EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: gooseEL34 on June 27, 2016, 11:33:51 pm
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Check the schematic attached. It is from a vacuum tube guitar amplifier. Fuse one is a 3A fast blow that is mounted in a chassis mount fuse holder. The 5A is a fast blow mounted to the pcb. From that fuse you go directly into the AC transformer. My questions:
1. What is the fuse in the dotted box that is third in the line? I do not see a corresponding component. Is it something internal inside the AC transformer ??
2. Why would someone design an amp with 3 fuses in series?
3. Is there a chance the on of switch is fused internally and this is a schematic mistake?
Thanks
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There is no technical purpose to having more than one fuse in series, unless the power cord is non-polarized (in which case, fusing both wires has some use when the primary transformer isolation goes short). But you should consider who the product customer is: if it is someone stupid enough to defeat the fuse by stuffing aluminum foil around it, then having an internal fail-safe with a higher rating may be good insurance.
Switches normally do not have internal fuses, but combined switch/inlets often do. Things like this: (https://images-na.ssl-images-amazon.com/images/I/61N9ClGWJ1L._SY355_.jpg)
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There is no technical purpose to having more than one fuse in series, unless the power cord is non-polarized (in which case, fusing both wires has some use when the primary transformer isolation goes short). But you should consider who the product customer is: if it is someone stupid enough to defeat the fuse by stuffing aluminum foil around it, then having an internal fail-safe with a higher rating may be good insurance.
Switches normally do not have internal fuses, but combined switch/inlets often do. Things like this: (https://images-na.ssl-images-amazon.com/images/I/61N9ClGWJ1L._SY355_.jpg)
I've never done it, but wouldn't one potential reason be to have a fast-blow and a slow-blow? For example, a fast-blow 5A fuse that will immediately fail when exceeded and a slow-blow 3A fuse that could sustain a current over 3A for a couple minutes at a time?
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Fuse 3 is a thermal fuse - the clue is the 157 deg C temperature rating. It is almost certainly buried inside the transformer in intimate contact with the primary winding and is there to cut the power if the transformer overheats due to being persistently overloaded.
Regarding the series fuses, with one concealed on the PCB, well that's just about the minimum to make it reasonably bass player and roadie proof - the designer anticipated some a===hole substituting a chunk of 1/4 UNC bolt for the externally accessible fuse!
The limit on the current carrying capacity of a 1/4" steel bolt 'fuse' is the wires desoldering themselves from the fuseholder contacts due to resistance heating, or at least that's what a friend who used one for an emergency field expedient repair of an Army radio told me. . . .
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I have seen fuses in series were the internal fuse was used by the design engineer to disable part of the system. It was left in to allow this part of the system to fail but allow the rest (supplied from before the fuse) to function.
The other possibility is that the PCB with the second fuse came from an existing system that was not re-engineered to remove it.