jimbob_400 wrote:Would anyone care to explain how the Cusack (or any) style relay switching design works?
I've also noticed the similarities between lovepedal gear and cusack gear... I remember reading somewhere that john cusack gets the switches made for him or something and so I was a bit surprised seeing the same style switches on a lovepedal design.
Hey, jimbob.
Just as there are a million ways to design an preamp, there are a million ways to design a relay bypass system.
Here are some ideas to consider:
The most common I've seen in "DIY" designs use non-latching relays (power applied to the relay to engage) and a latching footswitch. This is a cheap and easy way to make a relay work. You will switch power (or ground) to the relay coil, hopefully, there is some type of current limiting designed into the network. Once the power is applied to the relay coil, the switch engages and you're in business.
The downsides to these designs are plentiful (in stompbox application). In many modern amplifiers, you will see similar schemes, but the current consumed by the relay switching system is nominal when compared to the total current drawn by the rest of the circuit. In a stompbox, current consumption can often be a concern, and for this reason, non-latching relay bypass systems can be a bad idea; or if current consumption is a non-issue at best a moot point. To compound the issue, you still must use a latching switch, or design an bistable oscillator or flip-flop to manage the current to the coil, and thus the state of the effect circuit. This is where things get a little more complicated and cost becomes an issue. AND, you've gotta design a separate indicator circuit, if you want one. As I see it, why mess with all the extra parts if you're still using the same fundamental parts to control the addition?
Another alternative is a latching relay system. Latching relays require power to be applied to the coil for a short duration of time in order for the relay to toggle states and "latch" into the desired position. Sounds simple enough, right? It gets tricky, and here are a few reasons why:
Latching relays generally require either two separate coils (engage and disengage) or reverse polarity of the coil. It would make sense to just use the variety with two separate coils, but then you must design a flip-flop to select which coil you wish to engage. You also must keep the power applied for a short duration (recommended operating conditions can be found in the part specific datasheet). Simple enough, right? Kinda. Get a few and begin playing around with some designs. Its lots of fun!
The other flavor of latching relays require reverse polarity of the coil (that is to say, + and - must be alternated to toggle the relay). So, with this variety, you must now design a flip-flop that will swap POLARITY of the output, and be able to sink/source enough current, for a specified duration, to drive the relay coil. Sounds easy enough, right? We haven't talked about some of the nuances to inductors used in this fashion yet. Here's where it gets really fun (er, difficult).
The coil in the relay is really just an inductor. Inductors exhibit an interesting behavior when you remove power from them. They produce a spike in current as the magnetic field (produced by the current flowing through the coil) collapses. Its really quite simple when you think about it. However, this presents its own problems and considerations. Depending on how you've designed your control circuitry, this could present a major problem, when negative suddenly becomes very positive and positive suddenly becomes very negative. There are ways to compensate/adapt to this inherent behavior. Inductive Kickback diodes can be employed to allow a constant current path for the sudden reverse voltage. However, if this diode is placed as a permanent fixture in the coil's network, the relay will only be allowed to operate in one way (as reversing the current will circumvent the coil and simply flow through the diode--probably not a good thing). Now we're getting into the fun stuff. It gets better.
Another nuance to inductors and magnetic fields is the inductive nature of current. Think about an alternator: basically, it is just a magnet (or group of magnets) passing over a coil (or coils) of wire. When the magnet passes close enough to the coil, electrons are drawn toward the magnet. As the magnet begins to travel farther away from the coil, the electrons return to their "original" position. Well, something has to fill the void as all the electrons migrate towards the magnet, and what does? more electrons. Once they return to their original positions the electrons who took their place must return as well. So we have electrons moving around, changing the potential difference of the conductor resulting in what we call alternating current! Sorry for the side track, but its important to understand what happens in the relay. The same concept happens when the coil saturates. A magnetic field builds up around the coil (and whatever conductor happens to be in the vicinity) when power is applied to the coil, said field also collapses when power is removed from the coil. This results in the kickback voltage discussed above. The downside is that this voltage can also be induced into any nearby conductors. it will likely be a nominal voltage, but if there are sensitive inputs to amplifiers (in a stompbox, this can be a concern...) the amplifiers may pick up this small voltage and turn it into a BIG voltage, resulting in a sudden noise in the circuit.
Those are a few considerations as to the operation of a relay bypass circuit for stompbox application. Its up to the designer to make everything perform.
This is getting kinda long, but I think you can draw some conclusions based on the information above.
). It costs current, but I don't use batteries, and it is more reliable in the long term than the