Author Topic: Low Z Mode power dissipation?  (Read 283 times)

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Offline Phoenix6478

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Low Z Mode power dissipation?
« on: August 21, 2018, 10:42:05 pm »
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
 

Offline exe

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Re: Low Z Mode power dissipation?
« Reply #1 on: August 21, 2018, 11:16:17 pm »
I think they use PTC, 3kOhm only in cold state :). At least 121GW works this way.
 

Offline Phoenix6478

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Re: Low Z Mode power dissipation?
« Reply #2 on: August 21, 2018, 11:26:19 pm »
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.
 

Offline pigrew

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Re: Low Z Mode power dissipation?
« Reply #3 on: August 21, 2018, 11:37:53 pm »
Joe Smith on YouTube just posted .  I assume the 289 had a similar setup.
 

Online David Hess

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Re: Low Z Mode power dissipation?
« Reply #4 on: August 22, 2018, 05:29:53 am »
It might also use a high voltage constant current shunt like a couple of depletion mode MOSFETs so the resistance increases proportionally to voltage.
 

Offline mqsaharan

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Re: Low Z Mode power dissipation?
« Reply #5 on: August 22, 2018, 06:54:23 am »
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.
« Last Edit: August 22, 2018, 01:57:39 pm by mqsaharan »
 


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