I want to supply power to a device that runs on 12VDC and needs about 2 Amps. Preferably it will be fed by a 24V (solar) battery. But when the battery voltage drops under e.g. 25V, it should automatically switch to a PSU 240AC-> 24VDC. The detection of the battery voltage is done by a µcomp. (ESP32) It switches a relay (double pole) ON when the battery voltage is above 26.4 V (battery full) and switches the relay OFF when the voltage drops to 25V so that the PSU takes over.
Important is that my device needs constantly 12V. So I came up with following diagram :
View attachment 271917
When the voltage rises above 26.4V, the relay comes ON, and the 240VAC falls off. The 24 -> 12V converter gets its power from the battery. Works perfect. The capacitor is large enough to maintain the feed to the converter while the relay is switching. (As long the converter gets more than let's say 13 or 14 Volts, it will deliver the 12V at its output.
When the voltage goes below 25V, the relay falls off and switches on the 240VAC over its NC contact. I would suppose that the capacitor AGAIN would maintain the 24V to the converter, but it doesn't... I know that it takes some time before the PSU reaches 24V, but the capacitor together with the input capacitor in the 24->12 converter should keep up at least 12V. The voltage over the capacitor drops to +/- 6VDC and slowly builds up to 24V. (Although it WAS charged to 25 to 29V by the battery) My device is without power during approx. 1 second. And that is unacceptable... No matter how big I make the capacitor, I always get the voltage drop... (If the capacitor gets too big, the PSU cannot charge it at all, although the capacitor is not "empty" : 25V ...)
Where is my mistake ? Aren't the diodes supposed to keep the stored power in the capacitor from going in the wrong direction ?
Thanks in advance for your advice !!!
Important is that my device needs constantly 12V. So I came up with following diagram :
View attachment 271917
When the voltage rises above 26.4V, the relay comes ON, and the 240VAC falls off. The 24 -> 12V converter gets its power from the battery. Works perfect. The capacitor is large enough to maintain the feed to the converter while the relay is switching. (As long the converter gets more than let's say 13 or 14 Volts, it will deliver the 12V at its output.
When the voltage goes below 25V, the relay falls off and switches on the 240VAC over its NC contact. I would suppose that the capacitor AGAIN would maintain the 24V to the converter, but it doesn't... I know that it takes some time before the PSU reaches 24V, but the capacitor together with the input capacitor in the 24->12 converter should keep up at least 12V. The voltage over the capacitor drops to +/- 6VDC and slowly builds up to 24V. (Although it WAS charged to 25 to 29V by the battery) My device is without power during approx. 1 second. And that is unacceptable... No matter how big I make the capacitor, I always get the voltage drop... (If the capacitor gets too big, the PSU cannot charge it at all, although the capacitor is not "empty" : 25V ...)
Where is my mistake ? Aren't the diodes supposed to keep the stored power in the capacitor from going in the wrong direction ?
Thanks in advance for your advice !!!
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