VT100 PSU smelling.

Brent Hilpert hilpert at cs.ubc.ca
Fri Jan 22 21:44:32 CST 2016

On 2016-Jan-22, at 3:56 PM, Mattis Lind wrote:
> 2016-01-22 21:48 GMT+01:00 js at cimmeri.com <js at cimmeri.com>:
>> On 1/22/2016 2:54 PM, Mattis Lind wrote:
>>> Short: R27 in my VT100 PSU is hot and smelling. Why?
>>> Long: I think it has been 20 years since I powered up this VT100 so I did
>>> it carefully. Used a Variac and a bench supply. It switched just fine and
>>> delivered the steady 5V out when the input was at approx 50V (115V input).
>>> All the other voltages looked fine at full AC input. But there was this
>>> little smell from R27.
>> You don't use Variacs with switch mode power supplies... not unless you
>> want to burn them up.   I'll sometimes use one, but only to provide a very
>> fast soft start.  Even that's risky.
> Well. It depends if you read the schematics before you do so. In this case
> I powered the startup voltage from a 12 V bench supply. Normally the
> startup voltage comes from a small mains transformer and a 7812. Then I
> supplied the primary side voltage for the main switch transistor using a
> variac. Works perfectly well if you know what you are doing. Have done this
> practice with many SMPS supplies.

Speaking generally, I don't know that that will necessarily save one (in using a variac on a SMPS).

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.

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.
	- 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.

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