Need some more assistance from my Forum friends. I am building a circuit where I need a 1.5 Amp circuit to protect the transformer I will be using. (120VAC to 16 VAC at 1.875 Amp. The local electrician says to protect it at approxomately 75% of rating as I don't know the load I'm drawing. 1.5 Amp is 80%.) The inline fuse circuit I have uses an SFE fuse. It came with a high amp fuse. However, I am unable to find a SFE fuse that goes that low of amperage and from what I've read about SFE fuses it wouldn't be the right length I need either (1/4 x 1 1/4"). Can a fast attacking AGC fuse of the right amperage be subbed? Electronics like that are not my strong suit. As usual thank you for any assistance that can be provided.
Back when I used Bussmann 'Littlefuse' devices I believe nearly all of mine were AGC.
https://m.littelfuse.com/~/media/automotive/datasheets/fuses/automotive-aftermarket/glass-fuses/littelfuse_aftermarket_glass.pdf
AGC only denotes Automotive Glass Cartridge.
SFE fuses vary by length for their equivalent amperage and you need to match the fuseholder (length of spring) to the fuse amperage.
Good Luck, Ed
I wouldn't use a fuse at all; I'd spend a few dollars on eBay or electronic parts supplier and get a suitable 1.5A 120V breaker.
Now, be advised that 'conservative' rating may be cumulative. A "1.5A" breaker may only support something like 1A continuous current. If I understand correctly this is on the input side of the transformer, so any internal losses would further reduce the permissible amp draw from the secondary.
There are often concerns like inrush current that might make 'fast blow' fuses a poor choice for this application. If a transformer is rated 1.875A, short-term operation at that current would only produce a rated temperature rise, not immediate electrical damage, but a fast-blow fuse would promptly go. I think slow-blow (as in many automotive fuses) would be wiser if you retain fuse protection.
Overmod get a suitable 1.5A 120V breaker.
not quite the same thing.
2A at 16 V is 32W of power. 1.5A at 120V is 180W or 11.25A at 16V
greg - Philadelphia & Reading / Reading
Overmod Now, be advised that 'conservative' rating may be cumulative. A "1.5A" breaker may only support something like 1A continuous current. If I understand correctly this is on the input side of the transformer, so any internal losses would further reduce the permissible amp draw from the secondary. There are often concerns like inrush current that might make 'fast blow' fuses a poor choice for this application. If a transformer is rated 1.875A, short-term operation at that current would only produce a rated temperature rise, not immediate electrical damage, but a fast-blow fuse would promptly go. I think slow-blow (as in many automotive fuses) would be wiser if you retain fuse protection.
This is on the output side where I'm planning to put the fuse. I would prefer to have to replace a fuse than having to replace the entire transformer.
why not use a 2A agc fuse with a common fuse holder?
FRRYKidThis is on the output side where I'm planning to put the fuse. I would prefer to have to replace a fuse than having to replace the entire transformer.
Okay, now that all the experts have had their say, I was always under the impression that the purpose of a fuse or breaker on the output side of a power supply was to protect the devices downsteam of the supply, not the power source itself.
Am I incorrect?
maxman I was always under the impression that the purpose of a fuse or breaker on the output side of a power supply
I'm with the understanding that the fuse can serve several functions.
A short across the power supply can overheat the supply, damage the rectifier if there is one or melt the wiring inside the enclosure. Especially true of an automobile battery. A dead short across the starting circuit can be disastrous, thus the 'fusible link'.
Still, the fuse protects downstream by removing the power from the 'defective' circuit and alert the user of said defect.
Fuse = electricity's saafety valve.
Regards, Ed *not an expert*
maxman was to protect the devices downsteam of the supply
a ground fault circuit interrupter (GFCI) breaks the circuit preventing harm before a short caused by a person touching some electric device and a plumbing fixture which is the ground in home electric systems
home circuit breakers prevent drawing excessive current which could harm a person or result in a fire
a device on a model RR connected to a power source isn't damaged by a short unless it's internal to the device causing more current to be drawn thru a portion of that device than it is rated for
a short typically causes a drop in the power source voltage which wouldn't damage a device drawing currrent from a power souce
however, a short in a lamp circuit can of course damage to a power source which would be protected by a fuse.
DCC power supplies have protection circuits to prevent excessive current being drawn and being damaged by overheating.
regulated DC supplies could have a current limiting circuit
maxman FRRYKid This is on the output side where I'm planning to put the fuse. I would prefer to have to replace a fuse than having to replace the entire transformer. Okay, now that all the experts have had their say, I was always under the impression that the purpose of a fuse or breaker on the output side of a power supply was to protect the devices downsteam of the supply, not the power source itself. Am I incorrect?
FRRYKid This is on the output side where I'm planning to put the fuse. I would prefer to have to replace a fuse than having to replace the entire transformer.
You are correct. With residential and commercial wiring, the breakers are actually protecting the wire (except GFC breakers also protect people, as Greg pointed out).
That said, there are actually two sets of wiring in a transformer, the primary and secondary. To protect the primary, fuse the line voltage wiring from the wall outlet to the transformer. If this is 1.5A, then at 120V you'd need a 250ma fuse. These are common in 5 x 20mm for IEC inlets. For direct wiring, use an inline 5x20mm holder. They're 1-2 bucks, usually. A slo-blo is best for a transformer, because they can tolerate inrush currents and you don't want nuisance tripping, else it might induce you to bypass the fuse.
If you have no fuse whatsoever on the primary side, then it has no protection at all - from line surges, lightning strikes, etc.
If you place a fuse on the secondary, it's still there to primarily protect what's downstream. If you are distributing that 16VAC through direct wiring, then you should use a fuse or circuit breaker to protect that wiring and the devices connected to them.
This can all be confusing because outside of this hobby, "transformer" means a bare device - a metal core with two or more windings with input and output connections directly to the bare device. In model railroading, it could mean a box with protective devices already enclosed in the box, among other things, in addition to a "bare device with a metal core and windings."
IC_TomIf you place a fuse on the secondary, it's still there to primarily protect what's downstream. If you are distributing that 16VAC through direct wiring, then you should use a fuse or circuit breaker to protect that wiring and the devices connected to them.
how does it protect what's downstream?
think about it this way, you have a dozen lamps on some circuit supplied by a wall wart and a fuse. while wiring another device you create a short. the fuse burns out and you realize that what you just did causes a short.
if you just replace the fuse, the fuse burns out again
if you undo what you just did and replace the fuse, things work again.
if you don't have a fuse, the wall wart burns out (or the fuse inside the wall wart does, possibly preventing a fire,)
if you undo what you just did and replace the wall wart, things work again. nothing is damaged except the wall wart
a short across the power supply doesn't cause any extra current to flow thru the devices supplied by the power supply. the short routes all the current from the power supply back to the power supply
OvermodIf I understand correctly this is on the input side of the transformer,
No, he said it was on the output side.
"a ground fault circuit interrupter (GFCI) breaks the circuit preventing harm before a short caused by a person touching some electric device and a plumbing fixture which is the ground in home electric systems"
Not at my house, nor any other house that has plastic feed/drain piping.
"a short typically causes a drop in the power source voltage which wouldn't damage a device drawing currrent from a power souce."
I am not smart enough to agree or dispute this statement. However, since we are speaking about model railroad application I know with complete certainty that it is possible that a short can cause damage if the current draw is not quite enough to trip the breaker between the track and the control system.
It would be helpful if the OP told us exactly what he was trying to do and provided a circuit diagram. If the circuit is a published item, then I would presume that some competent individual verfied the components. If that is the case, then any advice to change components, especially without knowing the particulars, is ill-advised.
If the OP is designing the circuit (and he has already stated that electronics is not his strong suit) I don't see how anyone can give any sort of advice without knowing exactly what he is trying to do. Sounds down right dangerous to me.
gregc IC_Tom If you place a fuse on the secondary, it's still there to primarily protect what's downstream. If you are distributing that 16VAC through direct wiring, then you should use a fuse or circuit breaker to protect that wiring and the devices connected to them. how does it protect what's downstream? think about it this way, you have a dozen lamps on some circuit supplied by a wall wart and a fuse. while wiring another device you create a short. the fuse burns out and you realize that what you just did causes a short. if you just replace the fuse, the fuse burns out again if you undo what you just did and replace the fuse, things work again. if you don't have a fuse, the wall wart burns out (or the fuse inside the wall wart does, possibly preventing a fire,) if you undo what you just did and replace the wall wart, things work again. nothing is damaged except the wall wart a short across the power supply doesn't cause any extra current to flow thru the devices supplied by the power supply. the short routes all the current from the power supply back to the power supply
IC_Tom If you place a fuse on the secondary, it's still there to primarily protect what's downstream. If you are distributing that 16VAC through direct wiring, then you should use a fuse or circuit breaker to protect that wiring and the devices connected to them.
You're misinterpreting "downstream" and forgetting the wire. The distribution wiring is downstream of the transformer secondary.
The downstream device may not be a light, but a motor, instead. If allowed to run overloaded, it will draw more current. The breaker/fuse can protect that, even if the transformer can supply more current.
In any event, I was simply addressing whether it was a reasonable belief that a fuse protects the device/wire that comes after (downstream) the source of current, and it is.
Going back to the OP's original question - yes, a fuse/breaker on the downside will also protect the secondary windings in the power supply. In his particular case, that might be wise because as stated, the load is unknown. However, the transformer is still unprotected if there is no fuse ahead of the primary.
maxman Overmod If I understand correctly this is on the input side of the transformer, No, he said it was on the output side. "a ground fault circuit interrupter (GFCI) breaks the circuit preventing harm before a short caused by a person touching some electric device and a plumbing fixture which is the ground in home electric systems" Not at my house, nor any other house that has plastic feed/drain piping. "a short typically causes a drop in the power source voltage which wouldn't damage a device drawing currrent from a power souce." I am not smart enough to agree or dispute this statement. However, since we are speaking about model railroad application I know with complete certainty that it is possible that a short can cause damage if the current draw is not quite enough to trip the breaker between the track and the control system.
Overmod If I understand correctly this is on the input side of the transformer,
This is primarily because we don't fuse or breaker our locomotives themselves. It's probably not practical, but I've wondered why expensive decoders don't have built-in fuse protection.
It would be helpful if the OP told us exactly what he was trying to do and provided a circuit diagram. If the circuit is a published item, then I would presume that some competent individual verfied the components. If that is the case, then any advice to change components, especially without knowing the particulars, is ill-advised. If the OP is designing the circuit (and he has already stated that electronics is not his strong suit) I don't see how anyone can give any sort of advice without knowing exactly what he is trying to do. Sounds down right dangerous to me.
Great statement and sums things up well!
maxman It would be helpful if the OP told us exactly what he was trying to do and provided a circuit diagram. If the circuit is a published item, then I would presume that some competent individual verfied the components. If that is the case, then any advice to change components, especially without knowing the particulars, is ill-advised. If the OP is designing the circuit (and he has already stated that electronics is not his strong suit) I don't see how anyone can give any sort of advice without knowing exactly what he is trying to do. Sounds down right dangerous to me.
The circuit was discussed with a licensed electrician. He was the one that gave me the inline fuse cord. I originally needed info on the different types of fuses.
As to the grumble about the diagram, let's try this link: https://crcit.net/c/6c3001d4b00b453fa02a758aba3aa442
I don't have the plug on the input end and I don't have all the lighting on the output end. The toggle switches represent two Atlas Connectors that control all the lighting. All six switches don't have items hooked up. Only 1, 4, 5 and 6 have anything hooked up to the terminals.
IC_Tom However, the transformer is still unprotected if there is no fuse ahead of the primary.
However, the transformer is still unprotected if there is no fuse ahead of the primary.
I don't know if this counts as protected or not, but this is how the particular circuit is set up going backwards from the transformer: It plugs into a strip with a circuit breaker on it, that strip plugs into a GFCI outlet (as this is in an detached garage), that GFCI is protected by a 20 amp fuse and that fuse is also on a line that is protected by a breaker in the main box in the house.
FRRYKidThe toggle switches represent two Atlas Connectors that control all the lighting.
lamps, fuses are typically used to protect the supplies for accessories (lamps, switch machines, stationary decoders, ...)
IC_TomThe downstream device may not be a light, but a motor, instead. If allowed to run overloaded, it will draw more current.
the maximum current drawn by a motor is when the armature can't turn, there is no bemf and the current is limited by the wire resistance.
would you put a fuse inline with your track to protect a loco motor?
model RR track power supplies have currrent limiting circuits to protect the supply because shorts are common in model RRing.
FRRYKidThe circuit was discussed with a licensed electrician. He was the one that gave me the inline fuse cord. I originally needed info on the different types of fuses. As to the grumble about the diagram, let's try this link: https://crcit.net/c/6c3001d4b00b453fa02a758aba3aa442 I don't have the plug on the input end and I don't have all the lighting on the output end. The toggle switches represent two Atlas Connectors that control all the lighting. All six switches don't have items hooked up. Only 1, 4, 5 and 6 have anything hooked up to the terminals.
The "grumble"? What "grumble" are you talk'n about Willis? In order to get an accurate answer all the facts need to be presented. Had you shown the circuit up front and stated that you just want a fuse to fit, that would have eliminated all the circuit breaker, GFI, and other non-related answers.
Here is the circuit from the link:
So what you have is the xformer and the wire(s) to the two Atlas connectors. Once you make the conections to the Atlas items you will end up with 6 parallel circuits. I presume that the 1.5 amp fuse value was drawn in by you based on the electrician's advice.
You did not give the xformer output wattage value (volts X amps), but I presume that it is around 30 watts. This is based on the information you provided (16 v at 1.875 amps).
Anyway, if the transformer can put out 1.875 amps any fuse you use with a rating above 1.875 amps will not protect anything. You might as well replace that with a piece of wire. Hence the electrician's recommendation of 1.5 amps.
So what does the in-line fuse wire look like. Something like this:
The wire on the item shown is 22 gage. Is the wire on yours about the same, or a lot heftier? For what you are doing I don't think you need a holder any different than the one shown.
Can you ask the electrician where he got the fuse holder? If he knows the simple expedient would be to go to that place and ask for a fuse the fits.
So, yes, a 1.5 amp fuse will work. However, a more important question I would be asking is do you really want 1.5 amps available to go through the lighting circuits? Because the 6 circuits will be in parallel, each will have the same 14 volts across. However, the amperage through the 6 circuits will be additive. So with all 6 switches on and all with the same load each circuit will draw one-sixth of 1.5 amps, or 0.25 amps. (I think this is 250 miliamps). Assuming a 30 milliamp bulb requirement, you could have 8 of these on each circuit.
David Popp did an MR article that you might be interested in looking at: https://www.trains.com/mrr/how-to/expert-tips/11-easy-urban-lighting-tips-for-your-model-railroad/
For a bunch of lights on one of the project railroads he used a 12 volt, 1.25 amp wall wart.
Anyway where I was going with this is that maybe you might wish to install one of the in-line fuse holders in each of the 6 circuits and use a smaller value fuse in each. The fuse holders seem to be relatively inexpensive.
Oh, as to what the in-line fuse protects, any one who is interested can view the following link: https://monroeengineering.com/blog/the-beginners-guide-to-in-line-fuses/#Benefits%20of%20In-Line%20Fuses, the beginners guide to in line fuses. Among other things it states "You can install in-line fuses directly in circuits. A circuit may consist of an electrical wire running from a battery to a device. To protect the device from overcurrent, you can install an in-line fuse directly in the electrical wire."
maxmanAnyway where I was going with this is that maybe you might wish to install one of the in-line fuse holders in each of the 6 circuits and use a smaller value fuse in each.
so lets say the wall wart is rated at 1.5A and you put a fuse in series with each lamp. lets say each lamp draws ~0.1A and you use a 0.5A fuse
what happens when adding the 16th lamp. the 1.5A rating on the wall wart is exceeded and the wall wart may burn out. all those fuses didn't protect the more expensive wall art.
yes, you shouldn't add that 16th lamp. are you protecting the wall wart or a fuse.
as mentioned, you're more than likely to cause a short while wiring in another lamp.
it's common practice to wire a fuse next to the power supply ... just like the diagram
the benefits of an inline fuse is when you don't have place to mount a common fuse holder
gregco lets say the wall wart is rated at 1.5A and you put a fuse in series with each lamp. lets say each lamp draws ~0.1A and you use a 0.5A fuse what happens when adding the 16th lamp. the 1.5A rating on the wall wart is exceeded and the wall wart may burn out. all those fuses didn't protect the more expensive wall art. yes, you shouldn't add that 16th lamp. are you protecting the wall wart or a fuse.
Since you are still stuck on the protecting the power source thing, you obviously did not read the link I provided about what the in-line fuse protects.
Further, your hypothesis states that there would be a 0.5 amp fuse in series with each lamp. So when you add the 16th lamp (don't know where you got that number) there would also be a 0.5 amp fuse in series with it. So all that would happen would be that fuse would fail and everything else would be okay.
The scenario I talked about was 6 parallel circuits coming from the Atlas devices each with quantity 8, 30 milliamp bulbs. If each of these circuits was protected by the 1.5 amp fuse all that would happen would be that the 1.5 amp fuse would fail. Everything else would survive.
maxmanSince you are still stuck on the protecting the power source thing, you obviously did not read the link I provided about what the in-line fuse protects.
I did. it says:
To protect the device from overcurrent, you can install an in-line fuse directly in the electrical wire.
Overcurrent can cause catastrophic damage to devices. It will expose the devices to heat, which in turn can damage them. Too much heat, in fact, is a fire hazard. Devices can trigger building fires due to overcurrent. In-line fuses and other fuses prevent problems such as these from occurring.
i didn't see where it said the fuse protects the fused device from damage
you're suggesting that somehow the lamps starts drawing more current and the fuse blows to protects it. if it did, won't it do it again when the fuse is replaced?
-- the most likely cause of a short is while wiring --
maxmanSo when you add the 16th lamp (don't know where you got that number) there would also be a 0.5 amp fuse in series with it.
adding the 16th (0.1A) lamp increases the current drawn from the supply to 1.6A which can damage the supply if it's rated at only 1.5A
maxmanSince you are still stuck on the protecting the power source thing,
devices like lamps don't have built in fuses, but things that supply power to other devices: power supplies, DCC booster, higher powered audio amplifiers have fuses or current limiting circuits in their output to protect them from damage.
The inline fuse protects anything it is 'inline with' from overcurrent beyond a certain strength and duration. That presumes the fuse is not, as mentioned previously, in one parallel branch and not in one of the wires exiting the transformer.
If the transformer has multiple taps, each one would need a fuse of appropriate current rating for the 'whole' of the transformer to be output-protected.
All the initial discussion appeared to involve an electrician recommending a 1.5A maximum current for a transformer rated at 1.875A nominal, to "protect the transformer". An inline fuse or breaker will accomplish this by opening the secondary rather than the primary (which I assumed was going to be connected, like a doorbell transformer, to a typical 120VAC branch and its breaker, let's say 15A thermal rather than magnetic).