RC Oscillator - Page 1
Forum section: electronics / projects

author: Dandoni
author_rank: Contributor
timestamp: October 27, 2016, 02:23:07 am
content: Hey all,This is the circuit I have been playing around with and it seems to work ok and I messed around with digikey's sheme it to draw it up. From the reading I have done on RC oscillator circuits , the phase shift needs to be 360 degrees for oscillation to be sustainable. With one capacitor having a theoretical max phase shift of 90 degrees, how does this work on this circuit? What waveform does this produce?thanksps I hit the insert image but it doesn't seem to do anything, so I have attached schematic below

author: Zero999
author_rank: Super Contributor
timestamp: October 27, 2016, 08:02:20 am
content: Quote from: Dandoni on October 27, 2016, 02:23:07 amIt doesn't.Quote0V. The transistor will just turn on, dissipate lots of power and fail, possibly with lots of smoke.I think you're trying to make a blocking oscillator, which requires an extra feedback winding on the transformer.

author: Dandoni
author_rank: Contributor
timestamp: October 27, 2016, 08:38:42 am
content: It definitely works as I have the circuit in front of me and its transferring power from the transmitter to the receiver circuit as the LED is lighting up quite nicely.I got the idea from a video off youtubewith the same setup but with different components.I originally used the same transistor(2N3904) which also worked but was experimenting with different cap and resistor and voltage/current supply and I damaged the transistor. I settled on components that I posted and seems to work ok.I forgot to mention that this is a wireless power transfer circuit. Is the a different way you would represent the coils in the schematic?

author: Dandoni
author_rank: Contributor
timestamp: October 27, 2016, 10:33:10 am
content: Or does the coil in the circuit make it an LC Oscillator?Every where I read says more than a single stage RC network is required to produce a sustainable oscillation.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 27, 2016, 02:58:34 pm
content: Quote from: Dandoni on October 27, 2016, 02:23:07 amYour circuitas drawnwill not operateas an RC oscillator.The R & C arenotin a feedback loop between collector and base.To obtain feedback via CR,one,or both needs to be connected to the collector side of L1,not to the +ve supply,which byconvention ( which is pretty much descriptive of all real circuits) is,for AC, at the same potential,as the -ve side of the supply,& hence,the emitter of Q1.Just for curiosity's sake,disconnect C & check if the device still oscillates.

author: Dandoni
author_rank: Contributor
timestamp: October 27, 2016, 03:06:13 pm
content: Thanks for the reply. Its all soldered up. I will have to dummy another setup on the breadboard tomorrow. I will let you knowCould it then be a RLC or LC resonator?

author: danadak
author_rank: Super Contributor
timestamp: October 27, 2016, 06:59:04 pm
content: Capacitor not being ideal, will have < 90 degree phase shift, so 3 sections mandatory. Largely dueto ESR of cap.Wein bridge only requires 2 RC networks.Regards, Dana.

author: Dandoni
author_rank: Contributor
timestamp: October 27, 2016, 10:38:04 pm
content: Ok so this points in the direction of not being an oscillator, and more likely to be resonating between the coil and the cap

author: JimRemington
author_rank: Regular Contributor
timestamp: October 28, 2016, 04:04:13 am
content: LC oscillator. For a lark, I breadboarded the circuit using a 2N3904, 100K, 1 nF, loop of wire about 8 cm in diameter and powered by a 9V PP3. The scope shows it to oscillate at about 100 MHz. Obviously LC, because if you squeeze the loop the frequency changes. I assume that the collector-base capacitance is providing the feedback.

author: Zero999
author_rank: Super Contributor
timestamp: October 28, 2016, 12:19:52 pm
content: Quote from: Dandoni on October 27, 2016, 08:38:42 amThen the schematic is not a true representation of reality, especially if you're using a breadboard.As others have stated, you have an LC oscillator, rather than an RC oscillator there.The real circuit is probably similar to an FM bug oscillator, with the capacitor across the transistor, in parallel with the inductor (transformer) and emitter resistor being parasitic components. Breadboard is quite resistive and has parasitic capacitance which will make the circuit work. If you tried building it on a proper PCB it's quite likely it won't oscillate.

author: Dandoni
author_rank: Contributor
timestamp: October 31, 2016, 07:43:40 am
content: Thanks all.It works on a breadboard and when I soldered it up on strip board it also works. Hooking the Uni-t 139c up to my circuit( not the same components as the youtube vid) , I am getting a frequency around 4.5Mhz. I have yet to hook it up to an oscilloscope.I had to present my project in class, and you lost marks if it didn't work. I'm glad it did. Murphy is normally lurking around somewhere.