Re: 475 questions

Harvey White

As a precaution, some switching supplies don't work well with a voltage below the operating voltage.

For a linear supply, as the voltage to the supply rises, the regulator generally (if it has a decent reference) will turn on full until the input voltage to the regulator reaches a threshold where the output voltage would exceed the setting, then the regulator stops being a short circuit and starts regulating. Depending on the circuit, for example a 5 volt regulator may need anything from 5.25 volts to 8 volts to start regulating.

On the other hand, a switching regulator is not so well behaved.

Remember that a switching regulator has a transistor that, instead of being a variable resistor, is a switch.  It's either on or off.  The switching regulator works by pulsing the transistor on with a limited duty cycle.  That complete (on) dead short lasts for enough time to build up the output voltage, then it turns off.  There are several geometries of switching regulator, some of which, if the switching element is turned on, will draw a lot of current.

The problem in a switching regulator (or one that has two transistors driving a transformer as an inverter), is the question:  What happens when the input voltage is too low?

Some turn on and try to build a proper output voltage.  Some may not operate until they get enough input voltage to work properly.

The problem with some of them is that they draw a LOT of current before they start to operate properly (and switch on/off).

Tektronix uses both switching and linear supplies, and the switching supplies may or may not "like" low input voltages.

I have brought up scopes using a variac, but I have monitored the input current.  I've seen it go over the proper rating and then go back to a lower voltage when the supply starts to switch.

I've seen them work into a short (and the tick mode/overcurrent mode) not work well.  You find that out by monitoring the current the scope draws as the voltage goes up.

Each supply/scope may be different.  You'll need to watch what each one does.

I have a sencore unit that has an isolation transformer and a variac.  It measures both leakage current from the supply lines to ground (not neutral) and the input voltage, output to the UUT, and current.

It's been useful.  You could make one if you wanted, with the exception of the leakage current measurements.  Not sure how they did that.

I'd seriously recommend simultaneous output voltage/output current monitoring, but on this one, I know where the variac will put out full voltage, and can still monitor the current.


On 5/28/2020 6:39 PM, ciclista41 via wrote:
Still great advice for later when I'm working on a DUT with a SMPS. :-)

On Thu, May 28, 2020 at 03:34 PM, <> wrote:

Excellent advice, Eric!

Now I'm glad I just put that incandescent bulb in series when I turned it on
the first time. Not that I have a variac (yet).


On Thu, May 28, 2020 at 12:53 PM, Eric wrote:

I would test the rectifiers the silicon is usually pretty resilient and
normally grossly fails "open or short". I have only run in to one transistor
so far that has "kind of" failed that one took a 576 to find it, was out of
fluke 5200A. However do NOT bring up a 400 series scope slowly on a variac.
The 400 series have switching power supplies they are not the linear
They can over current when the line voltage drops to low to regulate and can
be damaged. Slow up on a varic is ONLY good for linear power supplies.

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