SawTooth generator - Page 1
Forum section: electronics / beginners

author: daniel4422
author_rank: Newbie
timestamp: June 11, 2016, 02:33:32 pm
content: Hello,I followed this great tutorial -and tried to simulate the circuit in orcad pSpice, but with no luck :/I attached the circuit schematic, I'm trying to do a "transient analysis while Voltage probing Rload, can see only 0.5 DC and not a sawtooth signalcan anyone take a look at it ? could it be that I choose a wrong PNP/NPN transistors ?I was wondering if the schmitt trigger Vin should be a pulse signal and not a DC signal. is that true ? (tried it also...)

author: danadak
author_rank: Super Contributor
timestamp: June 11, 2016, 03:35:29 pm
content: Some questions -1) Do you want a ramp symmetrical about ground or from gnd to some + value ?2) You stuck on this circuit ?3) Do you care about rise/fall time of ramp reset ?4) Single supply or split supply preferable ?5) Voltage controlled frequency ? If so range of V and F.Regards, Dana.

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 04:58:52 pm
content: Quote from: daniel4422 on June 11, 2016, 02:33:32 pmHiThe file you posted comes up flagging an error and will not display.Bob

author: rstofer
author_rank: Super Contributor
timestamp: June 11, 2016, 07:27:35 pm
content: Right, the attached .pdf above is hosed...I modeled the project with LTSpice and made a number of mistakes that needed correction. First of all, by default, the PNP transistor was backwards. Second, I was looking at the Switch signal instead of the Out signal. Nevertheless, it all worked out in the end.I have attached my schematic and waveforms as .pdf files.It's a pretty neat circuit. I like the bridge rectifier being used as a switch. Clever!

author: daniel4422
author_rank: Newbie
timestamp: June 12, 2016, 06:40:16 am
content: Thanks a lot for the help !I can open my pdf file, but anyway I uploaded a jpg of the circuit and the waveform.I changed the NPN/PNP to be the exact model in rstofer example and still no luck :/I am new to pspice (did the orcad pspice tutorial) so maybe I am missing some settings ? should I use a different simulation software like you are using LTspice ?

author: Chris Mr
author_rank: Regular Contributor
timestamp: June 12, 2016, 09:47:31 am
content: Obviously just an observation but the emitter of Q5 (PNP of high side constant current source) looks to be disconnected - everything else has a red line joining it up to something.

author: Chris Mr
author_rank: Regular Contributor
timestamp: June 12, 2016, 09:50:17 am
content: Ah, one more point of pedantry, this is a triangle wave oscillator rather than a sawtooth.To be a pedant requires a lifetime of vigilance

author: rstofer
author_rank: Super Contributor
timestamp: June 12, 2016, 03:54:02 pm
content: Take a look at the PNP transistor as mentioned above. However, the voltage readings seem to suggest it is conducting.I used a 'generic' op amp. I'm not sure about the wisdom of using a 741 because it is nothing like the author's rail-to-rail (output) op amp. If I wanted a 'real' part, I would look for a single supply rail-to-rail device that can work with 10 volts.It's odd that pspice doesn't force you to define the Vcc supply as a component like LTSpice.Your 'switch' signal should be either 0V or 10V, not something in the middle. You are doing 'transient' analysis, right? That's another thing about LTSpice, the type of analysis is included in the sketch.It only takes a couple of minutes to install LTSpice. I have attached my .asc file should you decide to install it. Open the file, click on the 'running man' icon and probe the Out signal and the Switch signal.I am truly a rank amateur with LTSpice. I use it more for simulating mechanical systems (differential equations) than for serious electronics work. There is a very active LTSpice group on Yahoo. Several true experts hang out there.

author: daniel4422
author_rank: Newbie
timestamp: June 12, 2016, 06:19:00 pm
content: The PNP is connected properly, it is just the orcad figure.anyway, thanks a lot for the LTSpice tip - work like a charm