I added the 100ohm resistor to drop relay voltage to ~8V. (160ohm makes it 6V) Power dissipation in that resistor is around 3/4W. So it most likely needs to be chassis mounted for ultimate reliability. Not a big deal since it's in series with the coil but thought I'd point that out. On a positive note, the 5ohm (transistor current limiter) and TIP31 are running well within safe margins.
AndrewT said:
Like I said, I think it could be as high as 17VDC once rectified. I wouldn't dare run a 12V coil at this so the relay saver will drop to 6-8V depending on what I decide to do. I'll probably just get a 2W resistor and solder in-line if it takes up too much real estate on the board.
Pedant alert: You can't get 81 in standard E12/24 ranges. You must mean 82 
Even before we started talking about this extra stuff, you would have needed some series resistor for the relays, and as each user will want to use a different relay, that's added complication. Personally I would fix the relay type by mounting them on the PCB (although that has the added hazard of mains on PCB tracks, and a loss in flexibility). As in all engineering, there is no right answer; it's up to you to choose the best compromise.
By all means add capacitors and increase the series resistor - give your users the choice. My opinion is that the added complication might not outweigh the benefits here - what if they get the wrong value resistor, or the cap is a little small for their monster relay? Or worse - it's normally fine, but one day the mains is a little low? When the mains relay doesn't quite pull in, the contacts will fry and they'll blame you. IMO, such a scheme is safer when the supplies in question are *much* higher than the nominal relay coil voltage because normal variations in supply voltage won't change the chosen currents significantly.
I wouldn't risk 1n4148 diodes for anything larger than a small-signal relay. A 1n400x is cheap enough to make it a neglibable cost saving. Regarding the resistors, they will fit the PCB, but while a 2W resistor might seem generous, bear in mind it will get very hot. Don't mount it/them near any electrolytic capacitors or similar...
EDIT: Just looked at the diagram - the capacitor should be returned to earth.
Even before we started talking about this extra stuff, you would have needed some series resistor for the relays, and as each user will want to use a different relay, that's added complication. Personally I would fix the relay type by mounting them on the PCB (although that has the added hazard of mains on PCB tracks, and a loss in flexibility). As in all engineering, there is no right answer; it's up to you to choose the best compromise.
By all means add capacitors and increase the series resistor - give your users the choice. My opinion is that the added complication might not outweigh the benefits here - what if they get the wrong value resistor, or the cap is a little small for their monster relay? Or worse - it's normally fine, but one day the mains is a little low? When the mains relay doesn't quite pull in, the contacts will fry and they'll blame you. IMO, such a scheme is safer when the supplies in question are *much* higher than the nominal relay coil voltage because normal variations in supply voltage won't change the chosen currents significantly.
I wouldn't risk 1n4148 diodes for anything larger than a small-signal relay. A 1n400x is cheap enough to make it a neglibable cost saving. Regarding the resistors, they will fit the PCB, but while a 2W resistor might seem generous, bear in mind it will get very hot. Don't mount it/them near any electrolytic capacitors or similar...
EDIT: Just looked at the diagram - the capacitor should be returned to earth.
I'm in the process of laying everything out in Eagle. I'm pretty busy with work so it may take a few days. I did implement the extra cap/resistor for low power relay holding. I won't know the actual requirements for voltage until I have time to test but resistors are cheap and I can just get a bunch. Total for parts is still under $10 at mouser.
On a side note, does anyone know software well enough to tell me how difficult it would be to get a program running that sent a trigger signal through a PC USB or serial port. Voltage can range from 5v to 24v (although I know it wont be higher than 12v.) If the program could run in the system tray and act like a push button that would be great. ~100ms pulse is what I need.
On a side note, does anyone know software well enough to tell me how difficult it would be to get a program running that sent a trigger signal through a PC USB or serial port. Voltage can range from 5v to 24v (although I know it wont be higher than 12v.) If the program could run in the system tray and act like a push button that would be great. ~100ms pulse is what I need.
Alex:
The USB modules from here might help:
http://www.hobbyengineering.com/SectionIN.html
But you still need a programmer.
The USB modules from here might help:
http://www.hobbyengineering.com/SectionIN.html
But you still need a programmer.
alexcd said:
First, thanks for sharing this, I'm very interested in using this once it's done on another system in my home.
As for software, there is a great home automation program called HouseBot from CEBotics that's used to control all kinds of things.
It has several hardware interfaces supported. Some via USB, some via Ethernet (GlobalCache) and some via RS-232 (ADI Ocelot)
Using the ADI products, there are a lot of Relay controls available. I actually use this in my balanced power environment, where I use the ADI SECU-16 to manage the 24v coil voltage of 20amp DPST relays that then switch my balanced power outputs for the various parts of my system (sources, amp, amp, amp).
Using HouseBot, one can control the sequence of relay triggers to get amp delay on turn-on and Shut amps off before sources. Works great.
I even use it to control a monster 30amp relay on the input to the 5Kva Equitech Toroid that supplies the balanced power to the rig. Been very reliable for >5yrs.
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