Fast and slow
After 50 years of working with audio gear, it’s easy for me to forget not everyone knows the basics. I’ll do my best to help remedy that.
A few days ago we got into fuses, a subject that helped me realize not everyone knows there are two basic types. Fast and slow.
A fast fuse saves circuitry. A slow fuse saves lives.
First, let’s talk fast blo fuses. You don’t see these a lot anymore. They are typically used in the DC “rails” (the power supplies) that feed circuitry. They need to be quick because they are there to stop the voltage feeding circuitry before something bad happens to that circuitry. Their most common use is to protect power transistors in the output of a power amplifier. Where small signal circuitry like that of a preamplifier doesn’t have enough “juice” behind it to cause damage to the silicon, a power amplifier surely does. The fuse must die faster than the transistor it is protecting.
A fast blo fuse is basically a whisker-thin wire inside a glass (or ceramic) enclosure. Here’s a picture of one.
Fuses are designed into the power supplies that feed the output transistors of amplifiers. They are chosen by their rating of how much power they can pass before the little wire inside heats up and vaporizes—thus breaking the connection to the power supply.
Most amplifiers don’t have these because there are now more modern means of providing the quick shut off (other solid state devices).
A slow blo fuse has the same characteristic as the fast blo, meaning it too is chosen by the maximum amount of current (power) that is allowed to pass through it. The difference is time.
When we first plug in a product to the AC wall socket, or flick on the power switch, a surge of power flows into the unit. Perhaps you’ve noticed the room lights dimming on first turn on of a power hungry something. This inrush of current (power) needed to fill empty power supply capacitors (or start a motor or heater) is only for a brief moment. Thus, what we want is a fuse that will tolerate that momentary inrush of power until the device settles down.
That’s why it’s called a slow blo. Here’s a picture of one:
Instead of the whisker-thin wire of the fast blo, we now have what looks kind of like a spring. Pass enough current for long enough through the spring and it too heats up and vaporizes.
Slow blo fuses are found at the very input of products. Before the power supply. They exist to make sure that your product doesn’t light on fire, or that the wires in your home don’t light on fire.
Of course, modern homes are doubly protected. An electromechanical version of the venerable fuse is required for safety. It is called a circuit breaker.
Hope that helps.