EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Phoenix6478 on August 21, 2018, 12:42:05 pm
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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
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I think they use PTC, 3kOhm only in cold state :). At least 121GW works this way.
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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.
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Joe Smith on YouTube just posted a video on the 189 which had a few minutes talking about the front-end of that meter (https://youtu.be/fHIPGIyLJ3M). I assume the 289 had a similar setup.
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It might also use a high voltage constant current shunt like a couple of depletion mode MOSFETs so the resistance increases proportionally to voltage.
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Hi Phoenix6478,
Please take a look at HKJ's review at http://lygte-info.dk/review/DMMFluke%20289%20UK.html (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.