Fuse on primary side of transformer

[ekdenton]

I am wiring half coil on the motors and parallel on the power supply. I got the power supply working today, I am getting a steady 35.1Vdc. I am thinking that when I talked to the guy at Gecko about the g203V's that he said to use a 3A slow blow fuse. Is that what you guys are using for the primary side fuse? I know the 203's have their own fuse but the primary side of the trannie should get a fuse also.

[ekdenton]

On the electrical plan there is a 1amp sloblow. I forgot about that being there. I think though 3amp slowblow would be correct for the 2amp motors.

[Richards]

When I build a new device, I normally use my multimeter to measure the current being drawn on the primary side of the transformer and then pick a slo-blow fuse that is rated about 25% higher than the current that I'm reading.

[Gerald D]

You cannot use the motor currents to decide the amps at the input side of the transformer - there is no relationship between those two factors. The transformer is transforming volts and amps :)

If you have a 300VA transformer, driven from 120 Volts, the Amps are 300 divide by 120 = 2.5 Amp.

Personally, I am not a great liker of fuses. In my case the transformer primary is connected directly to the main circuit breaker of about 20 Amp rating :eek:

[domino11]

Gerald,
What do you do if your transformer overheats and shorts out? You would have a full 20 amps available to the short circuit. What is your main problem with fuses? (If you don't mind me asking.) :)

Heath.

[Richards]

A fuse is the last resort to keep WIRING from heating to the point that it causes a fire. If ALL of the wiring in the circuit can handle the current of the circuit breaker of the main panel, then no fuse would be needed (technically); however, if ANY part of the wiring cannot carry the current of the rated breaker, then the wires MAY overheat and a fire MAY result.

As Gerald pointed out, "If you have a 300VA transformer, driven from 120 Volts, the Amps are 300 divided by 120 = 2.5 Amp." Putting a 2.5A fuse in that circuit will probably NOT work, assuming that you're actually pulling all the current that the circuit can handle because, most fuses are designed to blow at the rated current, not to pass the rated current. Most fuses will blow at a point up to 25% less than the rated current. (That's a safety feature to protect the wiring.)

In other words, if the wiring is rated at 15A (14 gauge, solid copper), then the fuse MUST blow before 15A is carried by the wiring -therefore the fuse is rated to carry a MAXIMUM of 15A at 120V.

About 25 years ago, one of my good friend's building burned to the ground because a fuse did not blow. He owned a professional photolab and he was using a little fish-tank heater to keep chemicals at the required temperature. To do that, he used a fish-tank heater that ran 24/7. Something happened and the fish-tank heater malfunctioned. The fire marshal stated that the cord of the fish-tank heater had burned through the cord's insulation and had ignited a wooden beam. When the wooden beam started to burn, the building caught on fire and my friend lost everything that he owned - all because the little 16-gauge cord overheated before it could carry enough current to blow the main panel's breaker.

[Gerald D]

Heath, we seem to have a reset culture as opposed to a replace culture over here. I sometimes do have to replace a fuse, and then it is a schlep to find the right fuse - not many people keep a wide range. Very tempting to fit a higher rating . . . . . .

You asked me what protects my transformer when it overheats and shorts out? Well, could I first ask what protects your cell phone charger when it overheats and shorts out? But, to answer your question, if my big transformer "overheats and shorts out", it is well on its way to protecting itself by burning like a fuse :) Seriously, at that point it is already beyond repair and in no need of further protection. (The secondary side fuses should be preventing the overloads)

If there is a known risk of something burning and causing collateral damage, we have to minimise that risk. Fuses and circuit breakers are a small part of that risk management.....using non-flammable materials, metal enclosures, quality wires and terminals, good insulation, are all part of that risk control.

[ekdenton]

Thought it might be cheap insurance for the torroidal to put one in. The fuses are readily avaliable here.

Richard, you are probably right that a 2.5amp would probably not work, but if you use what they call a slow blow fuse, it will work. The slow blow will take a higher sudden pull of current than a fast blow fuse.

[Richards]

Ed,
I agree partially that a 2.5A slow-blow fuse could be used to protect a 2.5A circuit. Usually there is a high-inrush current into the transformer. The slow-blow fuse is designed to allow that inrush current without blowing. In other words, it is a time-delay fuse that handles the unexpected for a certain amount of time. What actually happens is the fuse material just takes a little longer to get hot, initially, but if the high current persists, the fuse will blow.

Normally, a transformer rated for 2.5A on the primary side will almost never actually draw that much current. If it does, at least in my opinion, the transformer is undersized and should be replaced with a larger transformer.

The same principle is used with a garbage disposal unit in our kitchen sinks. A breaker protects the motor. If something is caught in the disposal and the motor can't turn, the breaker will 'pop' a few seconds after the unit is turned on. If you let the motor cool down a little, push in the breaker and try again, the breaker will pop again - until you remove the obstacle and allow the motor to spin. The breaker protects the motor, but more importantly, it protects the house. It will fail as long as any condition exists that allows the motor to pull too much current. It wouldn't matter whether you used a breaker or a slo-blo fuse, if everything were sized correctly, the breaker would pop or the fuse would blow before damage was caused.