The previous version of the Qwiic Quad Relay (COM-15102) specified a power supply from 7 and 15 volts and used an unspecified linear regulator . The current version (COM 16566) specifies 7 to 12 volts and uses a switching regulator based on an LMR14010A. The data sheet for the LMR14010A says it has an input range from 4 to 40 volts.
The application I have in mind would used a 12 V lead acid battery, which would be maintained by a solar panel. I could see voltages on the Vin rail being upwards of 14 or a bit more volts.
From reviewing the schematic for the Quad Relay, I can’t see or find any reason why it’s limited to only 12 volts. Does it have something to do with the size of the copper traces on the PCB? Are we really restricted to 12 volts? If so, why?
Thanks.
There’s a 10uF tantalum capacitor connected to the power jack that’s rated for 20 volts and to maintain a safety margin we decided to cap the max input at 12 volts. You’re technically supposed to be able to feed that cap 20V, but there’s a tolerance on caps where some are good above their max rating while others will fail below their max rating.
Since tantalum caps have a bad habit of[ bursting into fire when you exceed their maximum voltage it’s usually a good idea to de-rate them to prevent a mishap. We even did a [blog post about this a few years back that you might want to check out.
There’s a good chance you’d be OK at 14.1V but be aware bad things can happen if your specific cap is a bit out of tolerance and that anything over 12 volts would be at your risk.](https://www.sparkfun.com/news/1271)](https://www.youtube.com/watch?v=rVYGZ2hFUsw)
Well, that certainly answers the question of acceptable input voltages. My application isn’t critical or expensive and won’t endanger anything except some plants in a greenhouse.
But the next question then is what was the inducement to Sparkfun to put a tantalum cap in there in the first place? Real estate and circuit density couldn’t possibly be an issue on this board. The data sheet suggests using a low ESR ceramic cap on Vin. Seems like a simple ceramic would have been more than sufficient for this application and with a higher voltage ceramic you could have retained more of the capability/flexibility provided by the LMR14010A.
On this particular board there’s plenty of room for a different type of capacitor, but most of our boards are designed to be as small as practical. Rather than having to keep multiple types of the same value capacitor in stock, we standardized on what works for the largest number of our products.