View Full Version : Piggybacking

May 11th, 2009, 10:14 AM
right|thumb|If capacitor C1 has failed open circuit, piggybacking capacitor C2 (with the same value) restores the correct capacitance between (a) and (b).
Piggybacking is a technique used when troubleshooting components (for example ICs) in which a functionally identical part is placed in parallel to a suspect component in an attempt to effectively bypass the faulty parts of the circuit. If an otherwise faulty circuit behaves differently with the piggybacked part in place, this suggests that the original part is faulty and should be replaced.
Piggybacking is a useful means of testing whether a part has failed "open circuit," meaning that there is an internal disconnection within the component such that current in the circuit can no longer flow correctly. Consider, for example, a capacitor, which is simply two conductive plates separated by a thin layer of insulating material. A working capacitor holds an electrical charge across these two plates. On the other hand, a capacitor that has failed open circuit is in essence just a pair of unrelated plates and will hold no charge. Placing a known-good capacitor in parallel with the faulty one would be functionally equivalent to replacing the bad capacitor with the working one.
Note however that, due to the above reason, piggybacking is not useful for diagnosing problems caused by a short (closed) circuit or other failure. It is therefore by no means an exhaustive test for any component.
200px|thumb|right|DIP ICs, like this RAM chip, are easy to piggyback.
A particularly useful application of piggybacking is diagnosing faulty ICs. If an IC is soldered in (as opposed to being socketed) it is far simpler to place a similar part over the suspect one than to replace the part (which is not yet even known to be faulty) completely. DIP ICs are particularly suited to this technique due to their physical form: simply place the piggybacked part over the suspect component ensuring all pins make contact. The spring effect of the pins themselves, which can be increased by bending them slightly inwards, is often enough to hold the piggybacked IC in place. If not, apply light pressure to the top of the chip.