mahfuzmatyaacob

Hey guys,

Im having a problem on designing a circuit that produces a linear ramp from 0V up to 6.5V, where u can actually control the ramp frequency. Can someone give ideas on this subject matter.

Blueteeth

I'm unsure what you mena by 'ramp frequnecy', but I assume you mean 'saw tooth wave'. I believe a 555 timer chip can do this:

http://www.m.case.btinternet.co.uk/h...enerators.html

It uses the 555 in standard astable mode, but with a transistor to provide a constant current source to the charging capacitor. So it ramps up linearly. You can control the frequency simply by adding a pot to one of the timing resistors.

Blueteeth.

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Hero999

Use the NE566.
http://www.genieelectrique.ca/docs_u...s/NE_SE566.pdf

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mahfuzmatyaacob

the ramp doesnt start from 0V. It start from 1.9V at the minimum value. Any adjustment i should make for the ramp to start from 0V up to lets say 6.5V?

Blueteeth

Did you use the schmatic (second one down) in the link I posted?

That will start at 0V. If you wish to make it 'stop' at 6.5V the I suggest a comparator (LM393 should do) connected to pin 4 of the 555 timer. Connect the output of the ramp generator (pins 2,6 and 7) to the _ of the comparator, and use a voltage divider on the + pin. I do'nt know what your power supply voltage is but I'm sure you can work out the resistor values.

Then the output of the comparator, with a pull-up resistor to VCC should connect to pin 4 of the 555.

Your ramp output 0-6.5v will be the output of the 555 timer (pins 2.6.7). All this does is reset the ramp (discharge the charging cap) when the threshold of the compartor is reached...in your case 6.5v. It will then discharge the cap to 0v, and start to ramp again.

I will post a schematic soon if I have time.

Good luck.

Blueteeth

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mahfuzmatyaacob

Let me try again on your advice. I hope it'll work. Thanks man. Appreciate it

Roff

The ramp in the second circuit in that link will not go to zero volts except with small capacitor values, where the propagation delay of the 555 is not fast enough to turn off when the cap voltage reaches Vcc/3. With large cap values, it will oscillate between Vcc/3 and 2*Vcc/3.
Below is a sawtooth oscillator which does a pretty good job of ramping between 0V and 6.5V. It's actually two monostables which trigger each other.
The two 555s could be replaced with a single 556.

Mahfuz, if this is for your A/D test fixture, it will not approach 50MHz. You might get a decent ramp at 50kHz.

Attached Images

555 ramp gen.PNG (31.7 KB, 18 views)

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Ron

mahfuzmatyaacob

Thanks Ron, but if i may ask, what is the purpose of that regulator? If its to produce a constant current, you already have the transistor for it? Mind sharing.. :-)

Roff

Study the LM555 datasheet. The 555 has an internal voltage divider with two thresholds, one at Vcc/3 and one at 2*vcc/3. In this circuit, the ramp cap charges until it reaches the upper threshold, where an internal flip-flop gets reset, discharging the capacitor.
You want the ramp to reset at 6.5V.
9.75V*2/3=6.5V. Hence the adjustable voltage regulator.

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Ron

mahfuzmatyaacob

Roff, I have tried to simulate the circuit you gave me, but it turns out to be errors on the connection. Is it because of my simulation program (PSpice) or what? Need you opinion on this. Attached the circuit diagram.

Attached Images

test_circuit.JPG (58.1 KB, 13 views)

Roff

Do you get error messages from Pspice? What was the length (time) of your simulation? I don't see anything wrong with your schematic, except I would not set the pots to their maximum value. The pot to set the supply voltage (your Vin) actually needs to be about 30 ohms. For hardware, I would change the resistor in series with it to 680, or change the pot to 100 ohms.
I'm attaching the .ASC file for LTspice, which is what I used for simulation. You can just open this file with LTspice, and the schematic will appear, ready to simulate.

Attached Images

555 ramp gen wave.PNG (13.0 KB, 9 views)

Attached Files

555 ramp1.asc (3.4 KB, 3 views)

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Ron

mahfuzmatyaacob

Ron,

Thanks dude, seems like i need to install the LTSpice software.. Haha have to learn on how to use it, but anyhow thanks for your advice. need to find LTSpice library since there are certain libraries that are missing when you gave me the file. Thanks man for the help

mahfuzmatyaacob

I seem not to understand how does the LTSpice works, i mean the subcircuit. Based on the file you gave me, the software cant detect the LM317 device. I went searching around the forum on how the subcircuit works and it seems like you guys are editing/creating some sort of file to be rename as .sub file. Hmmm, confusing.....

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-Mahfuz-

Roff

Sorry about that.
In the attached zip folder, unzip LM317.sub into C:\program files\LTC\swcadIII\lib\sub.
Unzip LM317.asy into C:\program files\LTC\swcadIII\lib\sym\misc.

I created that symbol. It doesn't come with the LTspice download, as you discovered.

You will have to close LTspice and restart it so the program can find these files.

Attached Files

LM317.zip (1.1 KB, 7 views)

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Ron

mahfuzmatyaacob

Hey Ron, thanks man, it works properly.. I never used LTSpice before and that is why i dont seem to understand the way the software works. But i guess from the overview of it, i think that this software allows you to create your own device based on your own specification. Again, thanks man for the help.. Appriciate that

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-Mahfuz-