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Tear me apart: Relay Step Attenuator

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BurnedResistor:
Ouff. Busy with some school stuff but finally had some time again to start working on the things that actually matter   :-+



--- Quote from: OldEE on March 26, 2020, 01:30:40 pm ---On the 3V3 line for the relays consider using a much larger capacitor to handle the inrush current.  Also consider a separate regulator for the processor.

--- End quote ---

Good call. I will add a second 5V line for the realys.

I did some looking around regarding what relay to pick.


--- Quote from: OldEE on March 26, 2020, 01:30:40 pm ---Since there will be no wetting current, and that's important, have a look at Panasonic Electric Works DS2E-S-DC3V relays.  They are designed for small signal work and have gold flashed silver contacts.

--- End quote ---

Those look very fancy, but I am a little size constraint and I dont see those available in a smaller package  :-\
 

--- Quote from: floobydust on March 26, 2020, 04:31:47 am ---You need a relay with AgPd contacts, and a minimum switching capacity specification that is uA, not A.

--- End quote ---

But after some digging the Panasonic TQ2SA-5V seem like a good bet:

* 10 µA 10 mV DC minimum switching capacity
* Au-clad AgNi type contacts
* Relatively small smd package
Plus I saw some reports online that they are used in some high-end analog mixing boards, which gives me some confidence.
Is there any other spec I should be looking at?


One last question:
The TQ2 Series Datasheet mentions " In case of 5 V transistor drive circuit, it is recommended to use 4.5 V type relay." so If I have them on a 5V rail with low side mosfet switching I guess I really should pick the TQ2SA-4.5? Or what else would they mean by "5v transistor drive circuit"?

https://www.mouser.at/datasheet/2/315/mech_eng_tqsmd-1304769.pdf

max_torque:
One "trick" i do with driving, especially turning on,  high inductive loads (in my case, automotive fuel injectors) is to overdrive them for a short period, ie to apply significantly more than their rated voltage to kick them into movement, and as they start to move, their inductance increases which further slows their transition.  Now obvs, this is a compromise and if overdone could shorten the life of the devices, bu tan effective "peak and hold" driver can be doen very easily, especially if your load is fixed, which yours is (it's a known relay that doesn't change.

There are various circuits to achieve peak and hold, depending on how efficienct you want the driver to be, in your case, you probably don't care too much, because overall power levels are low?
You might get away with a series resistor up stream of the relay, feeding into the relay coil + and in parallel, a capacity of a few uF.  Then, when you switch the low side to ground, initally, there is more voltage (and current) to at first drive the relay open, but once the boosting cap is "empty" then the supply current is limited by the resistor in series (which of course also has to waste power when the relay in on) On the turn off side, simply use a high voltage fet/transistor, and put a suitablly high rated diode around the relay coil to break down just before your driver does. There are various schemes using various cunning architectures to achieve this voltate boosting without significant losses. especially for loads that dont' actually switch much, so don't actually consume much power during the switch event!

(my fave is the one used for high voltage (~100V) direct injectors on modern DI engines, where the inductance of the injector itself is used as the boost element in a boost converter, and in the injector off period, the injector is driven on/off at too high a frequency to actually open the injector (ie within its dead time) but that can act to charge a HV holding capacitor, ready for next time the injector needs to be opened (and that cap also is arranged to re-capture the inductive energy on injector closing too! 


of course, you need to be careful, because too much voltage and current in an audio application might actually be worse than slowly switching or mis-switching your attenuation......    :-DD

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