555 Constant Current Triangle Wave Generator Not Producing Clean Ouput

Thread Starter

Dario M

Joined Dec 30, 2017
5
I am using a 555 timer to produce a square and triangle wave with a symmetric current source although what I simulate in Lt Spice isn't really transferring into real life. The problem gets worse at higher frequencies around 30khz and improves a bit at around the 5khz range. Is there any way to modify this circuit to produce a clean output in the 30-200khz range?
I've attached what I get on my scope compared to LT Spice.
 

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AnalogKid

Joined Aug 1, 2013
8,101
Assembly/construction error? Can you post a photo of the circuit?

I get +/-4 mA to the timing capacitor. What did you get?

ak
 

Thread Starter

Dario M

Joined Dec 30, 2017
5
Im getting about 1.2 in the actual circuit. Im using a bc548 and b558 for the bjts and 1n4007 diodes. Cant get a picture of the circuit right now. It seems like the current source just doesn't want to behave properly with smaller caps
 

RichardO

Joined May 4, 2013
2,273
What is your power supply voltage? The current sources and diode current switch take a few volts to work right. I haven't run the numbers but I don't think the circuit will work with a 5 volt power supply. It might work at 9 volts.

For this circuit to work right you have to use a CMOS version of the 555 timer. The LTspice model for the 555 is (sort of) for the CMOS version.

Also, _never_ use the generic parts in LTspice. You must always select a specific part number for diodes and transistors. If you don't then you can get confusing results in the simulation.
 

Thread Starter

Dario M

Joined Dec 30, 2017
5
When I ordered a set of general purpose diodes I didn't really think about switching speed so I just got a bunch of 1n4007. Just ordered some faster 1n4148 and going to see how well this works with a regular 555 since I don't have any cmos ones lying around.
 

Thread Starter

Dario M

Joined Dec 30, 2017
5
Also I want to run this circuit off 12 Volts for a function generator but I also tested it on 5V and it didn't seem to change anything.
 

crutschow

Joined Mar 14, 2008
23,310
Everything works with faster diodes, thank you
Good news.
By the way, the LTSpice simulation would likely have looked more like the actual circuit if you had used 1N4007 diodes and real transistor models instead of the generic default diode and transistor models, which tend to be much more ideal.
A simulation is only as good as the models you use.
 
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