In SX Microcontrollers, SX/B Compiler and SX-Key Tool, mebikerider wrote:

What you could do is lookup the schematic of your Tech board, and lookup the specs of the regulator itself, and check the voltage and current range specified by the regulator data sheet.

For example, on the SX48/52 board, even though the Parallax SX4852 pdf document specifies 6-9 VDC, the LM2940 voltage regulator is specified for up to 26 volts. So you can take that as a maximum voltage, as long as no other components are using the unregulated input.

Then look at how much current you're pulling through the regulator. First try to make sure you're not exceeding the specs of the regulator, 1 amp in the case of the LM2940. Then figure out whether the current will vary much. If your current load is steady, then using the series power resistor would be fine, as long as the voltage at the regulator input is high enough.

But if the current varies, and you're using a series power resistor, then you run the risk of occasionally pulling the voltage too low, which can lead to some really nasty program behavior where code seems to work, then suddenly it doesn't, then it does again. My hard-learned advice - avoid this situation at all costs, if you value your sanity.

If your total current needs are high, but within the specs of the regulator, and you really want to use 12V, then you might want to think about increasing the size of the heat sink. The regulator specs go in some detail about allowable temperature rise and heat flow to the sink, which may be more information than the average user really wants to know, but does make the point that a decent heat sink is needed to reach the 1 amp spec of the part. 

I usually just use the finger test - if the regulator is too hot to touch then I figure that something ought to be adjusted.

But I've been running my SX52 tech board from 12V (actually a 13.8 volt gelcell) for a long time with no problems.

David

---------- End of Message ----------


You can view the post on-line at:
http://forums.parallax.com/forums/default.aspx?f=7&p=1&m=118564#m118684

Need assistance? Send an email to the Forum Administrator at forumadmin@parallax.com

The Parallax Forums are powered by dotNetBB Forums, copyright 2002-2006 (http://www.dotNetBB.com)