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Question
boost converter
boost converter  
In this boost converter , when i increase the load resistance to very high value , it act as a boost converter. otherwise it wont .

Why this is so ?

Can u tell what is the effect of load resistance on this circuit ?

What will happen to the output when i increase/decrease the value of L and/or C ?

which device (with device number) i can use as a switch inorder to perform good switching activity?

If u give the design equations along with the answer , it will be more helpful to me.

thank you so much...

Answer
This is a very complex circuit, even though it looks simple. The concept is when the current rushes in the coil induces a higher voltage and the diode allows a build up of voltage on the cap at the output.  The device allows discharge current from  the coil so it can build up again.

The LRC time constant is at work and is sensitive to the actual current flowing to the load.  Therefore, the loading is a critical part of the operation of the circuit.

The inductance is quite a low value so it is expected that this is a low current converter and with an output cap of only 100 mfd it is not going to produce a very big dc output. The load resistance of 1k is obviously only few milliamps with an 8 to 10 volt output.

My experience is that these circuits will operate effectively only over a very narrow range and should be used whence the output loading is not changing much during its normal operation due to the narrow window of operation.

When the load resistance value goes too low it swamps out the circuit operation and it comes to a halt or at best merely produces some spikes across the load and a low value of dc. Do you have a scope so you can see the waveforms?  That is the easiest way to get a handle on what is going on.

If you had a better handle on the switching input it would be possible to get the circuit to operate over a wider range of load currents.

If you google you can find lots of information on these circuits:

https://www.google.com/search?q=operation+of+dc+to+dc+boost+circuits&client=opera&hs=W62&tbm=isch&imgil=UuNpjXfoIm1v6M%253A%253Bhttps%253A%252F%252Fencrypted-tbn1.gstatic.com%252Fimages%253Fq%253Dtbn%253AANd9GcTxtmqCNlbHGqPyEWpT0gDe8wokUm4It_jCXa2JBzouoo0QGu1DKw%253B620%253B387%253BvGHDrKLWEhPOCM%253Bhttp%25253A%25252F%25252Fwww.instructables.com%25252Fid%25252FDC-DC-HV-Boost-Converter%25252F&source=iu&usg=__vsxwaqM17eU3yFppOt-dFxqo8bw%3D&sa=X&ei=J-c6U9_GAqTuyAHX5oHIBw&ved=0CEwQ9QEwAg#facrc=_&imgdii=_&imgrc=UuNpjXfoIm1v6M%253A%3BvGHDrKLWEhPOCM%3Bhttp%253A%252F%252Fcdn.instructables.com%252FFQ8%252FO3KK%252FFQCTKS8F%252FFQ8O3KKFQCTKS8F.MEDIUM.jpg%3Bhttp%253A%252F%252Fwww.instructables.com%252Fid%252FDC-DC-HV-Boost-Converter%252F%3B620%3B387

The above URL is a result of searching the words: operation of dc to dc boost circuits

Good Luck with your experimentation.  Hope this helps.

PS: This may be of interest to you as well:

http://en.wikipedia.org/wiki/Boost_converter  

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Electronics questions about AC, DC and digital theory.

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Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

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BSEE University of North Dakota

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