Is a x100 probe safe to use for this?

[JayMan07]

I'm trying to fix an old DLP projector and I believe the problem is in the bulb driver circuit.

I wanted to monitor the input and output voltages on a F10L60V.  With a DMM I get 110VDC on the input and 355VDC on the output.

My scope is rated for 20V.  What's confusing to me is that when I hook a x10 probe up to the scope it will decrease everything by x10...simple.  Well when I hook up that same x10 probe to my DMM it just halves the voltage (so a 9V reads 4.5V).  So what is the actual voltage coming out of the probe?

I don't want to blow up my scope.  I wanted to measure the 110V with a x10 and the 355V with a x100...is it safe even though my DMM is saying otherwise?  Any other tips for doing this would also be appreciated.  Thanks!

[AG6QR]

A x10 scope probe works as a resistive voltage divider, but the other half of the voltage divider is the input impedance of the scope.  Standard scopes have 1 Mohm input impedance (in parallel with some capacitance, usually 10-30 pF or so).

To make the voltage divider to cut the voltage by a factor of 10 into a 1Mohm input, the probe uses a 9Mohm resistor.

If you use that same scope probe to feed a DMM with a 10Mohm input, the divider will have a ratio of 10/19, or pretty close to the 1/2 you describe.  Based on this, my guess is that your DMM has an input impedance of around 10Mohm.



As far as safety, realize that a scope's input has a rating, and the probe also has a rating.  It's important to look up the ratings, and don't exceed either rating.  Sometimes a x10 probe can't handle ten times the voltage that the scope's BNC jack can handle, because the insulation of the probe tip is not up to the task.

[JayMan07]

Wow thanks for that answer - it makes sense I just never would have thought of it.  And I looked up the spec on my DMM and sure enough the input impedance is 10M Ohms 

I got a probe that's x100 and rated for 2000VDC so it sounds like I'll be fine measuring the 355VDC on that pin.

[tautech]

Wow thanks for that answer - it makes sense I just never would have thought of it.  And I looked up the spec on my DMM and sure enough the input impedance is 10M Ohms 

I got a probe that's x100 and rated for 2000VDC so it sounds like I'll be fine measuring the 355VDC on that pin.

As pointed out a 100:1 probe will be constructed in a much better manner to ensure safety at higher voltages. They also have a much better "derating with frequency" spec.

Wise choice to have one in your gear. 

I get mine out for anything over ~150V

[mzzj]

Please note that gas discharge bulb ignition voltage spike is in thousands of volts!
Will fry most probes and meters on first try.

[JayMan07]

You mean the voltage of a dlp projector bulb can spike to thousands at the connector?  And do these spikes happen just at start up and shutdown?

[mzzj]

You mean the voltage of a dlp projector bulb can spike to thousands at the connector?  And do these spikes happen just at start up and shutdown?

yes, 3-4kV typical. See for example http://m.electronicdesign.com/lighting/build-efficient-hid-lamp-driver-circuit
High voltage spikes should occur only on start but i wouldnt necessarily bet my head or meter for that.

[Psi]

Yep, just like a TIG welder there's an initial high voltage pulse to get the arc going.

[Shock]

What frequency is it? You could also use a HV probe if suitable frequency, the Fluke and various copies of the design come in a 6kV and 40kV, they work as 1000:1 divider. 20V scope inputs is rather limiting.

[Simon]

I though all scopes were in the 20-30V input range.

[Wim_L]

I though all scopes were in the 20-30V input range.

Many are in the 150 - 400 V range on 1 Mohm inputs. On 50 ohm inputs, it's often limited to a mere 5V (higher than that, and you get thermal overload in the termination resistor).

[Alex Eisenhut]

I though all scopes were in the 20-30V input range.

Yeah, per cm maybe. That's still 200V ish.

[tautech]

I though all scopes were in the 20-30V input range.

Yeah, per cm maybe. That's still 200V ish.

I'd really like to comprehend this perception of max scope input by Simon. 

Sure it looks like Simon forgot zero's, but twice?
Even then he's a hundred volts shy of scopes that are available.

For EE's not to know what the max p-p input of a scope is pretty poor.     

Many CRO's and DSO's have a max p-p input of 300V and are historically designed for/in countries with 110 VAC mains.
Today with the prevalence of SMPS that need repair, a scope with 300V inputs must be used with extreme caution on the primary side of SMPS.
Why?
A very large part of the world has 230 VAC mains and when recified in the primary side of SMPS is ~325 VDC. Should one forget to switch a probe to 10:1.....you've just exceeded the max input of your scope. It might not go pop, but should your mains be ~245 VAC, then your're well over max.

For those considering work on mains, be very carefull with scope and probe selection and as in the explanation above I'll always recommend scopes with 400V inputs, just as in the full Siglent range of DSO's.

end rant.

[Simon]

I though all scopes were in the 20-30V input range.

Yeah, per cm maybe. That's still 200V ish.

I'd really like to comprehend this perception of max scope input by Simon. 

Sure it looks like Simon forgot zero's, but twice?
Even then he's a hundred volts shy of scopes that are available.

For EE's not to know what the max p-p input of a scope is pretty poor.     

Many CRO's and DSO's have a max p-p input of 300V and are historically designed for/in countries with 110 VAC mains.
Today with the prevalence of SMPS that need repair, a scope with 300V inputs must be used with extreme caution on the primary side of SMPS.
Why?
A very large part of the world has 230 VAC mains and when recified in the primary side of SMPS is ~325 VDC. Should one forget to switch a probe to 10:1.....you've just exceeded the max input of your scope. It might not go pop, but should your mains be ~245 VAC, then your're well over max.

For those considering work on mains, be very carefull with scope and probe selection and as in the explanation above I'll always recommend scopes with 400V inputs, just as in the full Siglent range of DSO's.

end rant.

based on my experience of low end kit the inputs wont take more than a few tens of volts and rely on the probes x10 to reduce it although i may have misread specs that were not written in the best ingrish in manual written by chinrse manufactures like rigol and owon. sorry i cant afford more expensive kit with decent manuals

[Wim_L]

Even on the cheap ones, there usually is a brief summary of the most important values on the front panel near the BNC connectors. This will include input resistance (almost always 1 Mohm), input capacitance (a few pF), and maximum input voltage. These are the parameters directly at the scope input itself, without probes attached. For faster scopes, there may be additional specifications for the 50 ohm mode.

[Simon]

my owon it rated for 400V CAT II, so I'm not sure if we are talking about a 400V input range or that a 400V spike in the CAT II range will not cause damage.

[Wim_L]

No, the number given for the voltage rating would be the voltage it can handle in normal operation. If it's rated for 400 V, you can apply 400 V all the time. The CAT rating specifies what kinds of systems it may be connected to, and how strong pulses may be. See, for example, the table at: http://www.ni.com/white-paper/5019/en/

Keep in mind that the CAT rating is only safety information. It does not mean the instrument will still function properly at those voltages (and especially not the transient pulses)! For that, you need to look at the more detailed specifications in the manual, if it's specified at all.