Fuse resistances?

MBK · 24th April 2004, 02:51 PM

I measured a 100 mA fuse to check if it's blown because I couldn't see the thread. Fuse was fine but DCR was about 9 Ohms. Nine Ohms!! I was stunned. That's quite a lot in PSU impedance I'm adding with my fuse.

Comments?

sss · 24th April 2004, 02:55 PM

never used a 100ma fuse but i think for that current 9 ohm is ok

markp · 24th April 2004, 03:34 PM

At 100ma it needs that kind of resistance to heat up and fail with that little current.

ClassD · 25th April 2004, 01:29 AM

9 ohms x 100mA = 0.9V drop

Thats a pretty nasty drop!

peranders · 25th April 2004, 07:39 AM

Quote:

Originally posted by ClassD
9 ohms x 100mA = 0.9V drop

Thats a pretty nasty drop!

No not really if you place it right, besides I think the value is a little bit high. How about nulling the resistance for the test cables and and removing oxide and dirt from fuse?

Eva · 25th April 2004, 08:25 AM

I have measured some fuses I had lying out here...

100mA : 17 ohms [no brand, unknown specs]
200mA : 7.5 ohms ['dh' brand, unknown specs]
500mA : 2.2 ohms ['dh' brand, unknown specs]
1A : 0.1 ohms [no brand, unknown specs]
2A : 0.04 ohms [no brand, unknown specs]
4A : <0.01 ohms [no brand, unknown specs, copper filament]
5A : <0.01 ohms [no brand, unknown specs]

So DC resistance appears to reduce quickly with increasing current values

I've succesfully used those 100mA and 200mA fuses to protect the primaries of very small transformers connected to mains

MBK · 25th April 2004, 08:45 AM

Yeah well... I use this 100 mA fuse for the mains of a small transformer, and what to do, it should have a fuse... I was just surprised at the value. Thanks all for your input...

Eva · 25th April 2004, 09:24 AM

Take into account that the primary of a 230V 20VA EI transformer may have 100ohms DC resistance and the secondary may show another 100ohms equivalent DCR as seen from the primary, so in the end, a 20VA transformer will show something like 200ohms primary impedance [shorted secondaries] and this means it will produce more than 20 times the voltage drop caused by a 9 ohm fuse

For tiny 2VA transformers you may measure primary DCR as high as 5Kohm [plus effective secondary resistance] so 9 ohms is fully negligible here

MBK · 25th April 2004, 12:25 PM

ok, sounds reasonable.

That gives a strong reason though to not recommend fuses o speaker outputs, as some people use. Unless you use fuses as say, an Aleph's source resistor

PMA · 25th April 2004, 02:48 PM

Resistance of fuses by Schurter:

100mA 5.65 Ohm
1A 0.06 Ohm
5A 0.012 Ohm

Close to that described by Eva. And do not forget that the added resistance in the primary circuit of transformer is divided by N**2 to the secondary side, where N = V1/V2.

24th April 2004, 02:51 PM	#1
MBK diyAudio Member Join Date: Apr 2002 Location: Singapore	Fuse resistances? I measured a 100 mA fuse to check if it's blown because I couldn't see the thread. Fuse was fine but DCR was about 9 Ohms. Nine Ohms!! I was stunned. That's quite a lot in PSU impedance I'm adding with my fuse. Comments?

24th April 2004, 03:34 PM	#3
markp diyAudio Member Join Date: Oct 2003 Location: L.A., CA	At 100ma it needs that kind of resistance to heat up and fail with that little current. __________________ If it sounds good... it is good!

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24th April 2004, 02:55 PM	#2
sss diyAudio Member Join Date: Jul 2003 Location: Israel , haifa	never used a 100ma fuse but i think for that current 9 ohm is ok

25th April 2004, 01:29 AM	#4
ClassD diyAudio Member Join Date: Apr 2004 Location: Brisbane	9 ohms x 100mA = 0.9V drop Thats a pretty nasty drop!

25th April 2004, 08:25 AM	#6
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	I have measured some fuses I had lying out here... 100mA : 17 ohms [no brand, unknown specs] 200mA : 7.5 ohms ['dh' brand, unknown specs] 500mA : 2.2 ohms ['dh' brand, unknown specs] 1A : 0.1 ohms [no brand, unknown specs] 2A : 0.04 ohms [no brand, unknown specs] 4A : <0.01 ohms [no brand, unknown specs, copper filament] 5A : <0.01 ohms [no brand, unknown specs] So DC resistance appears to reduce quickly with increasing current values I've succesfully used those 100mA and 200mA fuses to protect the primaries of very small transformers connected to mains

25th April 2004, 08:45 AM	#7
MBK diyAudio Member Join Date: Apr 2002 Location: Singapore	Yeah well... I use this 100 mA fuse for the mains of a small transformer, and what to do, it should have a fuse... I was just surprised at the value. Thanks all for your input...

25th April 2004, 09:24 AM	#8
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	Take into account that the primary of a 230V 20VA EI transformer may have 100ohms DC resistance and the secondary may show another 100ohms equivalent DCR as seen from the primary, so in the end, a 20VA transformer will show something like 200ohms primary impedance [shorted secondaries] and this means it will produce more than 20 times the voltage drop caused by a 9 ohm fuse For tiny 2VA transformers you may measure primary DCR as high as 5Kohm [plus effective secondary resistance] so 9 ohms is fully negligible here

25th April 2004, 12:25 PM	#9
MBK diyAudio Member Join Date: Apr 2002 Location: Singapore	ok, sounds reasonable. That gives a strong reason though to not recommend fuses o speaker outputs, as some people use. Unless you use fuses as say, an Aleph's source resistor

25th April 2004, 02:48 PM	#10
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Resistance of fuses by Schurter: 100mA 5.65 Ohm 1A 0.06 Ohm 5A 0.012 Ohm Close to that described by Eva. And do not forget that the added resistance in the primary circuit of transformer is divided by N**2 to the secondary side, where N = V1/V2.

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