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Old 8th April 2004, 04:23 AM   #1
imix500 is offline imix500  United States
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Default Pulse Generator Schems?

Hi, anyone have a schem for a pulse generator that has a fixed On time of around 50ms and a max frequency of 10Hz? I basically need a PWM in reverse, adjustable off time. I am using a 555 in old reliable astable mode, but it doesn't like a duty cycle below 50%. Thanks!
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Old 8th April 2004, 05:00 AM   #2
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I've done it by connecting a PNP transistor as a CCS in place of "R1" (the resistor from +V to pin 7). This way it charges linearly (dv/dt = C * I... dv/dt = rate of voltage change) by the CCS and R2 and discharges just through R2 (the CCS' current is shunted through pin 7).

By varying base current to the CCS you can get an extreme duty cycle.

Tim
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Old 8th April 2004, 05:47 AM   #3
imix500 is offline imix500  United States
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Thanks Tim, that a great idea. So, would the base be tied to pin 7 with the E tied to R2 and the C to V+? I'm having a little trouble visualizing it. How would the base current be varied? Thanks!
Ricky
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Old 8th April 2004, 04:22 PM   #4
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Collector is the high-Z output, aka CCS. Follow the current too, I think you described it reversed.. B to gnd through variable resistor (500k pot + 10k current limit R), C to pin 7 (7 to pins 6+2 thru "R2" as usual), E to +V.

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Old 8th April 2004, 06:13 PM   #5
imix500 is offline imix500  United States
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Got it. Yeah, I did describe it reversed. It's a PNP device, I had the current flow backwards. This is why I don't design after a few pints. Thanks!
Ricky
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Old 8th April 2004, 08:17 PM   #6
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Hi Ricky,

Another way to do it is use a resistor in series with a diode (i.e. a 1N4148) from the output to the cap to charge the cap, this sets the fixed on time. Use another (variable) resistor from the discharge pin to the cap to discharge the cap and set the off time. This works best with CMOS 555ís.

Cheers
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Old 8th April 2004, 08:52 PM   #7
imix500 is offline imix500  United States
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Pjotr, do you happen to have a schem using that arrangement? Thanks!
Ricky
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Old 8th April 2004, 09:07 PM   #8
imix500 is offline imix500  United States
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Pjotr, just found a schem using exactly what you described. I will build both and see what works best. Thanks for all the help!
-Ricky
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Old 9th April 2004, 02:55 AM   #9
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The classic 555 astable circuit can be fixed to supply <50% duty cycle by shunting the discharge resistor with a diode. This is probably the simplest way.
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