Improve this solar controller?

[EccentricDyslexic]

I built this charge controller,

**broken link removed**

the problem is the leds light drawing 30ma all of the time, which makes the circuit useless. Can it be improved with a better comparator?

Cheers

Steve

 

[Nigel Goodwin]

The LED's certainly shouldn't light all the time, if they do you're done something wrong.

One dead obvious 'improvement' though would be to fit the missing resistor between base and emitter of the transistor - this should cure the slight leakage though the LED's, and should have been fitted originally (try a 10K or something, lower if need be).

 

[EccentricDyslexic]

Thanks for the quick reply.

The 8 leds i use light up all of the time using 30ma, but when the supply V goes above 14.2 they dramatically improve to full brightness 130ma or so.

i will try the 10k and let you know how i get on.

Thanks!

Steve

 

[EccentricDyslexic]

I tried the 10k and it dont make much diffecence. wont doing this tie the base up rather than down though?

 

[Nigel Goodwin]

I tried the 10k and it dont make much diffecence. wont doing this tie the base up rather than down though?

It's to prevent any slight leakage turning the transistor ON - and is a standard component.

Have you checked the voltages around the TL341?, and the voltage on the battery - by fitting a variable resistor in series with R1 you could make the switching point adjustable.

 

[Cicero]

I have a problem with the whole concept of this 'charge controller' and his explanation. His whole point was to limit the charging current by diverting excess current away from the battery if needed. However, a battery will pull the most current when its discharged, and he only diverts current when its voltage exceeds 14.2V. Surely he needs to do the opposite?

It'll probably slow down its charging once it reaches 14.2V a bit, but it doesn't limit the max charging current by any means.

Or am I completely missing something?

EDIT: It wont discharge the battery back to 14.2V, missed the diode. Doh!

 

[Nigel Goodwin]

Or am I completely missing something?

EDIT: It wont discharge the battery back to 14.2V, missed the diode. Doh!

Yep

It's a simple circuit to prevent overcharging - once the battery is fully charged an amount of further charge is diverted to the LED's as a crude dummy load.

 

[MikeMl]

Like a lot of the circuits posted to the internet, it has a serious design flaw. How do I know? I recently made the same mistake as discussed on this long running thread on another forum. I have used TL431s for all kinds of things involving regulation and voltage comparison.

The circuit is just a shunt regulator, which prevents overcharging the battery after the battery voltage rises to ~14.2V. The LEDs are purely incidental to the operation of the charge regulator. It would clamp the voltage across the battery to the voltage determined by the R1/R2 voltage divider regardless of what is connected between the collector of T1 and ground. You could connect a power resistor, lamp, or nothing at all... If you dissipate the panel power in just T1, then it would have to be on a heatsink.

The glaring error that the author made is that even when the TL431 cathode pin switches to the high state (when the voltage at the Ref pin < 2.495V), the current into the cathode pin is still about 400uA, which is more than enough to partially turn on T1. The solution is to add a 1K resistor from the base of T1 to its emitter (Panel +).

This circuit (with the fix above) will regulate the battery voltage within a few mV. The open loop gain of the comparator inside the TL431 is about 1000.

 

[Nigel Goodwin]

Like a lot of the circuits posted to the internet, it has a serious design flaw. How do I know? I recently made the same mistake as discussed on this long running thread on another forum.

The circuit is just a shunt regulator, which prevents overcharging the battery after the battery voltage rises to ~14.2V. The LEDs are purely incidental to the operation of the charge regulator. It would clamp the voltage across the battery to the voltage determined by the R1/R2 voltage divider regardless of what is connected between the collector of T1 and ground. You could connect a power resistor, lamp, or nothing at all...

The glaring error that the author made is that even when the TL431 cathode pin switches to the high state (when the voltage at the Ref pin < 2.495V), the current into the cathode pin is still about 400uA, which is more than enough to partially turn on T1. The solution is to add a 1K resistor from the base of T1 to its emitter (Panel +).

See post #2

 

[MikeMl]

See post #2

But 10K is an order of magnitude too high. I recommend 1K, and understand why it needs to be there, and took the time to explain to the TS what is going on.

 

[ronsimpson]

The TL431 needs some C-ground current for it to work. Data sheet said 0.5mA typ and 0.7mA max. (depends on who made the part and the temperature)
I think we need to think 1mA of current is flowing and that is enough to turn on the transistor.
If R3=1k then I think you need a 1k from B to E of T1. This should shunt 0.7mA around the transistor and keep it off when the batteries are low.
---------------edit----------------
Nigel said this in post #2.

 

[Cicero]

Yep

It's a simple circuit to prevent overcharging - once the battery is fully charged an amount of further charge is diverted to the LED's as a crude dummy load.

Well yes and no, my point was that the explanation is all about limiting the current to 180mA because he says his panel can give 325mA. When in fact it does no such current limiting at all.

It limits the battery voltage perhaps, but very crudely and only by overloading the solar panel once the battery reaches 14.2V.

What is the max voltage a 12V solar panel can produce? Does it grossly exceed 14.2V?

 

[ronsimpson]

1) He is not happy about 30mA all the time. Fix with a resistor.
2) It is a shunt regulator. It does not directly limit current on its own. It diverts current away form the battery. Indirectly limits current.

 

[EccentricDyslexic]

What is the max voltage a 12V solar panel can produce? Does it grossly exceed 14.2V?

17.9v

 

[MikeMl]

Here is a simulation of a simplified TL431 shunt regulator circuit. I simplified the circuit to show only the essential components. The solar panel is simply modeled as a current source. The battery is simply modeled as a huge capacitor. Time scale is arbitrary. The voltage divider R1/R2 is set up to clamp the battery voltage at 14.20V.

The green trace is the battery voltage V(plus). I am showing what happens as the battery charges from 14.00V toward 14.20V. Note that it takes only a 3mV change for the regulator to switch on, which begins happening when V(plus) first reaches 14.198V.

The red trace is the power dissipation in Q1; essentially all of the panel output power goes into Q1 once the battery voltage reaches 14.201V. Note that Q1 would have to be on heat sink...

Also note that R3 has been selected to guarantee that Q1 is off before V(plus) reaches 14.198V. 1K would work, but 680 gives a better margin.

Hope this clarifies how the TL431 shunt regulator is supposed to work...

 

[EccentricDyslexic]

Thanks for the simplification! what is value of r2? and will a small heat sink (clip on) do the job? i like the idea of no leds.

cheers steve

 

[EccentricDyslexic]

what is the current consumption when not conducting?

gotta love LT! must get more practice on it.

 

[MikeMl]

what is value of r2?

21661Ω. I calculated it to make exactly 14.20V out. 14.20* R2/(R1+R2) = 2.495V. To do this for real, use a trimpot or put two 1% resistors in series to get close to that non-standard value...

and will a small heat sink (clip on) do the job? i like the idea of no leds.

Try it. Spit on your finger and touch it. If it sizzles, it is too hot.

what is the current consumption when not conducting?

A few hundred uA, mostly through R1/R2 and a bit into the cathode of the TL431. I increased R1 and R2 to waste less power.

 

[dougy83]

and will a small heat sink (clip on) do the job? i like the idea of no leds.

If the panel provides 300mA, the transistor will dissipate ~14.2V*0.3A=4.26W. If you're happy with a 40 degree C rise in temperature of the transistor, then use a heatsink with a thermal resistance rating of better than 6.25 º/W (transistor has 3.125 º/W junction-ambient). Mike's spit method applies if you don't know the heatsink specs.

You may wish to connect another resistor between the TL431 collector and the TIP32C base; I don't think the TL431 has any over-current or over-temperature protection built in.
EDIT: resistor would only be needed if the TIP32C isn't pulling down the rail sufficiently... i.e. it would be, so it's not needed.

 

[EccentricDyslexic]

Thanks! I notice you are using a TIP32c not a TIP30c, are they interchangeable? i only have tip30c handy.

Cheers!

Steve