555 monoshot-- loading effect

Thread Starter

vinodquilon

Joined Dec 24, 2009
234
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 ?

 

Audioguru

Joined Dec 20, 2007
11,251
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.
 

Thread Starter

vinodquilon

Joined Dec 24, 2009
234
How can I vary the amplitude of 1ms pulse output from 0V to 5V in steps of 0.5V without adjusting 555 supply voltage?
 

Mike33

Joined Feb 4, 2005
349
How can I vary the amplitude of 1ms pulse output from 0V to 5V in steps of 0.5V without adjusting 555 supply voltage?
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! :)

http://www.allaboutcircuits.com/vol_3/chpt_4/2.html
 

Thread Starter

vinodquilon

Joined Dec 24, 2009
234
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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Thread Starter

vinodquilon

Joined Dec 24, 2009
234
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.
 

Audioguru

Joined Dec 20, 2007
11,251
Your English translation is horrible.

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.
 

ScottWang

Joined Aug 23, 2012
6,827
Your English translation is horrible.
Sorry about that, for your eyes, for your brain, and for your mood.

The output voltage of the darlington pair of transistors depends on their hFE, their Vbe, their temperature and maybe a few other things.
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?

Usually we put a transistor inside the negative feedback loop of an opamp so the flaws are cancelled.
Show me the circuit please, and thanks.
 

Thread Starter

vinodquilon

Joined Dec 24, 2009
234
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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Audioguru

Joined Dec 20, 2007
11,251
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)
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.
 

Audioguru

Joined Dec 20, 2007
11,251
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.
 
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