Solar voltage limiter?

zmoz

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I want to buy a 6 volt solar panel for charging my cell phone while I'm camping. The problem is I know it can get up to over 8 volts in full sun...that would probably give me a nice cellular door stop. /ubbthreads/images/graemlins/wink.gif Is there any way for me to limit it to 6v max, but not take away any of the voltage when it is below 6v?
 

Brock

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Depending on the phone you probably don't need it. I know the Nokia 51xx/61xx phones say they need 3.7v, but the AC plug from Nokia is putting out 7.5v with no load. If you can check the voltage of the walwart charger and see how high it is. Unless the solar panel is rather large the voltage will drop from the load of charging the cell phone battery, and most phones have built in charge controllers, to a certain voltage.
 

Doug Owen

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As another poster has pointed out you may not actually have a problem.

Otherwise, you can easily put a zener diode of the voltage you wish to limit to across the output (suitable power diode of course). Be sure it's above the maximum voltage your charged battery might be or plan for lots of discharge current.......

Solar cells have fairly limited ability to produce current, this is a safe and easy 'out'.

Doug Owen
 

zmoz

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Hmmm...the charger that came with my phone (Sanyo SCP 8100) is rated at 5.4 volts. I have no idea if it will take anything higher or not, it hasn't been out very long so I don't think anyone has really tested yet. I don't really want to be the first to do so. /ubbthreads/images/graemlins/wink.gif If I put a diode in wouldn't that also drop the voltage when the panel is producing lower voltages? The panel I want to get is rated at 6v/50ma.
 

Doug Owen

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[ QUOTE ]
zmoz said:
If I put a diode in wouldn't that also drop the voltage when the panel is producing lower voltages? The panel I want to get is rated at 6v/50ma.

[/ QUOTE ]

Yup. It's subtract roughly .7 Volts *at all times*. Even when you don't want it. This probably means slower charging under all conditions.

OTOH, a 5.4 to say 5.6 Volt one watt Zener won't......

Your call.

Doug Owen
 

Brock

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can you put a voltmeter on the supply that came with it? I would be willing to bet it is over 7v, but if you can meter it, that would be best.
 

Evan

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If you are concerned about the drop across the blocking diode, use a shottky diode, maybe even a high power one even though you're not drawing much current; a shottky will only drop about .2 to .3 volt.
 

zmoz

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I measured it...almost exactly 6 volts...so under load I would guess it is very close to 5.4 volts.
 

Doug Owen

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[ QUOTE ]
Evan said:
If you are concerned about the drop across the blocking diode, use a shottky diode, maybe even a high power one even though you're not drawing much current; a shottky will only drop about .2 to .3 volt.

[/ QUOTE ]

I think you missed it, the goal was to reduce the voltage...

(you'd just need more diodes and would still be inferior to a zener in the application)

Doug Owen
 

zmoz

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While we're talking about this...do you think it is at all possible to damage the phone with under voltage?
 

FreeBSDboy

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Couldn't you use a zener doide reverse biased between the pos and neg etminals of your solar pannel to limit the voltage output to 5v? It would dissipate everything over 5V through the diode, so you'd need a decently heafty one.
 

Brock

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Under voltage won't hurt it, but it will most likely just stop charging. Another thought is to get the solar panel in full sun and connect it to the phone and watch the voltage. Also if it is a bit high you could leave the phone on drawing a bit more power, which will drop the voltage a bit more. Or in full sun put a piece of masking tape over part of the panel. If you play with it and watch a meter while your doing it you will figure out how much to block, if you need to block any.

How big is the solar panel or how many watt or mA is it rated at?
 

UK Owl

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You could always try a fixed voltage regulator.

(or possibly a vairable one such as LM317 with a few resistors added)
 

Doug Owen

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[ QUOTE ]
FreeBSDboy said:
Couldn't you use a zener doide reverse biased between the pos and neg etminals of your solar pannel to limit the voltage output to 5v? It would dissipate everything over 5V through the diode, so you'd need a decently heafty one.

[/ QUOTE ]

Yup, sure could, think I might have mentioned it a couple of times here......

And given the 6 Volt, 50 mA output (.3 Watt), "decently heafty" isn't a factor since the smallest one you can buy (.5 Watt) is big enough for the job. Typical ones (1 Watt) are 'plenty big'.

Doug Owen
 

Doug Owen

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[ QUOTE ]
UK Owl said:
You could always try a fixed voltage regulator.

(or possibly a vairable one such as LM317 with a few resistors added)

[/ QUOTE ]

Actually not. We don't have the voltage 'head room' to support this. LM317s need about 2.5 volts 'excess' to work.

Even low voltage drop out regulators are a poor choice against the simple zener solution.

Doug Owen
 

Entropy

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[ QUOTE ]
Doug Owen said:
[ QUOTE ]
UK Owl said:
You could always try a fixed voltage regulator.

(or possibly a vairable one such as LM317 with a few resistors added)

[/ QUOTE ]

Actually not. We don't have the voltage 'head room' to support this. LM317s need about 2.5 volts 'excess' to work.

Even low voltage drop out regulators are a poor choice against the simple zener solution.

Doug Owen

[/ QUOTE ]
What he may need is some sort of switching regulator. I believe NatSemi has some flyback controller chips that are designed to work in situations like this.
 

Doug Owen

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[ QUOTE ]
Entropy said:
[What he may need is some sort of switching regulator. I believe NatSemi has some flyback controller chips that are designed to work in situations like this.

[/ QUOTE ]

Not really. If you check, the original problem was worry that the solar panel would go overvoltage and kill the cell phone. Such regulators don't work well at reducing voltages.

I still think the zener's the call.

Doug Owen
 

Burnt_Retinas

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Fortunately, the easy fix is the common old "shunt regulator" used with small power solar regulation systems. In simple terms, it is as per Dougs posting - use a zener. There is no diode voltage drop so you'll get maximum voltage. Connect the zener across the panel's output leads. Remember - a zener is connected reversed biased ie the anode to the -ve of the panel and the cathnode (the end with the stripe) to the +ve of the solar panel. Connecting it backwards will short out the battery and burn up the zener.

The panel is rated at 50mA so this would only be 300mW dissapation maximum when the zener hits it's breakdown voltage at maximum panel current rating. I'd use a 1 watt diode though, rather than a 400mW one. They are cheap too. Make the voltage of the zener 6V which will not damage the battery. The power from such a low power solar array is simply not enough to damage the battery, nor will the battery allow the voltage to go above it's 'normal' charged voltage with such small currents as once a battery reached it's charged status the excess energy supplied goes up as heat within the battery. Once again 300mW really is insignificant.

You could as is also stated do nothing, probably stated due to the low power of the solar panel, but I'l prefer to err on the side of caution anyway for the cents it will cost for a zener.

Chris
 

zmoz

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[ QUOTE ]
Burnt_Retinas said:
Fortunately, the easy fix is the common old "shunt regulator" used with small power solar regulation systems. In simple terms, it is as per Dougs posting - use a zener. There is no diode voltage drop so you'll get maximum voltage. Connect the zener across the panel's output leads. Remember - a zener is connected reversed biased ie the anode to the -ve of the panel and the cathnode (the end with the stripe) to the +ve of the solar panel. Connecting it backwards will short out the battery and burn up the zener.

The panel is rated at 50mA so this would only be 300mW dissapation maximum when the zener hits it's breakdown voltage at maximum panel current rating. I'd use a 1 watt diode though, rather than a 400mW one. They are cheap too. Make the voltage of the zener 6V which will not damage the battery. The power from such a low power solar array is simply not enough to damage the battery, nor will the battery allow the voltage to go above it's 'normal' charged voltage with such small currents as once a battery reached it's charged status the excess energy supplied goes up as heat within the battery. Once again 300mW really is insignificant.

You could as is also stated do nothing, probably stated due to the low power of the solar panel, but I'l prefer to err on the side of caution anyway for the cents it will cost for a zener.

Chris

[/ QUOTE ]

Oh...I must have missed something along the way here. So if I put in a 6v zener diode, that will limit it to 6v max, but will not drop the voltage at lower voltages?

I definately don't want to just use nothing...$200 phone. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/icon15.gif
 
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