Device to control current going into individual electronics

[jflorey2]

and PV panels are current sources.

I keep hearing this and there is some truth to it - but it's also somewhat misleading.

A current source (like a voltage source) is a mythical source in electrical engineering, similar to a frictionless plane or a massless rope. A current source delivers 1 amp no matter what, and will go to an infinite voltage to support that current. Likewise, a voltage source will always provide 1 (or whatever) volts no matter what, even if it has to source an infinite amount of current.

In the real world things can approximate a current or voltage source. For example, a battery is pretty close to a voltage source - it will give you 12 volts up to a lot of amps. But even so voltage will decline as currents increase, and there will generally come a point where the current drawn will cause something to melt or the battery to boil. Within limits, though, they come close to a voltage source.

Certain kinds of transformers will approximate current sources (like fluorescent tube ballasts.) They will go to very high voltages to keep supplying their ~300ma to the tube.

PV panels are both. Left of the current "knee" they are close to ideal voltage sources. To the right of the current "knee" they are decent current sources (although they are better voltage than current sources.) The reason that DC-DC converters often fail with PV panels is that that knee moves around based on insolation. An MPPT charge converter is great at 'tracking' that knee, which is where the most power is. But a plain old DC/DC converter doesn't know where the knee is. So it may work fine when you are getting full sun. The knee is at 5 amps and you are drawing 2 amps because you need 24 watts. But in the morning, when the knee is at 1 amp, the DC/DC will still try to draw 2 - and will thus move the operating point to the side of the VI curve where it looks like a current source. And the DC/DC will try to draw more and more current, thus keeping the panel on the far right of its curve. In this mode, you'd see the DC/DC converter drawing 2 amps but the panel's output might only be 4 volts - and you'd only be getting 8 watts. This can continue even later in the day when you get full insolation. And you'd be puzzled as to why this panel is getting full sun but you're only getting 8 watts out of it.

[jflorey2]

This one in particular is in the range of DC 4-38V, max output current is 5A, max output power is 75W.
By the range of "4-38V", that means it can take IN 38V max?

Yes.

If so... It means I can connect the panel to the INPUT ports?(Because panel's voltage is way less than 38V)
Then on the OUTPUT is where the pump/etc would go, 12V/etc.

Yes - PROVIDED the converter does not collapse the input (i.e. draw more current than the panel can deliver.) That can often be a problem.

[Mike90250]

You will not be likely to replace a battery with a cap, and most DC-Dc converters are likely to fail with a PV panel, because they are used to working with a Voltage source, and PV panels are current sources.
Believe me, a zener diode on a big chunk of heatsink, is going to likely be the simplest way to get 12V for phone charging.

[neweclipse]

I'm trying to avoid batteries. I don't need them just yet.

You could use any junk battery with a charge controller, like a old car battery, since you are good with no phone charging after sun hours anyways...

[Soleadus]

You could use a zener diode on a proper sized heat sink, as a crude voltage regulator. Use a diode with a wattage rating slightly lower than your panel wattage.

a 1N3313B comes to mind, about $10 if you shop around, and also need heat sink too

First time in my life that I see a Zener Diode...
I got some learning to do!

[Soleadus]

What you desire is a Charge Controller...it will maintain stable usable voltage just as a battery would, but needs a battery to function.
Excess DC Current will not be the uncontrolled issue, it will be uncontrolled or too high of DC voltage from straight out of PV panel that gives the issue for what you want.
Control or manage the peak DC voltage output and any and all currents will take care of themself's up to the full capacity of production available and battery size/capacity.

I'm trying to avoid batteries. I don't need them just yet.

[Soleadus]

12V pumps - you can regulate current taken by reducing voltage. You need a variable DC-DC for this.
Phones - most phones will reduce charge rates based on voltage drop at the input. Again, you need a variable DC-DC for this.

In all cases you will need a regulator so 12V devices don't see the 18V from the panel. Most 12V devices cannot handle more than about 15 volts.

In many cases you can get by with a large capacitor acting like a battery. Devices that cannot be regulated can be turned on and off to keep the voltage within (for example) 11-16 volts. Laptops, for example, cannot easily be regulated - their chargers are either on or off. By turning on at 16V and off at 11V you can do a pseudo-regulation of power. Choose a capacitor so that you get a reasonable time (say 30 seconds) of charging during this sort of operation.

Thanks for the reply!


I just saw some videos about replacing, or using, capacitors instead of batteries.
I will explore this in depth when I have the time.



So basically, regarding regulators, variable DC-DC, something like this.
This one in particular is in the range of DC 4-38V, max output current is 5A, max output power is 75W.
By the range of "4-38V", that means it can take IN 38V max?
If so... It means I can connect the panel to the INPUT ports?(Because panel's voltage is way less than 38V)
Then on the OUTPUT is where the pump/etc would go, 12V/etc.
regulator.PNG

[Mike90250]

You could use a zener diode on a proper sized heat sink, as a crude voltage regulator. Use a diode with a wattage rating slightly lower than your panel wattage.

a 1N3313B comes to mind, about $10 if you shop around, and also need heat sink too

[neweclipse]

Thanks, Bruce

I just don't want to fry a motor because it's attached to a too big of a panel on a sunny day.

That's another thing I should mention. I don't want to limit myself to smaller panels, I want to buy a 100w panel that I can later incorporate into a bigger set up.

What you desire is a Charge Controller...it will maintain stable usable voltage just as a battery would, but needs a battery to function.
Excess DC Current will not be the uncontrolled issue, it will be uncontrolled or too high of DC voltage from straight out of PV panel that gives the issue for what you want.
Control or manage the peak DC voltage output and any and all currents will take care of themself's up to the full capacity of production available and battery size/capacity.

[jflorey2]

Is there a single device I can connect to the wires coming from panels and specify, like dial in, set, the current I can supply to other devices?
For example, panel --> "device" --> phone charging, 12v mini water pump, and other small devices.

12V pumps - you can regulate current taken by reducing voltage. You need a variable DC-DC for this.
Phones - most phones will reduce charge rates based on voltage drop at the input. Again, you need a variable DC-DC for this.

In all cases you will need a regulator so 12V devices don't see the 18V from the panel. Most 12V devices cannot handle more than about 15 volts.

In many cases you can get by with a large capacitor acting like a battery. Devices that cannot be regulated can be turned on and off to keep the voltage within (for example) 11-16 volts. Laptops, for example, cannot easily be regulated - their chargers are either on or off. By turning on at 16V and off at 11V you can do a pseudo-regulation of power. Choose a capacitor so that you get a reasonable time (say 30 seconds) of charging during this sort of operation.

[Soleadus]

This can work well with devices that contain their own battery, or are tolerant of varying power such
as a DC motor. Bruce Roe

Thanks, Bruce

I just don't want to fry a motor because it's attached to a too big of a panel on a sunny day.

That's another thing I should mention. I don't want to limit myself to smaller panels, I want to buy a 100w panel that I can later incorporate into a bigger set up.

[bcroe]

This can work well with devices that contain their own battery, or are tolerant of varying power such
as a DC motor. Bruce Roe

[Soleadus]

Greetings from infernally hot, yet conveniently sunny, Puerto Rico!

Is there a single device I can connect to the wires coming from panels and specify, like dial in, set, the current I can supply to other devices?
For example, panel --> "device" --> phone charging, 12v mini water pump, and other small devices.

My goal is to not have to use battery storage yet not wire panels directly to electronic devices, keeping the current within a manageable range, nothing big in terms of output.
I understand I am limited by sun hours, that's fine for my needs.

So far from the little I know about this tech I suspect a charge controller might be used as that "device". Or is such a thing done using several components?