Salvaged choke for boost

Hi,

I managed to salvage a working condition power supply and desolder one of the choke (if I'm right the name of the component) as shown in the attached picture.

I tried to create a simple boost converter to see if I "boost" voltage above 3V using the following circuit (it's not my circuit by the way, I google it and found this is the simplest I could find)

View attachment 67646

My circuit is similar to the above except no switch, diode or PWM, just the choke, 3V battery and a 10uF capacitor.

To assist in measuring the voltage I add a dummy resistor load of 1k and measured the voltage drop but I can't see the voltage increases all I can see the voltage drop across the resistor as 2.6V.

I'm sure it's something that I did wrong, can anybody point me in the right direction.


Edit:

As there are 4 legs in the choke I connect leg 1 and 2 to the circuit

Thanks

A boost converter needs the switch to be turned on and off to work. Your circuit is just reducing the 3 V supply by the diode drop.

One solution is an IC such as a MAX771 and a suitable MOSFET.

How do expect the circuit to work when you remove the components that are essential to its operation? Did you think they were somehow not needed?

crutschow said:

Did you think they were somehow not needed?

Click to expand...

Yeah, my main intention was to just to see whether I can see the voltage double thinking (sorry newbie) that by using the choke it will increase. I was thinking the choke will directly put more volume and hence the capacitor will be charged with more volume than the source.

Looks like that was the wrong assumption ?

An inductor produces a voltage spike when its current is cutoff by the switch (or by an electronic switch). The diode feeds the high voltage spike to the capacitor and prevents the charge from being shorted to ground the next time the switch is turned on.

The inductance of your choke is small so the frequency of the switch must be pretty high.

Nick,

The choke you have, (good pictures) is used for filtering out noise in a switching power supply. (common mode filter)
It was not designed to store energy or be used in a switching power application.

ronsimpson said:

The choke you have, (good pictures) is used for filtering out noise in a switching power supply

Click to expand...

How do you differentiate between choke for inductor and choke for filtering (sorry I'm new in this). Googling around I found quite a lot of pictures like mine (with 4 legs) and also lots with 2 legs the only difference is the material that it is wrapper around with (some with toroid and some nothing).

Thanks

nicksydney said:

Yeah, my main intention was to just to see whether I can see the voltage double thinking (sorry newbie) that by using the choke it will increase. I was thinking the choke will directly put more volume and hence the capacitor will be charged with more volume than the source.

Looks like that was the wrong assumption ?

Click to expand...

Yes, it takes more than just an inductor and a capacitor to increase the voltage.

Here is the operating sequence of a boost converter:

1. Switch is closed and current starts to ramp up in the inductor with a ramp rate determined by the input voltage and the inductance. This current generates a value of energy stored in the inductor equal to 1/2 LI[SUP]2[/SUP] where L is the inductance and I is the current.

2. Before current reaches the saturation point of the inductor (which depends upon the inductor design) you open the switch.

3. The inductance (from the stored inductive energy) tries to keep the current moving through the inductor. With the switch open, the current path is now through the diode and into the capacitor.

4. After the current drops to zero and all the inductive energy has been transferred to the capacitor, the switch is again closed and the cycle repeats.

5. After multiple cycles the voltage on the capacitor can become arbitrarily higher than the input voltage.

Make sense?

crutschow said:

Make sense?

Click to expand...

Yes it make sense now, all this while I thought without switching it at least once I should be able to see the voltage goes up, but looks like its sequence of event that happen in succession, now I can get my head around it.


According to ronsimpson the choke that I have is used for filtering and not for storing engergy, how do I differentiate between the two ? any idea.

Cheers

nicksydney said:

Yes it make sense now, all this while I thought without switching it at least once I should be able to see the voltage goes up, but looks like its sequence of event that happen in succession, now I can get my head around it.


According to ronsimpson the choke that I have is used for filtering and not for storing engergy, how do I differentiate between the two ? any idea.

Click to expand...

Any choke will store energy but it may not be practical for boost converter use. It just depends upon the inductance, resistance, and saturation current of the choke, and your boost requirements. The choke information is generally provided by its manufacturer. It is not easy to measure except for resistance.

One physically observable item about common mode chokes to be used for AC line applications, is the presence of a mechanical insulation element separating the two windings. In this case, the white, ribbed plastic barrier between the two coils.

Here's a thread that ron helped me with. https://www.electro-tech-online.com/threads/14-to-40-or-50-volt-boost-circuit.124138/#post1026371
One of the easiest way to boost is with the LT1270 from Linear Technologies.
My explanation for how it works is you slowly charge an inductor, building up a magnetic field and then open the circuit the the field collapses very quickly and creates a much higher voltage for a much shorter time and you catch that high voltage with a diode and a capacitor, over and over and over again and you do this at a rate of around a 100 kilocycles, plus or minus a power of 10s or so

A very simple buck is the LM2576 shown in this **broken link removed**