Relay Shield actual input voltage

The Photon Relay Shield silk screen claims 7 to 20 Vdc as its input. However the RT8259 has a recommended rating for as much as 24 Volts. The input capacitors on the shield schematic appear to be rated for 50 Volts. Is there a reason other than conservatism or perhaps legacy values for the limit of 20 Volts?

1 Like


Let me see if I can ping someone that might know the answer. @rickkas7 do you happen to know this?


I do not know the answer. Indeed the RT8259 is rated up to 24V and the question hasn’t come up before of why the data sheet has a maximum of 20V. I’ll look into it.

Without reading the datsheets etc you can make some educated guesses.
Headroom - its always a good idea not to operate at the very top end of somethings operating range, a crappy 24V supply could easily be putting out 28V or more when only lightly loaded.
Thermals - the regulator is 80% efficient the other 20% is lost as heat, you should some info on that in the regulator datasheet along with some formulas etc.The RT8259 is of course way more efficient than something like a 7805 so it doesn’t get as hot quite so easily as something like that.

1 Like


7-20VDC input voltage is a conservative number. You can indeed go all the way up to 24VDC if you know your power supply well. We have found out that the regulator is sensitive to ripple on the input line and large capacitance introduced by long power cables.