solid-state-relay
With mechanical relays, they can be inserted anywhere in a circuit.
Can SSRs be used anywhere in a circuit or are there certain gotchas that exist ?
Consider this simple example where SW* is a SPST SSR with a on resistance of 3mOhm.
Can I expect the the resistors to be shorted (excluding the 3mOhm ) when the SSR fires ?
simulate this circuit – Schematic created using CircuitLab
Using SSR or discrete FET switches for an audio attenuator is not the best idea when far better solutions exist. You should investigate the use of a digital pot for the attenuation core of the design. If you search at a site like http://www.mouser.com for digital pot you will find thousands of parts to pick from.
To eliminate the effects of capacitance on the wiper of the digital pot it is highly recommended to buffer the wiper of the pot directly into the high impedance input of an op-amp configured as a voltage follower.
Use of a higher total resistance digital pot can help to reduce loading on the source driving circuit.
To take care of the various types of source signals, that could be in a wide range of characteristics, I would recommend that you provide an input connector for each general voltage range of signal. Wire each of these input connectors to a suitable fixed input attenuator that scales the maximum input signal to be within range of the digital pot. Each scaled input would then be fed through a wafer style rotary input selector switch.
The selector switch output would then connect to the digital pot for further scaling based upon the actual signal level at the selected input. It would also be wise to use a very low capacitance diode clamp chip on the signal line going into the digital pot. This will help to protect the low voltage circuits (digital pot and voltage follower) if a user plugs a high voltage source signal into the incorrect input jack. The diode clamp device wants to one made with low forward voltage drop Schottky diodes and then connected into the GND and SUPPLY rails that define the operating range of the digital pot.
Depending upon the nature of the source signal attenuators it may be necessary to place some series resistance in the signal line before it gets to the clamp so as to limit the maximum current flow that could happen for incorrectly connected input signal selections.
It will sort-of work. The capacitance between terminals when the relay is off is typically about 65pF, so you'd see something like the green trace below (5kHz 50% square wave input shown) across the 100K resistor. Red trace is the 'PWM' input.
Best Answer
Yes. In the circuit you have drawn you want to use SSR's instead of mechanical relays as you will be hot switching them (i.e. current or voltage is present when you open or close the connection) and this will decrease the lifetime of mechanical relays. SSR's do not have this effect.
However, mechanical relays have an advantage that when they are open, they are OPEN! You get great isolation when they are open. There is no leakage and low capacitance. SSR's have a tradeoff of on resistance vs capacitance. The product of that R x C is a constant for most SSR's product families. Thus if you want low on resistance, you also get higher off capacitance.
one other advantage of SSR's is they open and close much faster than most mechanical relays. An advantage for mechanical relays is their on resistance is usually much lower than SSR's. So it all depends on what you are trying to optimize for...