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Forum: Analog Circuits How not to fry an oscilloscope on old equipment?


von Barny_G. (Guest)


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I want to measure the characteristics of an constant current source from 
the 60s/70s
It is able to deliver 10A to 300A.

I want to measure the characteristics of this constant current source 
without frying my oscilloscope.
Is a 10:1 Voltage divider with an 24V zener diode and a 10:1 probe 
enough protection against huge spikes?

Is a 2m long 175mm² cable in water linear enough to use it as a shunt 
resistor?

The reason behind this all is that I want to know how fast the 
regulation is and how large the voltage / current spike is when the 
constant current source get switched on/off
And I'm interested how far of the delivered current is compared to the 
label.
The constant current source was made for nominal 380VAC grid voltage 
which is now nominal 400VAC.

von OldMan (Guest)


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I would recommend a scope from the Tektronix 530 series.

They need not much protection.

von Barny_G. (Guest)


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Why should I buy an slow, Tek when I already own a old Rigol (150MHz, 
monochrome) which fullfills all my needs.

And a Tektronix oscilloscope gets damaged too if the voltage spikes are 
to high.
Specially when it is connected to 50+Kg metal.

I dont want to damage my mesurement tools with one stupide move.

von lulului (Guest)


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Barny_G. wrote:
> It is able to deliver 10A to 300A.

I assume DC. In general, is it really too difficult to tell us your 
brand and type of current source and oscilloscope? And, in case you have 
them, to provide links to specifications, manuals, etc.?

Because then I wouldn't have to assume DC, but would know.

> The constant current source was made for nominal 380VAC grid voltage
> which is now nominal 400VAC.

You are looking at the wrong data. You need to know what is called the 
"compliance voltage". This is the maximum DC voltage the constant 
current source is capable of generating to drive its 10A to 300A current 
through a load.

Then you match the compliance voltage to the specifications of your 
scope. And then you think about adding additional protection in case the 
current source exceeds its compliance voltage. E.g. because of glitches 
or defects when the load changes abruptly.

von Barny_G. (Guest)


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The scope is an Rigol DS5152MA.

The brand of the current source is unknown.
Its an large, green hammer-enamel box on wheels with an large hand crank 
on top.
Some parts (diodes) are probably CCCP made.

The constant current source is delivering DC.

The no load voltage is 80 to 100VDC, depending on dialed in current and 
the position of the current range selection busbars.

The oral passed on recomendation is to stay below 15V under long term 
load.

The compliance voltage isn't troubling me.
The voltage spikes which are occurring giving me headache.

And this voltage spikes, overshoots, undersoots,... should get measured 
without frying my scope.
I want to know if I'm able to use this current source for other things 
then charge large lead-acid batterys, let steel bars glow,... without 
risk to kill everything.

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