[Shaleradio]

Another newbie question.  I want to add an in- line fuse to a 38-12 (115v, 40w) and a  39-6 (115v, 30w).  If my math is correct, the 38-12 pulls .35 amps ( is a .5 amp fuse adequate?) and the 39-6 pulls  .26 amps (also a .5 amp fuse?).  I've seen videos where the advise was to use 1 amp fuses but will this be too high?  Also saw one where a resistor was used in series with the fuse...is this necessary?  If so, how've to figure for that?
Mom was right...no matter where you go, there will be math!

[morzh]

Use 1A fuse, though 0.5A will do just fine, but the chance of accidental blow of the fuse is higher with it.
1A is adequate as the wiring inside is perfectly capable of more than 1A without going on fire so in case of a short it will blow.

[jcassity]

with low utility mains simulated..which actually happens more than people know,,,, utility sags and such......

40watts/95vac= .421A
to size the fuse its .421 x 1.20= .505A

thats where the half amp came from... prob they used the voltage low voltage value on the prints.

115v is nominal

1/2A on both but its possible the fuse would be to senstive so 1A as said is a better choice.

[morzh]

Jcassity

The old analog radios (or amps or what have you before the advent of the switching supplies) are not constant piwer devices so as your voltage goes down, your power does also.

[Shaleradio]

Thanks for the quick responses. Shout out to Morzh for help in the past, always a great help! Jcassity, just a question, your multiplication factor of 1.20 was a factor of 20% tolerance? Just trying to put this all together. Plus you introduced a voltage under 115 was that also an example of the low end of 115 voltage?
Finally, I guess a resistor is not necessary in this recipe?
As always...great forum for help for us new guys!

[Radioroslyn]

Just as an aside I found these online. Physically they are fairly small abt 3/4"long and come in .5,1,and 2amp sizes among others. .79c for a box of 5. A bit of tubing over it and it's good to go, no holder to mount.

[jcassity]

Jcassity, just a question, your multiplication factor of 1.20 was a factor of 20% tolerance?  

not a tolerane actually....
its a standard with nfpa to size an over current protection device.
no matter what your protecting be it analog or digi power supply, if you have some basic info than you can size any dc or ac fuse/breaker.

fuses dont open the circuit as good as a breaker,,, unless its an RK type fuse with a very expensive calibrated filiment.  Breakers always have a ink stamp near the handle of "value" but turn it sideways and look at the name plate data and there you will see thier actual "trip" rating. 


The reason i used 95vac was because thats what your power company will tell you they guarrantee you for voltage.
you could use your prints or wherever you find the operation range manf data.

you could use 100 or 105 for math pruposes only,, not for the application

if this were a college/tech school test question then i would never jot down 1amp for your 40watt device.
its much more electrically correct to use a 3/4amp but........

[Shaleradio]

Thanks for clarifying, that makes sense.

Terry, great advise on the fuse.  Thanks again for your help!

[Jim Dutridge]

I pretty much use a 1 amp fuse in all of my radios, no slo-blows.

[WallaceRoger]

I pretty much use a 1 amp fuse in all of my radios, no slo-blows.

Me too.

[Phlogiston]

One thing to keep in mind is, that the correct fuse will probably blow if the radio is turned off and on rapidly. (kids in display area?) which might be an advantage for a slow blow, or at least a fuse holder.

[morzh]

A few things to keep in mind when sizing up a fuse: (and yes, for regular radios, up to 7-8 tubes, I also use 1A fuses)

1. A fuse is not there to protect your equipment, it is to protect you and your property from the equipment.
2. There is no such thing as exact value you should be using - it is always a compromise.
3. A fuse loaded up to and some 80% above the rated value does not blow, slo-blo or fast acting. Reliable blow-up from 1 to 10s occurs at 2-3 to 5 times the rated current.

Now, as long as your wires are thick enough and do not start going on fire before your fuse blow it is OK to have a higher value fuse. The wires of AWG 20 which many of us use are good up to 11.5A of normal operation, and then some.

Components when blow might release the smoke but rarely will draw enough current to blow a fuse, even properly sized.

In the transformered radios except the primary of the transformer and the filter capacitor (back in the days - today the safety caps are guaranteed to fail open) there is little that could cause a fuse to blow. And what can blow the fuse (the primary short or something very bad on the secondary side - in a regular radio I am not even sure what) will create currents that will blow 1A fuse as well as 2A fuse, as well as 3A fuse.
So, for a small table radio 1A is good, even a Slo-Blo, plus it will withstand the accidental transformer saturation (which Russ mentioned) and for a 37-116 or 37-690 2A fuse if good.

Again, a radio is not a constant power device so if the AC voltage goes down it does not mean the current will go up - no, it will also go down, proportionally more or less.

Those who want to protect as much as possible could try to install the fuses of smaller values commensurate with the current they protect after the transformer, like protecting filament winding or B+, (Philco used a fuse protecting the B+ feeding the output audio transformer, not that it helped much) but that could be tricky and all kind of scenarios should be considered. I would not do it.