Gerry O’Hara

I asked Bill Russell, a contact I have in Santa Barbara who is is an analogue circuits engineer and familiar with many types of test equipment. He provided an attachment (Capacitor Connection jpeg file) to illustrate the pictorial of the functionality of this connection approach and the following explanation. It’s effectively one capacitor: the two-leads approach is an attempt to minimize the effect of a capacitor’s typical lead inductance. The connection shown provides less coupling of the circuit’s left side AC signal appearing on the rightside. With the capacitor’s rightside signal pickup made very close to the capacitor, the signal source of C12 will look highly capacitive, and less inductive. If both circuits shared the same capacitor lead, then there’s an inductance going to the capacitor. This connection approach is often used with switching power supplies operating at high frequencies and high ripple currents. The induced voltage in an inductor is e= L x di/dt. In a circuit where there are large currents existing (di) and of short time periods (dt), significant induced voltages can be created. There are tantalum caps that have a low ESR and along with short lead lengths can provide a very low impedance to high frequency signals. Depending on the amount of current dumped in and out of the capacitor and the speed of the current changes will determine the amount of induced voltage in the capacitor leads. It doesn’t take much L to mess things up – just like using a scope probe with a long ground lead results in lots of ringing in the signal viewed on the screen. In practicality with the subject circuit, the large C used may render this technique not unecessary, but I’ll give the designer the benefit of the doubt. Not seeing more of the circuit makes it difficult to see if the value of C is an overkill or not. In power circuits, a signal waveform can contain both fast and slow current variations. The big C could be needed for the slower current variations.

In a nutshell, the left side of the circuit probably contains high frequency signal components and the left side of the circuit (probably the load side) contains low frequency components requiring the large C. The capacitor wiring technique is used to reduce the coupling of high frequency signals from the left side coupling over to the right side of the circuitry. There are low ESR aluminum caps, too by the way – Nichicon and United Chemi-Con have low ESR and very space efficient caps designed for power supplies. I think some radio restoration hobbyist use these caps inside of gutted electrolytic cans. Several 10uF / 450WVDC Nichicon caps could fit inside of an old 8uF chassis mounted cap can. Digi-Key and Mouser are a good source for these parts.

Hope this helps (and thanks Bill!!)



Download Capacitor Connection.jpg. (Caution: This file may not be virus scanned.)