Low Z Mode power dissipation?

[Phoenix6478]

Hello

I have got a question regarding the low impedance mode on multimeters like the Fluke 289:

The manual of the Fluke 289 states that it has an input impedance of 3kohm in the low z mode and it can go up to a 1000V. But if i measure for example 1000V, there will be a current flow of 333mA through the meter at 3k according to  ohms law. But 0.333A at 1000V leads to a power dissipation of 333W. That is quite a lot of power to dissipate inside a multimeter.

So how exactly does the multimeter handle the current flow? Is there a detection feature which switches to the usual 10Mohm impedance if the voltage does not drop down like in the case of ghost voltages?

Greetings,
Phoenix

[exe]

I think they use PTC, 3kOhm only in cold state . At least 121GW works this way.

[Phoenix6478]

Thank you for your reply 

Using a PTC makes a lot of sense. As soon as the current starts to flow it starts to heat up and its resistance increses. The current gets limited by the rising resistance.

[pigrew]

Joe Smith on YouTube just posted a video on the 189 which had a few minutes talking about the front-end of that meter.  I assume the 289 had a similar setup.

[David Hess]

It might also use a high voltage constant current shunt like a couple of depletion mode MOSFETs so the resistance increases proportionally to voltage.

[mqsaharan]

Hi Phoenix6478,
Please take a look at HKJ's review at http://lygte-info.dk/review/DMMFluke%20289%20UK.html. He has covered the answer to your question very nicely in the form of a graph. He is also a member here on EEVblog.

Regards,
Qasim.