AlfredHitchcock Posted July 12, 2015 Report Share Posted July 12, 2015 Thanks Roger When you look at the adverts for these they sing the virtues of solid state reliability. You could therefore assume that the unit would already have these caps in place. Ive dissected mine and find its just a box containing a regulator and three wires soldered on - in a silicon gel. Anyway.......Ive ordered TI LM2940T and associated recommended input and output tantalum capacitors and I am going to try and find a small metal case to act as an heat sink to build it up - any recommendations? I dont want to butcher my old mechanical one. Also Ive had to order 5 lots so if anyone wants to build their own I will have 4 spare - which will be at cost around £3 to 4 + p&p. Nick Quote Link to post Share on other sites
stillp Posted July 12, 2015 Report Share Posted July 12, 2015 On the input (12V side) try a 47 or 100uf capacitor and on the output (10V side) use a 0.1uf. One end of each cap goes to the relative in/out terminal. The other end goes to earth. Isn't that the wrong way round Roger? The 0,1uF will have less inductance and hence be better at removing the HF noise on the input to the regulator. Pete Quote Link to post Share on other sites
RobH Posted July 12, 2015 Report Share Posted July 12, 2015 This is the recommended arrangement from the LM2940 data sheet: Rob Quote Link to post Share on other sites
stillp Posted July 12, 2015 Report Share Posted July 12, 2015 Hard to see from that Rob, but is that .47uF on the input? Quote Link to post Share on other sites
RobH Posted July 12, 2015 Report Share Posted July 12, 2015 (edited) Yes - 0.47uF on the input and a minimum of 22uF on the output. Edited July 12, 2015 by RobH Quote Link to post Share on other sites
Lebro Posted July 12, 2015 Report Share Posted July 12, 2015 Isn't that the wrong way round Roger? The 0,1uF will have less inductance and hence be better at removing the HF noise on the input to the regulator. Pete +1 Quote Link to post Share on other sites
AlanT Posted July 12, 2015 Report Share Posted July 12, 2015 One of the component types we religiously avoided on aircraft was Tantalum capacitors. 1. failure modes produce a small fire and toxic fumes 2. lifetime is low in high temperature environments 3. lifetime is lowish anyway These are more for throwaway IT things in offices etc. Very high capacitance is not required. I'd be using Mylar film and ceramic. You want an alloy enclosure, NOT steel - thermal conductivity is too low. Quote Link to post Share on other sites
AlfredHitchcock Posted July 12, 2015 Report Share Posted July 12, 2015 Alan the data sheet recommends tantalum caps due to their stable ESR over a wide range of temperatures. Apparently ESR increases 30x at low temps on cheap caps. Dont know about ceramic and mylar film. I'm thinking of cutting a cube size of aluminium square section tube and mounting the regulator on the inside with the leads exiting through a hole. Then glueing some end covers on. Quote Link to post Share on other sites
AlanT Posted July 12, 2015 Report Share Posted July 12, 2015 They say this as an alternative to normal wet electrolytics. Quote Link to post Share on other sites
Lebro Posted July 13, 2015 Report Share Posted July 13, 2015 Multilayer ceramic would be my choice - these cover a wide range of capacitance these days. Bob. Quote Link to post Share on other sites
stuart Posted July 13, 2015 Report Share Posted July 13, 2015 Or stop trying to re-invent the wheel and buy a s/hand original one. They last for donkeys years. Stuart. Quote Link to post Share on other sites
AlfredHitchcock Posted July 13, 2015 Report Share Posted July 13, 2015 I will have a look out for the ceramic and mylar ones then. I'm determined to get this built up and working. The original one is in the shed somewhere. Quote Link to post Share on other sites
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