Good evening. I deal with n scale and have several sound equipped locos with Loksound I've installed. Most I've used 16v 220uf but recently I've seen people use 20v caps. Question is, should I be using 20v instead of 16v caps? Sent from my LG-H918 using Tapatalk
Benjamin, I just responded to one of your Facebook posts asking the same. The reason for using 20, or 25, volt caps is to give you room in case your track voltage gets too close to the rating. Most Ta caps have either a 10% or 20% tolerance on the spec. A -20% on 16V puts you at 12.8V. Many layouts are running at 13-13.4V. With the SMD caps, when they get an over-voltage condition they can explode, with flame. I do have a number of locos with 16V caps as well. There was a point when they were the best one could do.
The 20% tolerance is just for the capacitance. The working voltage is the minimum guaranteed working voltage. In other words, it might or might not withstand a higher voltage, but it is guaranteed to withstand the rated (DC) voltage. I generally spec a 50% higher voltage than the maximum expected voltage to provide a margin of safety.
Huh, I always thought the tolerance related to both units. Either way, works to the same: 16V is just too close to the operating voltage when 20V and 25V caps are now available in sizes usable for N. Here are some Ta and polyTa SMD caps which are available from DigiKey or Mouser Type Mfg Vendor P/N Tantalum 150 μF 20 V KEMET DigiKey 399-4794-1-ND Tantalum 220 μF 16 V KEMET DigiKey 399-10429-1-ND Poly-Ta 220 μF 20 V AVX Mouser 581-TCN4227M020R0100 Poly-Ta 330 μF 20 V AVX Mouser 581-TCN4337M020R0100 Poly-Ta 150 μF 25 V AVX Mouser 581-TCN4157M025R0070 Poly-Ta 220 μF 25 V AVX Mouser 581-TCN4227M025R0100
Thank you for all of the info Rick. I do greatly appreciate it. I'm still full of questions if I may ask a few? You list several options here, 150, 220 and 330. Wouldn't 330 be the best to use for a keep alive? My understanding is that the higher number uF, (I tried finding the correct code to change the U to the symbol needed using the Alt+number combo but failed!!) is better. Am I corrected in thinking this? Is it more a limitation in space allotted/ room for the caps that make us use the 220uF? I usually use 2 220uF 16v caps in my installs. Thank you for your help again.
It is all a matter of price and space. The 220uF 16V, or 150uF 20V KEMETs are about $2 ea, while the 330uF AVX is >$6 ea
I know this is an old thread, but is there anything else to watch out for, or can I just select by price and size, i,e. the cheapest smallest 16V 220 microfarad tantalum? I assume leakage current, lead/package inductance etc. do not really play a role for us, and the locomotives do not really run hot, either. I' m thinking of this capacitor here, which seems to be the cheapest on digikey, and also one of the smallest, if I'm not mistaken: 7.3 by 4.3 by 2.8mm https://www.digikey.com/en/products/detail/avx-corporation/F931C227MNC/4005195, a bit over a dollar a piece if one buys 50. Almost forgot to add: most of the smoke alarms (huge remote inaccessible electronics installation) at my old job were exploding tantalum caps. Always exciting!
These days, most of us are now using PolyTa caps. Much more forgiving on voltage range. There are some 25V 220uF caps at Digikey, in a slimmer package, than the ones listed above. And, at 25V ratings, using 80% of rating for PolyTa, they are perfect for any N scale or HO use. 440uF is sufficient to handle buffering most any dropout. Doesn’t provide extended running through dead zones, though
Thanks, Rick! They seem a bit more expensive (>$3/cap), wider (6.1mm), but less tall (2mm). I like the 'Benign failure mode under recommended use conditions' ... Shouldn't it be 'Will not fail under recommended use conditions'? Update: also, out of stock; I'll search a bit more.
There are over 1800 in stock https://www.digikey.com/en/products/detail/avx-corporation/TCN4227M025R0100E/9919931! They are also available at Mouser 581-TCN4227M025R0100 (25V 220uF)
I was wondering about the slightly more expensive -E part. Case dimensions seem to be identical to the non-E part, despite the E-parts being advertised as 'highest energy density'. Are the TCN4227M025R0100E more reliable than the TCN4227M025R0100? I did look through the data sheets, but haven't found the difference yet (except in the first page description). Edit: Mouser labels the non-E part 'end of life', whereas on digikey both have status 'active'.
They are probably "active" since they show "6 in stock". Had never noticed the difference, before. But I did notice the "price increase" on my last order, now I know why LOL
