Discussion in 'General Electronics Chat' started by vinodquilon, Dec 13, 2012.

1. ### vinodquilon Thread Starter Member

Dec 24, 2009
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I want to give 5V-1ms pulse to a resistive load using 555 monoshot.
First I am connecting 33 Ohm, then 68 Ohm and finally 100 Ohm as load.

Can I use the same 555 circuit for all the above three cases without any loading and voltage dipping problems ?

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
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5v at 33ohms is 150mA, so you should be ok as it can give out 200mA max. Or use a transistor output slave driver.

3. ### Audioguru New Member

Dec 20, 2007
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Look at the datasheet of an NE555 or LM555. When it has a 5V supply its output high IS NEVER +5V, with no load it is +3.7V and it is typically about +3.4V into a 33 ohm load but its minimum is lower, near +2.75V. Its output voltage is lower when its load current is higher.

4. ### vinodquilon Thread Starter Member

Dec 24, 2009
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How can I vary the amplitude of 1ms pulse output from 0V to 5V in steps of 0.5V without adjusting 555 supply voltage?

5. ### Mike33 AAC Fanatic!

Feb 4, 2005
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Are you looking to get to some specific voltage with the output? As mentioned, you can use a transistor to act as a switch. This will allow you to connect whatever voltage divider you like to the BJT output, getting the desired voltage up to your supply minus about .7V for the BJT. The 555 output pulse will only turn the BJT on, and have nothing to do with what voltage actually appears at the BJT output.

This link should get you started. You just need to hook up the base to the output of the 555 with something like a 1k-10k resistor (to prevent loading) and figure out where you need to be with your collector resistances....this is a handy trick, esp. with timers, because you can source MUCH more using a transistor!

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Aug 23, 2012
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7. ### vinodquilon Thread Starter Member

Dec 24, 2009
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Here is my final schematics....(one using OP AMP and other using BJT).
Kindly give me necessary advice on selection between the two ones that is better?

Is there any mistakes in my design and selection of components ?
After getting a clear answer I will go for hardware implementation of the schematic.

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• ###### 555(OPAMP).jpg
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Last edited: Dec 16, 2012
8. ### vinodquilon Thread Starter Member

Dec 24, 2009
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I got a power supply that can vary from 0-60V manually with 0-7A providing over current and voltage protection.

9. ### vinodquilon Thread Starter Member

Dec 24, 2009
234
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I am using ZUP 60-7(TDK Lambda) 0-60 VDC , 0-7A , 420W output power supply with Voltage/Current, OVP/UVP manual rotary adjust.

10. ### ScottWang Moderator

Aug 23, 2012
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11. ### vinodquilon Thread Starter Member

Dec 24, 2009
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Does this works !!!!!

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Aug 23, 2012
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13. ### Audioguru New Member

Dec 20, 2007
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The output voltage of the darlington pair of transistors depends on their hFE, their Vbe, their temperature and maybe a few other things.
Usually we put a transistor inside the negative feedback loop of an opamp so the flaws are cancelled.

14. ### ScottWang Moderator

Aug 23, 2012
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Is there something wrong?
The big difference between darlington pair and the pdf that I linked is the output voltage of darlington pair have to reduce about 0.9V, but the circuit in the pdf, the output voltage close to Vcc-0.2V, that's what I like, does that any different to you?

Show me the circuit please, and thanks.

15. ### vinodquilon Thread Starter Member

Dec 24, 2009
234
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Pls find the modified schematic.....Is it OK ?

Can I produce the low values like 0, 0.5, 1.0 V by turning 5K ? (also 1.5, 2.0, 2.5, 3 medium values & 3.5, 4, 4.5, 5V high values at output of darlington)

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16. ### Audioguru New Member

Dec 20, 2007
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The circuit does not produce accurate voltages. The output voltages change when the load current changes and they change when the temperature changes. The voltage even changes as the output transistor warms up.

The output high of a 555 is a darlington that has a voltage drop of 1.3V at low current or 2.5V at high current. Its datasheet also shows how its voltage changes when the temperature changes and when it warms up.
The two transistors making a darlington will perform the same.

17. ### vinodquilon Thread Starter Member

Dec 24, 2009
234
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Can you suggest a schematic that can manually control the output of 555 mono(1mS) from 0 to 5V in continuous fashion for loads 33 / 68 /100 Ohms.

18. ### Audioguru New Member

Dec 20, 2007
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If your 5V supply is accurate at +5.00V then use a Cmos 555 (LMC555, TLC555 or ICM7555). Its output goes to +5.0V when its load current is small.
It feeds an opamp that drives a transistor with negative feedback so the output voltage is what you want.

19. ### ScottWang Moderator

Aug 23, 2012
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Move your Load 33 / 68 /100 Ω to replace 22Ω, and the E connecting to GND.

20. ### vinodquilon Thread Starter Member

Dec 24, 2009
234
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555-OPAMP-BJT
Here is 555 oneshot with LM324 & 2N2222 for 0-5V continuous amplitude variation of 1ms pulse.

Any rectifications ?

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