Re: Allowable voltages on Scope
- From: dnichols@xxxxxxxxxxx (DoN. Nichols)
- Date: Sun, 26 Mar 2006 04:31:02 +0000
According to Sierevello@xxxxxxx <Sierevello@xxxxxxx>:
I have a Kikusui COS6100M Scope and want to look at the voltages coming
out of my RPC to see how out of phase each waveform is from the correct
120 degree phase angle. Someone told me not to apply the 240v to the
scope that it will damage it. They advised to get a large resistor and
drop the voltage across that and then use the 5v and look at the
waveforms. That would be a large resistor and I would not know where to
find one that large that would handle the watts.
*What* watts? It only needs to be a reasonably high *resistance
value. A typical 10X scope probe should be all that you need on each
channel. The input impedance of a typical scope is 1M(egohm), and a 10X
probe is 10M. The 240V (RMS) is about 340V, and applied to a 10M load
will pump all of 0.0116 Watts into that.
The manual says that the scope has allowable inputs of:
Channel 1,2,3: +- 250volts at 20KHz (DC+AC Peak)
So -- a 10X probe would allow you to look at things up to 2.5KV
(actually, probably a lot lower, with full scale being 8 divisions times
20 V/div, or about 160V Peak (assuming ground being set to the bottom or
top grid line.
Of course, a probe typically has a lower voltage limit than that
worst case calculation.
EXT Trigger Channel 4,5: +-50volts (DC+AC Peak) at 1KHz and lower.
Probe Inputs (10:1): +- 600volts (DC+AC Peak) at 1 Khz and lower
There you are -- with one of *their* 10;1 probes, you can handle
a swing of 1200 volts (-600 to +600), so your voltage from the RPC will
be within that.
Z Axis Input +- 25volts (DC+AC Peak) at 1KHz or lower.
Also, what about grounding the scope? I heard someone mention about the
chassis of the scope becoming charged and a possible shock hazard. What
causes this to happen and what is the remedy?
There is where the problem is. Normally, you want to look at
the difference between two of the three phases, assuming a delta wiring.
You probably can't get to the joined center of the windings in the
motor, if it is wired as a Wye format (which is pretty likely for US
made dual voltage motors. Unfortunately, the ground side of *all* of
the probes are connected to a single point, so you can't do this without
six probes feeding into three differential input pairs, which your scope
does not have.
So -- what I would suggest is that you get three filament
transformers which will take the 240 V on the input side, and produce
something like 12V on the output side. Hook each primary between two of
the three wires, and join one side of each secondary to the same point
on the other two. Connect that joined point to the scope's ground, and
hook the probes to the other side of each transformer. (A quick check
for whether you have one of the transformer secondaries reversed
relative the other two is to hook an AC meter between 1 & 2, 2 & 3, and
3 & 1. If all three measure fairly close to the same, things are fine.
If one of them is way higher or lower than the other two, then one of
the transformers is reversed, and you need to swap it around until all
three are fairly close.
Once you have this done, you can measure the phase relationship
between the three outputs.
Good Luck
DoN.
--
Email: <dnichols@xxxxxxxxxxx> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
.
- References:
- Allowable voltages on Scope
- From: Sierevello@xxxxxxx
- Allowable voltages on Scope
- Prev by Date: Re: Lincoln SA200 Welder
- Next by Date: Re: compressor auto drain kitquestion...
- Previous by thread: Re: Allowable voltages on Scope
- Next by thread: Re: Allowable voltages on Scope
- Index(es):
Relevant Pages
|
