ramp generator

[Thunderchild]

problem is if I use a comparator that detects it is charged and so discharges after a fractional discharge it will stop as it will go under the comparators threshold, basicall solving the problem would mean making a descrete version of the 555 IC timmer so i might as well use that. I treid doing a simulation with electronics workbench but it just jammed I tried using a current source to charge the capacitor. perhaps someone could give me a diagram of one that will feed from the positive supply, ie the capacitor is connnected to gnd annd the current source.
unfortunately I have very little equipement bread boards are unobtainable and even matrix board take time to arrive and costs an arm and a leg it takes me weeks to put things together and is very difficullt to experimment as everything is soldered on the fly

 

[audioguru]

There are lots of ways to make Pulse-Width-Modulation and a 555 timer can be used for a few of those ways. The simplest way is to use two steering diodes and a pot. Another way is by using the 555 as a triangle wave generator and a comparator. It doesn't need a linear ramp.

If you want a linear ramp circuit then look at the datsheet of the LM555:

 

[raybo]

that is almost a current source. why a change in power will change I. To make one just replace the resistor across the voltage to base with a zener and the I will remain constant as zener voltage +Vbe then you have a real current source it can only change with within those terms.

 

[audioguru]

that is almost a current source. why a change in power will change I. To make one just replace the resistor across the voltage to base with a zener and the I will remain constant as zener voltage +Vbe then you have a real current source it can only change with within those terms.

It has only 5V for a supply, probably from a voltage regulator.
The voltage at the base of the transistor doesn't change, and is only about 1.7V.
Zener diodes with a voltage of only 1.7V are not available.
The transistor is a PERFECT current source. Look at how straight the ramp is on the 'scope trace.

 

[raybo]

you never heard of diodes not as good as zeners but close enough

 

[audioguru]

you never heard of diodes not as good as zeners but close enough

The voltage divider across the regulated supply is better than diodes.

 

[raybo]

just get a life and every body else a break will you i am tired of your picking BS because you have nothing in your head and nothing to contribute.
people ask for help here this is not a pissing contest.

 

[audioguru]

Raybo,
What is your problem?
Grow up!

 

[Roff]

just get a life and every body else a break will you i am tired of your picking BS because you have nothing in your head and nothing to contribute.
people ask for help here this is not a pissing contest.

Raybo, you sound just like the Ray I shot down in **broken link removed** after you sniped at me there with a similar comment. Funny, I never heard from you again there.

 

[Nigel Goodwin]

I've just PM'd him warning him about his behaviour.

Moderator.

 

[Thunderchild]

and here it is simulated on electronics workbench 8 I just have to see what effect the comparator will have on it but doubt much thankfully version eight is very realistic