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Old 1st April 2016, 09:00 PM   #1
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Default Fuses and heat dissipation

Hi. Nearing completion of the PSU for my valve phono stage. The primary fuse will see both heater and HT current, I'm using a 1.6A fuse to start with, May have to go to 2A.

I measured the resistance through the fuse at 2ohms, though I strongly doubt the accuracy of my multimeter at that level.

If 2ohms is correct, then at 2 A the fuse would dissipate 8W...? Can that be correct? Seems surprisingly high to me.
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Old 1st April 2016, 09:05 PM   #2
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The fuse will read almost zero. It is your multimeter leads playing games.
I have just built and tested an RIAA pre-amp with Baxandall tone controls. Uses two ECC83 and two EF86. Very quiet, very high gain from the EF86 and I use a 125mA Time Delay fuse which is quite sufficient.
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Old 1st April 2016, 09:25 PM   #3
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Originally Posted by JonSnell Electronic View Post
The fuse will read almost zero. It is your multimeter leads playing games.
I have just built and tested an RIAA pre-amp with Baxandall tone controls. Uses two ECC83 and two EF86. Very quiet, very high gain from the EF86 and I use a 125mA Time Delay fuse which is quite sufficient.
Thanks Jon - yes I did wonder. it's getting on a bit (and was a cheapie), might think about buying a newer, decent quality one if I see a bargain somewhere.
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Old 2nd April 2016, 09:35 AM   #4
AndrewT is offline AndrewT  Scotland
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Rh,
what does you ohm-meter read with the probes touching?

That reading can be subtracted from any resistance reading you take across other low value resistors.

A very few DMM do this subtraction electronically when you "set up" the ohm-meter.
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Old 2nd April 2016, 09:54 AM   #5
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Originally Posted by AndrewT View Post
Rh,
what does you ohm-meter read with the probes touching?

That reading can be subtracted from any resistance reading you take across other low value resistors.

A very few DMM do this subtraction electronically when you "set up" the ohm-meter.
It varies between switch ons, sometimes up to 20. I've changed the battery and reseated the plugs, but it still varies.

Not to worry, I only measured the fuse out of interest. I think I'll keep an eye out for a decent MM on sale.
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Old 2nd April 2016, 10:52 AM   #6
DF96 is offline DF96  England
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A decent MM will still have difficulty measuring low resistances. For that you need a 4-wire connection.
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Old 2nd April 2016, 11:06 AM   #7
infinia is offline infinia  United States
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testing a fuse non destructively with any meter is a go nogo type of test for us mere mortals. generally the 'buzz out' mode is sufficient on a decent DMM for debugging bad AC connections. edit > reminds me of the Bugs Bunny cartoons> testing bomb fuses on a conveyor belt with hammer blows .
what's a MM /moving meter?
if you haven't used a meter in a long time replace batteries and test the meter 1st.
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Old 2nd April 2016, 11:21 AM   #8
AndrewT is offline AndrewT  Scotland
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Originally Posted by RhythMick View Post
It varies between switch ons, sometimes up to 20. I've changed the battery and reseated the plugs, but it still varies.

Not to worry, I only measured the fuse out of interest. I think I'll keep an eye out for a decent MM on sale.
a new DMM should have resistance scales from 199.9 ohms to 19.99Mohms, 199.9mVac to 600Vac and 199.9Vdc to 1000Vdc
Any others are just for guides and are rarely accurate. I don't place any value on them.
But handy if they offer continuity buzzer, hFE and capacitance
A basic DMM offering 199.9Vac can be found in the UK for <<10.
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Old 4th April 2016, 09:59 AM   #9
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Yeah thanks all. I realise my initial test was pants. More for interest than anything else, but turned out to be a distraction.
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