How to correctly power an electronic device

[kavan]

Hello Forum,

Some electronic devices specify a certain, specific voltage that they need to work properly. Some other devices specify a specific current. Why don't they all specify the voltage or the current?

If the voltage is specified, the current will be automatically fixed by the internal impedance of the device. At the end of the day all devices require power, which is the product P=IV.

As a beginner, I am familiar with two types of sources: constant voltage sources and constant current sources. Regardless of the type of load and its impedance, a constant voltage source always applies the same voltage across the device. The constant current source, instead, makes the device draw the same amount of current, regardless of its impedance.

I am asking these questions because I have a small, basic personal project: power, using a small solar cell (max two connected in series) an electronic device like a smart phone.

a) I am not sure if the solar cell behaves more like a current source or voltage source.
b) The voltage from the solar cell(s) is usually much smaller that the electronic device required voltage. We can get, using a couple of cells (I don't want to use many) about 1 or 2V. A USB device I think needs 5V.
I guess I will need a DC-DC converter between the cell and the device....Is that all I need to trickle charge the phone, for example?

For a fixed solar illumination, I wonder how I can transfer the most power from the cell to the phone. I am familiar with impedance matching but I don't know if I need to worry about it and where in the circuit. So far I have the cells, the phone and I need to get the small DC-DC converter which I don't think does the impedance matching between the solar cell and the phone...

How should I look at this situation?

thanks,
Kavan

 

[ronv]

Here is some reading.

https://en.wikipedia.org/wiki/Maximum_power_point_tracking

 

[kavan]

Thank you Ron.

I did see that. This max point tracking device is needed because the solar illumination may change and we want to continue to transfer the max power to the device....

I am assuming the illumination is fixed, for now.

The mppt is not cheap either....

I have seen this small solar gadget online:

**broken link removed**

It does not have a mppt and it surely powers electronics


thanks
kavan

 

[ronv]

If you don't need maximum power from the solar panel then you can just use a boost regulator to supply 5 volts to your charger. The trick in your case is to find one with low enough input voltage like this one.

http://www.datasheetcatalog.org/datasheet/sipex/SP6641.pdf

If you had a little more voltage from your panels you could use some cheap ones from EBay.

I think there is also a circuit here posted by ()blivian that might work. You can search for it or he may come along with some help.

Specs on the panels would be needed to make sure it would work.

 

[kavan]

Thanks Ronv,

forgive my ignorance once again. We have a source (solar panel) and a device to power (smart phone). We surely want to send the max power to the device, otherwise it would seems like a waste... or not?

Imagine the sun gives the panel (based on weather condition and panel area) 100 W of power. The cell is 20% efficient so it only gets 20 W of electrical power can be squeezed out of those 100 W of solar radiation.

Our electronic device (phone) needs its own voltage and power to work ok. Imagine the needed voltage is 5V and the electrical power is 30W instead (which means a current of 6A "should" be drawn). We surely want to match the voltage and transfer allll those 20 W to the device, even if we are short of 10 W and we will probably only trickle charge the phone.

In essence, we match the voltage (5V) and want to assure max power transfer to be able to transfer all those 20 W available and nothing less..

So both a voltage booster and a max power transfer (impedance matching) network seem necessary, correct?
Otherwise we would only match the voltage but not transfer as much power as we could...

Best,
Kavan

 

[Nigel Goodwin]

Thanks Ronv,

forgive my ignorance once again. We have a source (solar panel) and a device to power (smart phone). We surely want to send the max power to the device, otherwise it would seems like a waste... or not?

Imagine the sun gives the panel (based on weather condition and panel area) 100 W of power. The cell is 20% efficient so it only gets 20 W of electrical power can be squeezed out of those 100 W of solar radiation.

Our electronic device (phone) needs its own voltage and power to work ok. Imagine the needed voltage is 5V and the electrical power is 30W instead (which means a current of 6A "should" be drawn). We surely want to match the voltage and transfer allll those 20 W to the device, even if we are short of 10 W and we will probably only trickle charge the phone.

In essence, we match the voltage (5V) and want to assure max power transfer to be able to transfer all those 20 W available and nothing less..

So both a voltage booster and a max power transfer (impedance matching) network seem necessary, correct?
Otherwise we would only match the voltage but not transfer as much power as we could...

It depends on how much you want to spend - bearing in mind you will lose energy in the electronics anyway.

It's probably far more cost effective to just have a larger solar panel than to use an MPT system.

 

[kavan]

Hi Nigel,

I see your point. But I wonder if this small gadget

**broken link removed**

has a MTP inside... I don't think so...what is the basic assumption on how this device works then?

thanks
kavan

 

[ronv]

It is not so much impedance mathing as it is mathing the load to the solar panel. They have a power curve like this:

So for a given amount of light there is a best place to run it to get the maximum power. To always know this point is a moving target depending on the amount of light hitting the panel. That is why maximum power point is expensive.
So usually what is done is a panel is selected to provide more than enough power for the load. In your case probably 0.5 amps at 5 volts.
Or the charger simply shuts off if there is not enough light.
And yes it wastes power, but it is easier.
To do what you suggest requires MPPT. It is best, but not easy.

 

[Nigel Goodwin]

Hi Nigel,

I see your point. But I wonder if this small gadget

**broken link removed**

has a MTP inside... I don't think so...what is the basic assumption on how this device works then?

Exactly as I said above - the panel is made big enough not to need MPT - you're greatly over thinking your project.

 

[MikeMl]

Solar Panels are more like a constant-current source if operated above the "knee" voltage.

 

[kavan]

Hi MikeMI,

So solar panels behave more like constant current sources, as we can see from the I-V curve that ronv provided.

Now, I am still slightly confused about this point: providing the correct voltage to the device and providing the most power possible. Once we provide the smart phone with the correct voltage by using a boost regulator that converts DC-DC, the phone should draw its own, needed and sufficient current, automatically...

Where does the max power transfer play a role then? Why do we need that or the MPT? It seems that the voltage booster would suffice....I am missing something

thanks
kavan

 

[ronv]

Suffice yes, maximum power no.


https://www.electro-tech-online.com...ower-an-electronic-device.132938/#post1109963

 

[audioguru]

The lithium battery in a cell phone is not supposed to be trickle charged so the charging circuit inside the phone will not do it.
The charging circuit might not work if the input current is too low. The charging circuit completely disconnects when it detects that the battery is fully charged.