On 9/4/05, Daniel Chia wrote: > Hi, > SMD tantalums seem to offer a great advantage in terms of size > over normal electrolytic caps in stabilising a power supply. However I'm > concerned with voltage spikes killing the cap. > > Basically my application is a small robot with 2 motors > (Minimotor 1724 6volts version) with a current draw of around 300mA > each, through a MOSFET H-bridge. Power is provided by a LiPo pack > providing 7.4V nominal. What I'm concerned about is whether a 47uf 20V > SMD tantalum will help stabilise the supply adequately for my 5V LM2940 > reg as well as whether motor spikes might kill the cap. > Daniel, low ESR tantalum or aluminium polymer capacitors have been discovered just for your needs. Check with a scope how large are the spikes you're talking about and dimension the capacitor voltage according. As low will be the ESR as low will be the spike, but the price of an ESR is quite big looking at the Digikey price. One good solution is to add a ceramic capacitor in parallel with your electrolitic capacitor on the LM output. The cap must be added also near the +VCC of the bridge and suppression diodes must be connected between both motor ends and VCC/GND. cheers, Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist