The TVS is fast (Picoseconds). It is the diode of choice for protecting the transistor because it can shunt the transient before the transistor or FET is damaged. The idea is to be faster than the transient. A snubber across the relay coil will only work if it is actually across the relay coil. If it is on the circuit board it will be relatively ineffective.Transients are secular events that can produce thousands of volts from a 12 volt relay coil. Since they are secular, they are different every time so some can be more destructive than others. The best bet is to put the snubber across the coil and the TVS across the semiconductor. Rich ----- Original Message ----- From: "Richard Prosser" To: "Microcontroller discussion list - Public." Sent: Wednesday, February 08, 2006 9:23 PM Subject: Re: [EE] Transient Suppression - was Re: [ee] 3904 base resistor >> Options so far - >> >> TVS across transistor >> RC snubber across relay coil >> RC on transistor base to slow switch-off >> Diode (of some sort) across relay coil >> >> Is there any consensus as to the preferred order ? >> >> eg if cost is not an issue / if cost is an issue >> eg if components are not to be grossly over-rated >> > ...snip > > My 2c worth - > > Note that anything across the switching device (TVS / snubber etc) > will include the power supply and decoupling cap in the current loop. > This is likely to induce more EMI in adjacent circuits than the > shorter path when just a diode is used. > > In general I'd use a diode to take care of the relay current and > possibly add a snubber to sort out feed inductance effects. And > decouple as close to the hot side of the coil as possible. > > If switching speed is important I'd look at using a TVS across the > relay coil (in place of the diode) to kill the loop current as soon > as possible but would take a bit of care selecting the correct voltage > ratings etc. I've done a similar circuit with a switched, zener > controllered bipolar to kill the coil current quickly but that was to > meet a special speed requirement. > > Another possibility is to connect a zener from the collector to base > of the switching device. A high voltage on the collector then turns > the switch on slightly and the power is dissipated in that - rather > than the zener (useful for higher power levels). You have to have a > some impedance in the base drive to avoid the current pulse feeding > back to the driver however. And a snubber to sort things out until the > zener turns on. > Adding an RC to the drive circuit will work but it can also oscillate > if you get very unlucky with stray inductance & capacitances and fast > devices. > > Richard P > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist