Re: Simple but precise ramp/triangle generator

From: Tilmann Reh (tilmannreh_at_despammed.com)
Date: 09/14/04 

Date: Tue, 14 Sep 2004 16:48:49 +0200

Fred Bloggs schrieb:

> Sounds like a 555 application to me-

You're so right. :-)

In the meantime I think I found the simplest solution:
The (CMOS) 555 in its basic astable circuit, however the discharge
pin is connected to the cap by a schottky diode - so the voltage
at the cap will not fall below about 0.2 V.
Charging is done by BJT current source, or eventually with a
resistor connected to a larger voltage available.
Supply is 12 V, but the control voltage (Pin 5) is drawn to a low
impedance 4.8 V potential which I can easily make from the 5 V
supply with two resistors.

The voltage at the cap is a nice ramp from 0.2 to 4.8 volts, and
tolerances should not make it vary too much. A connected comparator
safely outputs 0 to 100% PWM for 0..5 V input voltage (with some
margin at both ends, as intended).

(I didn't look deeper at the 555 before since the tolerances of
the internal control voltage divider seemed to high for this
application. However, since the upper treshold can be set by
external voltage and the internal circuitry is slow enough to
completey discharge the cap, this really is the solution.)

Anyway, thanks to all for your comments. 

-- 
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de

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