PWM module driver in switching circuit does not work right - Page 1
Forum section: electronics / beginners

author: tastychickenwings69
author_rank: Newbie
timestamp: November 26, 2021, 07:00:29 am
content: I have a problem with my Pwm switching circuit.It is just a class A npn pwm switch with a Bipolartransistor and a LED load with a current limiting resistor (RCollector), one base resistor, a power supply with 12V and a pwm module that gets powered from the same power supply which has a 12V output at 1Khz and around 50% duty cycle.The switching circuit kinda works. The problem is that it also works when only the connecters for PWM and Ground at the input side of the switch and +V but without -V for the input power of the PWM module are connected.The pwm module has no input/output isolation, the input output resistance is close to 2Ohms, but its only DC anyway so does that even matter?The LED load seems to be responding right when I change the frequency or duty cycle but it still does not make any sense to me.I checked everything and everything seems to be connected right.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2021, 08:00:23 am
content: A diagram is worth a thousand words!What PWM module? What switch? What is a "class A switch"?What pins/connections/wires do these modules have? I gather from your description, some have power and ground connections, some have +V/-V, signal output/input -- but which and where, is not clear.Minus this information, it simply sounds to me like you've got the power-return or common terminals connected (GND and -V),as you should(so you're not doing anything wrong; or at least, not apparently so), and maybe just, no one explained it to you, or showed in the diagrams or whatever, how they're really connected internally, and what all that means for external connections and how to operate it all. (I gather you expected that breaking some connection would cause it to stop, and it hasn't?)And the thing is, if these are like, no-name off-the-shelf modules, the documentation might be abysmal, if present at all -- so this could very well be a point of confusion for a beginner, you simply don't know how circuits are usually wired internally, and you don't have the diagrams showing that, so what do you do, right? Well, you'd probably ask someone else. And so you've come here.Tim

author: tastychickenwings69
author_rank: Newbie
timestamp: November 28, 2021, 05:10:18 am
content: Quote from: T3sl4co1l on November 26, 2021, 08:00:23 amSorry for the late answer, it didnt show new messages for some reason.Yes I meant a normal bjt switch.And there is a voltage divider of course to reduce the voltage to under 3V for the base of the bjt. R1 is 1.2k and R2 is 400ohm for 3V and 10mA max.Here is the circuit in a simulator. It does work in the program and I added the connections for the pwm module to the power supply.Its this pwm module:http://www.icstation.com/signal-generator-square-wave-generator-channel-150khz-dual-mode-pulse-frequency-duty-cycle-adjustable-module-p-13752.htmlhere a picture with 4 connections:http://www.icstation.com/images/big/products/13755_5_4402.JPG

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 28, 2021, 06:18:41 am
content: Ah yeah, they just show power and output, but who knows what the relationship between them is like, right? In all likelihood the ground wires are tied and they could use a 3-terminal block for all that matters (4 is just a bit more convenient).Tim

author: tastychickenwings69
author_rank: Newbie
timestamp: November 29, 2021, 05:08:33 am
content: Quote from: T3sl4co1l on November 28, 2021, 06:18:41 amWould it make sense to maybe try powering the pwm module from another (isolated) power supply?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2021, 08:19:23 am
content: Sense for what, what are you looking to show?An isolated supply will unpower the PWM when either power or ground are cut.Tim