Scope high volt probes

[willyrivero]

I have an old Tenma scope 30mhz. and sometimes I would like to use it to check for ripple voltage on SMPS or for testing components in this area.
Would buying a couple of high voltage passive probes be enough for this?
What precautions should I take?
How can I connect the probes for testing?
Anyone that can give me some good advice on this matter?

 

[Nigel Goodwin]

Normal x10 probes are fine for SMPS - if you working on the primary of a supply you need to take account of the fact it's live to the mains. If you don't know how to do that, you shouldn't be touching it in the first place.

 

[willyrivero]

I do know the fact the primary is live to the mains, but I only use my DMM for servicing this area. I just wanted my scope to be a part of it too when required. I bought it used, no manuals. I also didn't find much info regarding to this type of testing with the scope.

 

[Nigel Goodwin]

I do know the fact the primary is live to the mains, but I only use my DMM for servicing this area. I just wanted my scope to be a part of it too when required. I bought it used, no manuals. I also didn't find much info regarding to this type of testing with the scope.

A scope is just a tool, they don't come with instructions on how to use them - it's something you need to learn, or be taught. You can buy books about using scopes.

 

[BeeBop]

What precautions should I take?

You should at least get familiar with this:
In electronics servicing an Isolation transformer is a 1:1 power transformer used for safety.

 

[KeepItSimpleStupid]

In order to use a HV probe, you need to know the input Z it works with and the input Z of your scope. DMM's typically have 10 M ohm and scopes typically have 1 M ohm shunted by 1 pf (typical) or 50 ohms, sometimes selectable.

That will make a big difference.

Even if you don't know the resistance of your scope, you can figure it out. Scope probes have an RC attenuator which will be able to null out the capacitance of the scope. That's the little screwdriver adjuat for the 10x, 100x etc probes. With a high rise time square wave in, you adjust for a nice square wave. This is why scopes have a calibrate TP on the front.

 

[Nigel Goodwin]

In order to use a HV probe, you need to know the input Z it works with and the input Z of your scope. DMM's typically have 10 M ohm and scopes typically have 1 M ohm shunted by 1 pf (typical) or 50 ohms, sometimes selectable.

Sorry, but you are way out there - scope inputs are normally 1 M ohm shunted by 25pf, and this is usually actually printed next to the socket - standard probes are all designed to work with that specification.

I can't say I've ever seen a scope with 50 ohm inputs, but it would simply switch a 50 ohm resistor across the normal 1 M ohm input.

 

[KeepItSimpleStupid]

I think there was no mention of type of HV probe, whether DVM or O-scope. I hate to assume anything.

See Oscilloscope - Wikipedia, the free encyclopedia and look at the Input section. I was wrong about the 1 pf spec.

Here is a real live example with switchable inputs:**broken link removed**

 

[Nigel Goodwin]

I think there was no mention of type of HV probe, whether DVM or O-scope. I hate to assume anything.

I have an old Tenma scope 30mhz

You also wouldn't need an HV probe for a DVM just to measure mains voltages.