Just put the shunt regulator directly across the solar panel. You can't use a series resistor. What is the series resistor going to do?
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Just put the shunt regulator directly across the solar panel. You can't use a series resistor. What is the series resistor going to do?
Do you ever read the preceeding posts? If you had, you might have learned that justwantin hooked a shunt regulator directly across his power supply and likely blew it up!Originally Posted by colin55
Mike ML.
Thanks for that I'll see what I can do given that info. I had a feeling the current out of my supply might be the problem but I didn't know enough to know why it would be though, or even what to google for.
I haven't thrown a resister at this yet. I've been digesting your reply. I've just about understand how you have come to your answer for resistor sizing but why max at .5W?The maximum power that a LM431 should dissipate is a 1/2W
There's probably a little variation between manufacturers but dissapation seems to be spec'd at better than .7W on the sheets I've looked at. In my case I have a National in a TO-92 package and max is .78W. Have you factored in a safety margin? and if so is there a general rule of thumb for such margins?
Thanks Mike,
I believe I have a handle on this now, at least off my benchtop supply, and have learned allot too. One last thing. would I place a diode between the tl431 and the solar panel or battery?
Yes, I put in a safety factor. At 0.78W, the TO-92 will be stinking hot.
With a shunt regulator, the regulator itself dissipates most when the load is the lightest. OTOH, the series resistor dissipation doesn't change as a function of load; only as a function of input voltage...
Mike ML.
If you want to prevent the battery discharging either into the resistors around the 431, or into the panel itself, then put a shottky diode between the reg and the battery, pointing at the positive pole on the battery. Set the regulator voltage ~0.5V higher than the battery chemistry dictates.
Mike ML.
Are you talking about something like a 1n5817? The system I was involved with had 1n4004 or 1n4001 diodes. I take it there is a difference and it matters.
Shottkys have a lower forward drop and have a sharper knee; Silicon diodes will work as long as you offset the voltage higher.
Mike ML.
Thanks again I have somewhere around .4V forward drop using a 1n4004 but will look at a schottky next time I'm in a shop as my stock of resistors leaves me with using a 1k and 1.8k to get around 7.2v before the diode. The breadboard circuit is charging off the 9v panel as I write. Almost time to move on to something else.
The tl431 is on the hot side of warm but everything seems fine. I am not using a resisitor to regulate Vin as was the case off my benchtop PS. I have noted in other discussion that people refer to a resisitor on the solar panel output. Is that standard for panels? Neither my 9V or 12V have anything but wires off the backside of the panels.
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