Prompted by a posting on the MERG forums about capacitor performance, I returned to the test bench to see what happens in the real world.
The MERG posting pointed to the performance of Ceramic capacitors as the applied voltage is varied. At very low voltages (1 or 2v) they achieve their rated capacitance. But, as the voltage rises towards the rated voltage, the capacitance declines quite dramatically. In contrast, Tantalum capacitors have a nearly flat graph, with full capacitance at their rated voltage.
My use of capacitors in stay-alive devices uses capacitors at or near their rated voltages, so the ceramics used in earlier postings were 16v rated and are used at voltages of around 13v.
A typical Tantalum capacitor is about twice the volume compared to a Ceramic for the same uF and same 16v rating. So, with a given volume in a loco, one can fit twice the uF of Ceramics to a loco.
Which brings me to the test bench. My 4mm Scale Y6 Tram loco, fitted with a Mashima motor, High Level gearbox, and underneath, an 0-4-0 with coupling rods linking the wheels. It was built with a CT DCX75 decoder, and originally fitted with 22 of 100uF Ceramic capacitors.
An equivalent volume is achieved with 6 of 220uF Tantalum capacitors, or 1320uF in total.
This was tested on the bench, and appeared to show a small but significant increase in how far a wheel revolves when power is removed.
So, the first conclusion is that for equivalent volume, the Tantalum capacitor performs a little better than the Ceramic.
The next test was to increase the Tantalums to 2200uF by using 10 capacitors. On the track, this gives about 1/4 of a wheel turn without power at modest running speeds. This is substantially more than the about 1/10th of a wheel turn which the Ceramics would show. So, clearly for the same uF rating, the Tantalums work much better than the Ceramics, which is consistent with the maker's specification sheets of declining performance mentioned above.
Overall conclusion for model building - for the same unit volume, Tantalums work better than Ceramics. For the same distance travelled, about 0.3 of the uF in Tantalum will give the same movement as 1 unit of uF in Ceramics. Tantalums are cheaper and available from more suppliers than Ceramics. So, I'll be using Tantalums first and only if stuck on awkward space going over to Ceramics.