How to protect Oscilloscope inputs from overvoltages?

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As in subject... Do you know any circuits I could use to protect my oscilloscope? I don`t want to blew my oscilloscope...
Measuring coils while operating may produce some transient spikes...

Do you know any commercially available circuits?
How would you make them. I thought first abut TVS diodes...

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In general use probes with built in attenuators.

Typically x10 probes.

The front end of the oscilloscope in general has inbuilt protection in the form of protection diodes clamping the input signal to some safe reference ( +ve and -ve ).

If you need something beyond this then by all means do design and build it.Just keep in mind the input resistance and capacitance of You oscilloscope and make sure YOUR protection circuit is properly frequency compensated so that neither the INTENDED voltage nor bandwidth are compromised beyond usable state.
NOTE commercial attenuated probes also have an upper usable voltage limit. DO NOT EXCEED IT

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You can get 100:1, 1.2kV probes fairly inexpensively (not sure about in Poland). Even 1000:1, 30kV probes are available, although they get pretty expensive.

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Ahh, THE classic 'scope probe book! Back when Tek knew what it was doing.

It is correct that most well designed 'scope inputs include over-voltage protection. The cost to repair such damage is simply too high, and the errors happen too frequently.

The actual amplifier input should be able to handle about +/- 1V linearly. There will be clamping diodes that turn on somewhere about +/- 1.5V. The amplifier will probably have limited before then, but the diodes keep it from being damaged. These numbers may vary according to the gain distribution within the vertical amplifier and attenuator chain.

In short, the input ought to be able to handle several hundred volts on the most sensitive range. The manual should tell you what the limits are. Use a 10X probe and improve that by 10X BUT there are voltage breakdown limits in the probe, itself. You need to check the probe for its own peak voltage rating.

Jim - former Tek scope designer

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Jim,

How interesting, I perceived you worked for Tek or HP, in the golden years.

It all starts with a mental vision.

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Thank you guys for information. I will study pdfs. George your posts are as always very informative. Thank you. I see the key of the success is to take good Oscilloscope Probe with some voltage reserve.

Adam

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Hello George, I am pleased to hear somebody jamed in overvoltage limitators/protections for Rigol's scopes, I already have looked around for a smart solution but still not very confident in home made probes.

To me to fiddle on my 230vac mains as watchig signals on a gate/anode Triacs I intend do an "interface" probe with a 220/2x12v I know that won't be perfect but images of voltages is probably enough to me, + galvanic protection.

Now, how to compensate effects of extra capacitors I don't have any idea yet but let's see latter. By the way I'm looking for a DS1102E Rigl's scope (490€) quite cheap for me and looking convenient for what I need to do in my electronics projects.What do think about Rigol's scopes?

About voltages protection I guess also use transil bidirectionnal diodes which clamp very fast or even Zener diodes in series + limiting current resistor...May be somebody has a better idea so I'll be glad to know what is better but to to buy an differntial probe which is to expensive to me for a very occasionnal use.

Thank's to all and regards.

Jessyfrench

Jessyfrench

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SpiderKenny is absolutely  correct. This is probably a greater cause of damage, especially to probes, than over-voltage on inputs. 

 

Consider this dumb scenario. You have some line powered hardware. You need to measure AC current. So, you put a current-sense resistor in line with the hardware. In the U.S., it is hard to tell which side of the line is neutral (nearly ground) and which side is hot. Other parts of the world float both sides of the line. So, without thinking, you clip the probe ground clip to one side of the sense resistor and the probe tip to the other side  of the sense resistor, and plug the whole thing in. In the U.S., you have a 50-50 chance of it all going boom (or, at least, a VERY loud POP). Been there, done that (many years ago, but well remembered). When I looked back at the scope probe, the ground lead was GONE, turned to a vapor of metal and plastic; the now hanging ground alligator clip was just that - no longer connected to anything.

 

Around Tektronix, where I was working then, there were numerous power cords with the ground pin cut off, just to prevent such little embarrassments. But, it also made the scopes, themselves, very hazardous as the metal housing COULD be at line potential. 

 

There is NO transient protector that will protect against this type of fault. You CAN isolate, using a current transformer, and that will save much hazard and/or embarrassment. 

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Mon. Feb 15, 2016 - 01:36 AM
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ka7ehk wrote:
You CAN isolate, using a current transformer, and that will save much hazard and/or embarrassment.

And the cost of a replacement scope.....

 

East Side Jim

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Correct. In comparison, a current transformer is (really) cheap. Also, consider the value of hospitalization or other medical care if you should happen to do things really wrong. And, what value do you put on NOT having pain?

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Mon. Feb 15, 2016 - 01:35 AM
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And do not forget to shunt the CT secondary.

It all starts with a mental vision.

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Mentioning EEVBlog and oscilloscope safety, this is a must-see:

 

EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

https://www.youtube.com/watch?v=xaELqAo4kkQ

 

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Hello Georges, I'm still searching about differential probe, and in fact is a simple voltage divider which drops your 230Vac  to less than 1

Vac. Then that little volage goes to inputs of an measure amp ....So there is the complete doc available at the foffowing address:

http://www.dgkelectronics.com/bl...

I began to make 2 probes but stoked by components missing in my workshop. I already verified the voltage divider with other resistors

values and the dividing ratio is good, good luck and be carefull when testing .

Regards and pleased to read your posts

Jessyfrench

Jessyfrench

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Years ago, had to design this 18KV fence charger for a start-up:

They were really paranoid about their DSO and accidents, so we did this:

- Wear rubber gloves

- Used mostly plastic tools

- Everything behind a 1:1 mains isolation transformer (not autotransformer), including oscope, device and power supplies.

- Probes had 100:1 carbon resistor dividers.

- TVS and MOV between probes and DSO ground.

We had to measure DC mostly so we didn't care too much about freq. response of protections.

 

while(!solution) {patience--;}