Voltage limiter for solar charger

OK, I am very new to electronics and am just starting to learn the basics. I ordered a solar panel from eBay to charge my phone and ipod. It was a 6v 400ma panel and had a usb connector on it already, so you can just plug it in and off you go. That works fine but the phone needs +5v to start charging, and the ipod needs about +5.4v. But they will keep on charging until the voltage drops to about 4.5v.So in my office or outside on a cloudy day they do not start to charge but they would continue to charge once they started.

I thought that i could get a 7.5V+ solar panel which would have a better chance of having more than 5v in low level lighting conditions. The problem is that I don’t want to fry the phone.
Is there a way of limiting the voltage to 6v without regulating it?
I don’t care if it drops below 6 volts, just not above. I looked into zener diodes and switching regulators but they all seem to restrict the voltage or waste power.
I did read something about using a couple of transistors but I didn't really understand it.

Any Ideas?

hi,
Using a linear voltage reduction circuit usually means 'wasting' some power as heat.

For your case I would use a simple zener diode and resistor configured as a shunt regulator.

A 5.6V zener diode rated at about 3Watts, calculate the value of the series resistor for the current required by your phone/ipod.

Ok, but does that mean that the voltage will always be 5.6v? I understand that it will limit it to 5.6v but will it let any less than that through? does it have to get to atleast5.6v before it will let any electricity pass?
Also do I have to use a resistor? if I am happy for whatever ampage the solar panel produces to be passed through or does it have to be there?
what resistor would I have to use if I wanted it to be restricted to say 300ma? how would i work it out? sorry for all the questions but I have a lot to learn.

leefurness said:

Ok, but does that mean that the voltage will always be 5.6v?
No.

I understand that it will limit it to 5.6v but will it let any less than that through? does it have to get to atleast5.6v before it will let any electricity pass?
No.

Also do I have to use a resistor?
Yes.

if I am happy for whatever ampage the solar panel produces to be passed through or does it have to be there?
what resistor would I have to use if I wanted it to be restricted to say 300ma? how would i work it out? sorry for all the questions but I have a lot to learn.

Click to expand...

Use ohms law to calculate the resistor.

Rs= 7.5V - 5.6V = 1.9V drop across the resistor for a current of 300mA.

that gives approx 6Ω, rated at least 1W, I would use a 3Watt.

The 5.6V zener will pass 300mA without the phone.ipod connected, so thats

5.6V * 0.3A = 1.7W, so choose a 3Watt zener.

The voltage will be clamped by the zener to 5.6Vmax. For voltages less than 5.5V the zener will not conduct.

Connect the phone across the zener.

Perfect thanks. Just out of curiosity why do you have to use the resister? What would happen if you didn't use one?

Thanks again.

leefurness said:

Perfect thanks. Just out of curiosity why do you have to use the resister? What would happen if you didn't use one?

Thanks again.

Click to expand...

Over 5.6V the zener would effectively short out the solar panel.

So either the panel or zener would fail.

Whats the problem with fitting a series resistor.

a 6 volt 400mA solar panel would take ages to fully charge an ipod and mobile phone and provided you dont leave it unattended for days on end a simple blocking doide would probably do the job. as the Op doesn't say where he/she is from so unless he/she is right on the equator 250mA would about the max for any period the pv panel would produce. What a heap of people dont understand about PV panels when hooked to a battery the battery will regulate the voltage untill fully charged where the pv panel can be just short circuited. Eric's idea of a zener diode is probably the best idea to be totally safe.

Cheers Bryan

hi Bryan,

I think the OP is planning to use a 7.5Vdc, 400mA solar panel.

Yeah, that’s right. I do have a 6v panel which works fine with just a diode but I wanted to use a 7.5v or higher so I had more of a chance to hit the 5v threshold needed for the phone to start charging.

I don’t have any problem with fitting a resistor I just wondered Why I needed one.

I know that it will take ages to charge the phone or ipod but I plan to put the solar panel on my backpack and just have it charging whilst I’m walking about. I also thought it would be good for festivals. I also thought That I could put 2 panels together to double the amperage. As long as it doesn’t go over 500ma I should be fine as that is what the nokia charger outputs. I think regular PS USB slots are the same.

Thanks for everyone’s comments, this is really helpful.

Well guys on my nife bank that I run @16 volts I just use a 12 volt 80 watt PV panel and for the last 4 years that 1 panel has kept my 10 nifes on float 99% of the time. For a trial try just hooking up your 6 or 7.5 volt solar panel open circuit to a volt meter and put in full sun this will give you the open circuit voltage. Next connect the solar panel via a the dpm on current and put the neagtive to a battery for load. Put it out in the full sun again that will give your max current. After all thats done just do what Eric suggested and with a bit of maths you will know just how much charging power you can expect.

Hope this helps some

Cheers Bryan