I have a good old fast Tektronix scope with a slower 100x probe and a pair of 10x probes. This scope is new to me and has some nice features my old Heathkit didn't.
I want to see the output of the power transformer in-use. It uses a bridge and no center tap. Only the output of the bridge is grounded at one side. The bridge puts out 550v at the first main fitler caps.
These are not differential probes. One side is grounded thru the oscilloscope.
I do have some isolation transformers sufficient for the oscilloscope and for the amplifier, so I guess I could float one or the other or both.
But is it necessary or even desirable? Can I just clip the probe grounds to the amp's ground and use two probes, one on each secondary lead, and press the "Display A - B" button? Will that need two X100 probes (right now I only have one, but a matching one is on order) or can I get away with the pair of X10 probes I have?
I want to see the output of the power transformer in-use. It uses a bridge and no center tap. Only the output of the bridge is grounded at one side. The bridge puts out 550v at the first main fitler caps.
These are not differential probes. One side is grounded thru the oscilloscope.
I do have some isolation transformers sufficient for the oscilloscope and for the amplifier, so I guess I could float one or the other or both.
But is it necessary or even desirable? Can I just clip the probe grounds to the amp's ground and use two probes, one on each secondary lead, and press the "Display A - B" button? Will that need two X100 probes (right now I only have one, but a matching one is on order) or can I get away with the pair of X10 probes I have?
normally the scope is isolated from mains by its internal transformer and using a 3-wire mains cord the chassis is connected to protective earth.
And the same applies to the amp - even with a 2-wire mains cable it should be isolated from the mains.
If this is true, you do not need any isolating transformer.
But if you are not sure about the amp, insert an isolating transformer between mains and amp for security.
Anyway I do not see any point in inserting an isolating transformer between mains and your scope, because you would loose the connection to protective earth of scope chassis - no good idea at all.
And the same applies to the amp - even with a 2-wire mains cable it should be isolated from the mains.
If this is true, you do not need any isolating transformer.
But if you are not sure about the amp, insert an isolating transformer between mains and amp for security.
Anyway I do not see any point in inserting an isolating transformer between mains and your scope, because you would loose the connection to protective earth of scope chassis - no good idea at all.
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Most scopes have a 1000v max input (you should check. Sometimes it's printed right on the scope ) so the ten times probes should be fine. And the A-B setting is the way to do it. And I would suggest testing the accuracy by putting both probes on the same transformer output and seeing if the scope shows close to zero ( 1/100 of the input voltage should be close enough ) if not you can use the adjustment on the input to zero the probes.
Often an amplifier (especially a guitar amplifier) does have its single-ended input grounded at the input jack where it also connects to chassis which is connected to the 3rd pin on the power plug. On a guitar amp that also gounds all the metal on the guitar to the 3rd pin on the power plug.
So I guess I could try to measure the resistance of the grond pin on the scope to its chassis or gound pin of its powere plug...
What you say is certainly true of some of the modern digital scopes. The Tektronix portable scope inputs are incredibly well isolated, in fact it seems at some point they are optically isolated. But this is an old Tektronix 485 analog 350 mhz scope.
In those days, there was a big difference between 'differential' probes and ordinary probes which had a ground clip and a hot tip.
I do know people who, in the past, floated their scope on a power transformer, but had to have everything pre-set because they couldn't touch the metal knobs. That tells me those scopes really grounded the ground lead of the probe to the third pin on the power plug. Maybe they just had the channel set to 'gnd' in stead of 'AC' or 'DC'????
Now that's the opposite of what voltwide said. But it's really like making a balanced differential input, except for that each channel is still limited in how much DC offset it can handle. It certainly seems safe, but is it necessary?
I assume I'm using the 1Mohm setting on the channel with the 10X or 100X probes, not the 50ohm setting.
The specs say that in the 1Mohm setting AC coupled it will tolerate 500 V peak-to-peak (DC + peak AC). So the 10X probes should be able to tolerate 5000 V? I'll have to see whether that's also within display range, or just the limit before damage or the proteciton relay kicking in. Maybe I didn't need to get the X100 probes. I was really looking for 20X probes, but they're not very common.
So...using the "display channel A-B" mode is kind of a poor man's balanced input when there's not too much DC offset... So then on a new but related subject, why can't I get a high-quality moving-coil cartridge transformer or a Jensen microphone transformer backwards and attach 2 BNC connectors and try to make a poor-man's differntial probe? Now sensitiv enough? Impedance still too low and would affect the system being measured too much? How come the Chinese don't make a cheap differential probe (yet)? Is there a DIY kit anywhere, or has interest dropped since the new scopes have isolated inputs?
Many old Tek scopes have been ruined by power transformers failing due to voltage breakdown from floating them. This is also very dangerous.
You can build a low frequency x 1/1000 differential probe with a few high value resistors and a unity gain stable, high cmr op amp like the AD845.
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