A twisted tale about a logarithmic relay attenuator

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rhysh

rhysh

Member
Joined 2008
MrMajestic said:



Not sure, I was more referring to the fact that a linear motorpot was available. Better wait for the TPA guys to have a say before ordering. I used a 10K linear RK27 for the JT 1.0 and that worked great.
Click to expand...


Indeed. Yes thats what i was thinking, i had seen people use RK27's with 1.0, so i just assumed it would be okay with 2.0
 
S

sjalloq

Member
Joined 2006
Russ/Brian,

I sent you guys an email to the contact address on tpa.com and have subsequently found this thread.

One of my questions was regarding the type of controller you seem to be building for your AC1 and I'll repeat it here.

I was planning on using my own encoder along with MCU board. If I want to integrate it with your relay driver board what do I need to know?

Do you have a PIC on this board that connects via I2C to Volumite? I assume I just need to know whatever protocol you've used?

Oh, and what VFD were you planning on using. I had considered this but haven't looked into it further. Would be good if you could point me at something that would stimulate my thinking.

Thanks.
 
PB2

PB2

Member
Joined 2004
linkedin.com/in/petebasel
BrianDonegan said:
I'm working another angle on this, will test tonight and report back.

The relays are rates at 20mA each. 20mA * 14 (all on) = 280mA. PIC16F685 is 25ma max per pin * 7 = 175mA. Max current draw is 300mA. This keeps us well below the 1A that the 7805 can supply.

So, what else is drawing current? BC546 transistors, currently driven through 1K resistors. I will try making these 10K to see if we can drop current draw on the PIC and the entire circuit.

khaho: if you have the time, you could try it too, as you have a none problem (my relay boards don;t currently have the motorboating problem).
Click to expand...


I know that this post is old but I'm looking at LOG attenuators and just wanted to offer a comment.
While the individual spec for the I/O pins above is correct, the absolute total for the PIC16F685 PIC
is 200 mA for all the ports. Your calculation:
20mA * 14 (all on) = 280mA
obviously exceeds it. See page 211 in the spec sheet here pg 231:
http://ww1.microchip.com/downloads/en/DeviceDoc/41262E.pdf
Try to add a mute, relay, or power on relay and things start to get far worse.

Best practice is to use a true open collector relay driver such as the ULN2803 type. And to run the relays off
an independent regulator, not for better regulation to the relays but just to isolate the switching relay noise
from the digital supply. Something as simple as a 10 ohm resistor and say 100 uF cap to the relays might provide enough isolation.

There was talk of paralleling two relay boards for balanced XLR I/O configs and this would overload
even the individual pin I/O drive limit of 25mA.

A buffer of some type is really the best way to go on each relay board.

Pete B.
 
Russ White

Russ White

Member
Joined 2005
twistedpearaudio.com
PB2 said:



I know that this post is old but I'm looking at LOG attenuators and just wanted to offer a comment.
While the individual spec for the I/O pins above is correct, the absolute total for the PIC16F685 PIC
is 200 mA for all the ports. Your calculation:
20mA * 14 (all on) = 280mA
obviously exceeds it. See page 211 in the spec sheet here pg 231:
http://ww1.microchip.com/downloads/en/DeviceDoc/41262E.pdf
Try to add a mute, relay, or power on relay and things start to get far worse.

Best practice is to use a true open collector relay driver such as the ULN2803 type. And to run the relays off
an independent regulator, not for better regulation to the relays but just to isolate the switching relay noise
from the digital supply. Something as simple as a 10 ohm resistor and say 100 uF cap to the relays might provide enough isolation.

There was talk of paralleling two relay boards for balanced XLR I/O configs and this would overload
even the individual pin I/O drive limit of 25mA.

A buffer of some type is really the best way to go on each relay board.

Pete B.
Click to expand...


We don't (and never did) drive the relays from the PIC. The relays are driven by buffers. The newer version actually uses an AVR and not a PIC.
 
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