# Help with TL431

Discussion in 'General Electronics Chat' started by spinnaker, Feb 15, 2010.

1. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I am just full of questions today and getting a lot of great answers.

I am looking into using a TL431 shunt regulator as the regulator for my solar panel. The panel puts out around 25 VDC at around .6 amps max and I need to regulate that down to 13.6 and 14.5.

I figure I could use the PIC in my project to select the proper voltage at the appropriate time.

I found this calculator on line. They don't allow you to play with R2. That is the one that is calculated.

Also they don't suggest a value for R. What value should I use? What wattage?

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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The power rating for a TL431 is only ~0.7W (TO-92). To regulate your panel, the power dissipation of a shunt regulator could be as high as (12*0.6)=7.2W, so you obviously cannot use the LM431 directly. You can couple it with a power transistor or Power FET to create a compound circuit. See page 8 of the LM431 datasheet for a suggested circuit.

3. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Thanks.

Wouldn't I be trying to fit a square peg into a round hole?

Is there an easier option than using the TL431?

Do I even need to worry about it I just did an experiment and hooked up my power supply to a current limiting resistor. I managed a little better than .4 amps with the resistor that I had on hand. I jacked the voltage up to around 24V and the voltage across the battery never exceeded the battery voltage.

If I went with that circuit on page 8, what would the value of the unmarked resistor be? What about the wattage? Would it need to be 7 watts?

Last edited: Feb 16, 2010
4. ### MikeML AAC Fanatic!

Oct 2, 2009
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Well, that just proves you need a regulator. Think of your panel as a constant-current source of ~0.6A all the time it is in direct sunlight. If the battery is discharged, then a charging current of 0.6A is all going to charge it. However, as the battery voltage approaches 13.7 to 13.9V, you want the shunt regulator to cut-in, and to divert the 0.6A to ground. If you let the current keep flowing into the battery, you are destroying it by drying out the electrolyte.

Here is a circuit/sim of an LM431 shunt regulator. It is adjustable. Goal is to have the battery voltage climb to 13.8V and no higher. Note how the current into the simulated battery drops to zero as the battery voltage reaches the set point, and is shunted to ground. The simulation is at five different positions of the pot wiper.

The PFet is dissipating ~7W once the battery is fully charged, so it will have to be on a heatsink.

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5. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Hey thanks much for the circuit. That does not look too bad to build.

What kind of wattage rating are we talking about on those resistors? What kind of tolerance for R2 and R3? 2%?

Can I use a digital pot or won't they be able to handle it? Or is that what you have in mind (you labeled it U2)

BTW that is .6A at open circuit.

But I might be upgrading the panel and or the battery at a later point anyway so a regulator is probably the way to go.

Last edited: Feb 16, 2010
6. ### MikeML AAC Fanatic!

Oct 2, 2009
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The three discrete resistors can be 1/4W. Since the pot trims it to the final voltage, they could be 5%.

When the battery voltage is being regulated, the voltage at the Ref input of the LM431 is 2.50V +- a few mV. If you can set up a digital pot to sample the Battery voltage and scale it to 2.5V under digital control, then you could control the set point with a digital value. I would still use fixed resistors external to the digital pot so that the output voltage is constrained at ~13.7V +-ΔV, where ΔV is like 1 V.

The U designator for the pot is because it is modeled as a Spice "subcircuit"

7. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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I was not two worried about the 3 resistors. I was thinking the 1M in series with that cap but I can see it probably does not draw all that much.

My original plan was to use fixed resistors. I can just have a a couple of 2n2222s to select the appropriate voltages?

My battery says 13.6-13.8 V for standby and 14.5 - 14.9 cyclic.

Should I just charge it at the standby voltage?

With the regulator, do I need to have a method for disconnecting the solar panel or just let it continue to charge?

8. ### MikeML AAC Fanatic!

Oct 2, 2009
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The 1mΩ (0.001 Ohm) resistor in series with the 50 FARAD capacitor is how I modeled the lead acid battery in Spice. You do not build the part inside the box, anymore than you would build the solar panel.

I would set the regulator adjustment for 13.7V, which is the correct float voltage for 99% of SLA batteries.

The REASON for having this regulator is so you do not HAVE TO DISCONNECT the panel from the battery.

9. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Wow that is great news thanks. It will make things easier.

Would I get a faster charge, if I charged at 14.5 until the battery was charged then have the PIC switch to float?

Or just stick with 13.8? It would make it much more simple. That way the PIC would not even need to be involved with the charging process. It's only job then would be to turn the light on when needed.

10. ### MikeML AAC Fanatic!

Oct 2, 2009
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If the goal is to charge your battery is the shortest possible time, without damaging it, you have to implement a complex charging algorithm in your PIC. If you just want to get it charged easily without coming up a complex learning curve, and avoid damaging the battery, then just voltage limit (as shown in the circuit I posted) at 13.7V and call it good.

11. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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Thanks

I think maybe rev 1 will be the circuit that you provided fairly easy and it will get this project off the ground.

Will see how it goes after that. If the battery is still not charging satisfactorily then I can always enhance the design. Plus at that time I will have a bit more experience.

Thanks for all of the help!!

12. ### spinnaker Thread Starter AAC Fanatic!

Oct 29, 2009
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This project is now getting closer to being built.

In looking at this circuit, to looks to me like I will need a blocking diode?

I guess I should use a Schottky diode?

On hand I have a 1n5820, 1n5818 and a 1n5817

Looks like the 817 has the lowest forward voltage.

Should this work for me? Or should I pick up something else?