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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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8th Apr 2015, 4:47 pm | #1 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Fuse with a difference!
I have a Mesa guitar amp in for repair and found this in the mains fuse position - a 2R 1/4W !
It's a new dodge for me, what were they thinking? |
8th Apr 2015, 4:52 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,005
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Re: Fuse with a difference!
In some ways it's a sensible idea.
See the spring at the end? That's under tension - if there's a sustained overload not-quite-enough to blow a traditional fuse, it will nevertheless heat the resistor, melt the solder, and the spring will then retract, breaking the circuit. A similar idea was used in a lot of 1950s/1960s TVs: a chunky wirewound resistor with a soldered-closed leaf-spring on the side. Prolonged overload melted the solder and the spring did the rest. |
8th Apr 2015, 5:26 pm | #3 |
Dekatron
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
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Re: Fuse with a difference!
I have some similar fuses in the workshop, I think they were removed from a video recorder I scrapped.
Mine too are spring loaded at one end. Mark |
8th Apr 2015, 10:18 pm | #4 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Fuse with a difference!
thanks guys
interesting as it obviously has been working, but if someone said to me can I put a 1/4W resistor in the 240V mains supply of a 20W valve amp (probably drawing 30+ watts with the heaters etc) my gut instinct would be - NO! But as ever, we live and learn |
8th Apr 2015, 10:36 pm | #5 |
Nonode
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 2,008
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Re: Fuse with a difference!
1/4 watt resistors are normally rated to 250V DC. Under short circuit conditions I suspect that the resistor would burn out before the the temperature could rise enough to melt the solder. In that case the carbon blackened resistor would be subjected to 350V peak. Not 100% happy with this idea.
Al
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I won't tell you how I discovered that. |
8th Apr 2015, 10:53 pm | #6 | |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,130
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Re: Fuse with a difference!
Quote:
In this case 2 ohms. If the mains input current was 0.1 amp, then 0.2 of a watt would be dissipated in the resistor, within its rating. At 0.15 amp mains input (more probable IMHO) the dissipation would be 0.3 of a watt, somewhat in excess of the nominal rating but arguably acceptable if the purpose is overload protection rather than a stable resistance as part of a circuit. |
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9th Apr 2015, 12:15 am | #7 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,273
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Re: Fuse with a difference!
That's quite a common type of commercial fuse, it's a time delay fuse and has a normal fuse wire and surge spring attached to the resistor. I suspect it's to modify the fuse's I2t characteristic.
http://uk.farnell.com/littelfuse/031...slo/dp/1149695
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Kevin Last edited by McMurdo; 9th Apr 2015 at 12:22 am. |
9th Apr 2015, 12:26 am | #8 |
Heptode
Join Date: Jan 2008
Location: Oslo, Norway.
Posts: 632
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Re: Fuse with a difference!
As far as I can see the resistor is 1.2 ohm.
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9th Apr 2015, 10:28 am | #9 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Fuse with a difference!
Thanks Kevin
I've never seen one before, certainly not in an amp. Little Fuse know their stuff so it must be a sound idea, and from what mark pirate said it's not a new idea either? Hans, yes you're correct but the fuse measures 2R |
9th Apr 2015, 10:49 pm | #10 |
Heptode
Join Date: Feb 2011
Location: Washington DC, USA
Posts: 619
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Re: Fuse with a difference!
I have come across this before in VCR's, they used a low value low wattage resistor as a type of Slow-Blow fuse. Took me some time to figure it out as in my case the resistor was soldered into to the PCB and looked like a normal component
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David |
9th Apr 2015, 11:32 pm | #11 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
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Re: Fuse with a difference!
Could it be a fuseable resistor? I have used these in place of a "proper" fuse on projects over the years. They are available from RS and Farnell etc and come in various values to suit different overload currents.
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