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Can someone just check my calculations for this project please?

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Well thank you Diver.
It was good of you to bring up the turn on/off times of the components, but i think there is a much simpler explanation about the 5.5uH inductor. The minimum Ton time of the chip is spec'd at 19us, and with that much time and a 12v input voltage at the end of the Ton period the current in the inductor would have reached something like 40 amps, and we know that the transistor will never be able to handle that kind of current as it is limited to 1/2 amp. As a matter of fact, due to the input voltage and min Ton time the minimum inductance for this app would be about 456uH. Any lower than that and the current demand exceeds the internal transistors rating. Make sense to you?

That makes perfect sense.
 
Hi again,

Yeah i'd like to see a schematic of the finished circuit and a photo of the real life board would be nice too :)
 
Hi Guys,

Here is a scan of my original design for the step up and a pic of the actual device finished.

The finished product works brilliantly (LED pun) and differs from the original design only in the fact I used a larger(47uF) smooting cap on the output.

Credit for the finished product goes to MrAl, Diver300 and Ronv, without whose help I would never have completed it, due to conflicting and possibly erronious information in the datasheet. Thanks Guys. Al

PS the circuit is slightly untidy but was intentionally designed as small as possible to fit into the smallest box I had. Thats why I made some of the links under the IC.
 

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Hello again,

Nice pics, thanks. So i guess you ended up using a 1.1mH inductor then? Do you happen to know its series DC resistance?
 
Hello again,

Nice pics, thanks. So i guess you ended up using a 1.1mH inductor then? Do you happen to know its series DC resistance?

Hi mate,

Do you mean measured with the DMM?

I have an inductance and capacitance meter that I made some years ago and that particular inductor was the nearest I had measurement wise - measured 1.087mH and seems to be just the job.
 
Hi mate,

Do you mean measured with the DMM?

I have an inductance and capacitance meter that I made some years ago and that particular inductor was the nearest I had measurement wise - measured 1.087mH and seems to be just the job.

Hi,

Yes you can measure the series resistance with the ohm meter just like you would do with a resistor. The ESR of the inductor is said to be in series with the pure inductance so when we measure this we see the actual series resistance (ESR). It could be low like 0.1 ohms or higher like 10 ohms.
 
Hi guys,

Mr Al, the res measures 1.3ohms but my DMM seems a bit high on small ohms, the dozens I have measured all seem to be about twice the marked value! So the inductor could well be 0.7ohms or anything in between.

Wish I could find a simple and accurate way of measuring the small ohms!

Al
 
Hi guys,

Mr Al, the res measures 1.3ohms but my DMM seems a bit high on small ohms, the dozens I have measured all seem to be about twice the marked value! So the inductor could well be 0.7ohms or anything in between.

Wish I could find a simple and accurate way of measuring the small ohms!

Al


Hi again bigal,

You can pump a known current through the resistor and measure the voltage across it and use Ohms Law.
If you use a regulated current source you can do this pretty quickly so you can check lots of resistors.
Just have to be careful with inductors because they can generate a high voltage when disconnected from a current source. When you test it you short it out and then unshort it, then short it out again before disconnecting the current source.

You can also use a battery and a larger resistor if you have a way to measure current. Same thing, use Ohms Law. You can measure the current by measuring the voltage across the larger resistor and again use Ohms Law.
 
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