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Solar Power Supply Question

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pmolsen

New Member
I have a circuit that I want to be able to power from either solar or from a regulated power supply. The circuit runs an LED based light that draws 1A.

For solar I am using a solar panel with an M083 solar controller. The controller will charge the battery up to 13.8V.

I want to limit the light input to 12V so as not to damage the LED's, but I want it to keep working even if the battery voltage drops below 12V.

I did some experimenting with a 7812 and a 9v battery. Without the 7812 I get a reading of 8.1v across the battery under load and the light is still quite bright. When I insert the 7812 I am only seeing about 6.8V across the output under load and the light is very dim.

I don't think I can use a zener due to the current drawn.

So how do I limit the voltage to 12V without losing so much when the voltage goes below 12V?
 
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premkumar9

Member
I have a circuit that I want to be able to power from either solar or from a regulated power supply. The circuit runs an LED based light that draws 1A.

For solar I am using a solar panel with an M083 solar controller. The controller will charge the battery up to 13.8V.

I want to limit the light input to 12V so as not to damage the LED's, but I want it to keep working even if the battery voltage drops below 12V.

I did some experimenting with a 7812 and a 9v battery. Without the 7812 I get a reading of 8.1v across the battery under load and the light is still quite bright. When I insert the 7812 I am only seeing about 6.8V across the output under load and the light is very dim.

I don't think I can use a zener due to the current drawn.

So how do I limit the voltage to 12V without losing so much when the voltage goes below 12V?
In your case you want to get 12V 1A when the supply voltage from battery varies from less than 12V to 13.8V. One solution may be to use a DC-DC converter meeting the above requirement.
 

MikeMl

Well-Known Member
Most Helpful Member
You need a Buck-Boost Switching Regulator. Go to this link, put in your numbers.
 

pmolsen

New Member
Had a look at the buck boosters. Afraid I am after something a tiny bit simpler than that. The LTC3780 for example requires the 24-pin buck booster IC plus over 20 discrete external components according to the sample diagram.
 

MikeMl

Well-Known Member
Most Helpful Member
Ok, try SEPIC switch mode powersupply
 

colin55

Well-Known Member
If you are using a 12v - 12watt LED light, it has its own internal buck regulator and will allow for 11v to 13v supply.
 

pmolsen

New Member
I am using a light that has 180 LEDs, in series strings of 3 with a 295 ohm resistor on each string. (Yep, that's the actual R value). Total measured current is right on 1 amp. It has no electronics on it at all other than that.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Presumably it's designed to run from a car battery?, the nominal voltage of which is 13.8V - I don't see as you have a problem?.

If you're really bothered, then stick a couple of 3A rectifiers in series between the battery and the light.
 

Sceadwian

Banned
If you want simple and don't mind a bit of wasted power just use a 2 ohm 5 watt power resistor or a couple of rectifier diodes in series. If you're LED's are drawing 12 watts I think the 2 watts a 2 ohm resistor would burn is relatively reasonable, a switch mode supply 80% efficient is only going to be the slightest smidgen more efficient.
 

premkumar9

Member
Presumably it's designed to run from a car battery?, the nominal voltage of which is 13.8V - I don't see as you have a problem?.

If you're really bothered, then stick a couple of 3A rectifiers in series between the battery and the light.
The OP says that the input varies from <12V to 13.8V. If he use 2 diodes to drop the voltage, then at low input the current through LEDs (which has got 300 Ω series resistor with each 3 LEDs) may not be sufficient to give the required light output. Using additional series resistors also will give the same problem. Right?
 

MikeMl

Well-Known Member
Most Helpful Member
If it is all about efficency, why not simply add a LED or two to the string when the input voltage is high, and then switch out the extra LEDs as the voltage sags. Sure, you loose a few lumens, but the remaining LEDs still get their rated current. You could automate this using some voltage comparitors.
 

Sceadwian

Banned
Re-arrange the LED array to use a constant current source instead of a resistor, it will work at any voltage within the compliance of the constant current source. It will be especially effective at lower voltages when a fixed resistor is just sapping power.
 
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premkumar9

Member
Re-arrange the LED array to use a constant current source instead of a resistor, it will work at any voltage within the compliance of the constant current source. It will be especially effective at lower voltages when a fixed resistor is just sapping power.
Can you pls suggest a practical circuit which will supply a constant current to the LEDs with an input from 11.5V to 13.8V?
 

premkumar9

Member

Sceadwian

Banned
You might need to use a rail to rail opamp to regulate close to the +VCC rail but the + voltage is only limited to the opamp and mosfets voltage requirements.
 

Sceadwian

Banned
It depends on the voltage the mosfet gate needs to fully turn on at the required current. If it's used as a low side driver and VCC to the opamp and to the load are the same it would never be a problem I was just being paranoid. If the voltage the FET is switching is much higher than the opamp is supplied with at high currents the mosfet may require more voltage than the mosfet can deliver, but that's rare for a low side driver.
 
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pmolsen

New Member
Hmmm. Methinks me just gonna leave it like it is and hope for the best. The last thing I want is lots of extra electronics. Another brand of light that I was using had that. It ran the LEDs at 8V and had an inverter circuit that would accept anywhere from 9V to 24V. Problem is within a week I had 2 of them fail completely. That shouldn't happen when driving the LED's direct in parallel since there are no common components that can fail. I might lose a few strings due to the 13.8v stressing them a bit too much. Will just suck it and see.
 

Sceadwian

Banned
Pick resistors for 13.8 volts and suck up the dimming that's going to occur down to the lower voltage limit. Then nothing will fry ever and you'll just get a slight dimming at the lower voltage.
 
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