> > Speaking generally, I don't know that that will necessarily save one (in > using a variac on a SMPS). >
I for one think it is a good practice to start carefully. I like the idea of bringing up the voltage over capacitors and other electronics in a controlled manner. Electronics that had been sleeping for decades. I use a protection transformer, variac and also often means to limit the current. Then I could easily measure how the switching transistor behaves. The base/gate drive etc. > A SMPS functions as a constant-power converter in response to varying > supply voltage. For a given load, as the supply V goes down the supply > current goes up, to keep the load delivery constant. (This is in contrast > to linear-regulator supplies which maintain headroom V into the regulator > and simply limit the output V - the only thing that changes as the supply V > varies is the headroom V and how much energy is wasted as heat in the > difference.) > > AIUI, the concern for SMPSs is that if the supply V is too low, the supply > current and consequent factors may go too high for parts such as the driver > transistors, etc. > Whether it's a catastrophe depends on a variety of factors: whether the > design detects & incorporates shut-down under these conditions, how large > the load is (how much power the PS is trying to convert) (if the PS is > lightly loaded relative to it's max capability there may be no problem), > whether the pulse-width/switching characteristics are wide enough to become > a problem under low supply V, and so on. > All this depend on what load your are using. When running on the bench with variac I have a very modest load. In this case 100 mA at full 5V. Just 1 % of rated output. In that case there is very unlikely to overload the switch transistor. > > Supplying an external start-up V would strike me as a crap-shoot, > dependent on the design of the supply: > - On the one hand, if the PS was designed so that an > early-energised control circuit would shut-down > or limit the main switching under low supply V, then good. > That was the case for the VAX-11/750 supply which didn't enable switching under low main input conditions. Something I deliberately disabled to be able to test. In the case of the VAX-11/750 PSU I was able observe and detect a number of failures in the supply at low and non-harmful voltages. Regardless of protection transformer or not I don't like the idea of working with a PSU with 300VDC everywhere when probing with the scope probe. > - On the other hand, if the PS was designed such that the control > circuit wouldn't be energised > by the startup supply until the supply V was in the safe region > for the main switching, then bad - > the external startup supply may fool the control into thinking > the supply V is in the safe region. But "safe" depend on the load applied to the supply. On the other hand my question to the list was not if the use of variac is a good practice or not. I will continue to use this method since it has served me well and I am not forcing any one else to use it if they feel it is wrong. The question I have is why R27 in the snubber network is getting what I think excessively hot. The schematic for the primary side can be found here: http://i.imgur.com/VlInF90l.png One input that I had that if C19 is marginally bad then that might happen. Like if the dielectric as deteriorated over time and cannot whit-stand the voltage in the circuit. The capacitor is not cracked like the RIFA ones. It looks perfectly fine. It is a SPRAGUE 0.033uF 1600VDC. I don't have an capacitance / ESR meter so I cannot check it. Maybe I should just go ahead replacing it. But I don't like idea of replacing things without really knowing they are bad. Another question is whether the R27 is normally getting hot or not. What is VT100 owners experience here? /Mattis