VT100 PSU smelling.

Mattis Lind mattislind at gmail.com
Sat Jan 23 04:24:01 CST 2016

> 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

> 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

>         - 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?


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