood is that you don't have enough copper in your traces to handle the load current and you're getting drop across the PCB trace resistance.

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 19, 2023, 08:11:03 pm
content: Quote from: tommys on March 18, 2023, 10:07:15 pmQuote from: Siwastaja on March 19, 2023, 01:34:34 pmI think Siwastaja is correct. The gate drive for the IRLZ34N is marginal (will be less than 3.3V) So as FETs are turned on, there is a voltage drop in the ground leads to the FETs (raising the source voltage slightly) which has the effect of decreasing the gate to source voltage. Since the FETs were already barely turned on to begin with, as more current is drawn, they get into their linear regions and the power dissipation increases.You could test this by putting a voltmeterdirectlyacross one of the FET's source and gate leads (second one to turn on) and observing the voltage there as the first, second, and third motor FET is turned on.I wouldn't be surprised to see the gate-source voltage of FET #2 drop when FET #3 is turned on. The voltage reading might even go "negative" when only FET #1 is on.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 19, 2023, 09:53:11 pm
content: Quote from: tommys on March 18, 2023, 10:07:15 pmThat is almost certainly because of insufficient gate drive for your MOSfets.