05-05-2015, 10:13 PM
Hi David,
By way of much-needed distraction, I just knocked up the transistor circuit on a breadboard, and it works fine for me. I upped the timing resistor from 100k to 330k to stretch the delay to about 3 or 4 seconds, and I used a pair of BC108s. The relay coil takes about 30mA. I tested it with a bench power supply, and would probably test it with an unregulated noisy supply before committing to a build.
Which reminds me - without re-reading the whole thread, I don't think that anyone has mentioned an important detail about the power supply: if you take the 12VAC from the existing transformer and full-wave rectify it, you'll get about 16 to 18 volts DC, depending on how close to 12V the transformer actually is. The 555 will probably be OK, but the relay coil will take more current than you expect. Rather than going to the complexity of a voltage regulator, I'd use a series resistor, chosen to result in 10-12 volts across the coil.
Your earlier PCB layout suggests that the smoothing capacitor will be 100uF - if so, it's worth checking the ripple on the supply rail, and ensuring that it's not confusing the 555 IC. Of course, the sudden voltage drop when the relay coil energises might be a factor here...
Right - back to real life
Cheers,
Mark
By way of much-needed distraction, I just knocked up the transistor circuit on a breadboard, and it works fine for me. I upped the timing resistor from 100k to 330k to stretch the delay to about 3 or 4 seconds, and I used a pair of BC108s. The relay coil takes about 30mA. I tested it with a bench power supply, and would probably test it with an unregulated noisy supply before committing to a build.
Which reminds me - without re-reading the whole thread, I don't think that anyone has mentioned an important detail about the power supply: if you take the 12VAC from the existing transformer and full-wave rectify it, you'll get about 16 to 18 volts DC, depending on how close to 12V the transformer actually is. The 555 will probably be OK, but the relay coil will take more current than you expect. Rather than going to the complexity of a voltage regulator, I'd use a series resistor, chosen to result in 10-12 volts across the coil.
Your earlier PCB layout suggests that the smoothing capacitor will be 100uF - if so, it's worth checking the ripple on the supply rail, and ensuring that it's not confusing the 555 IC. Of course, the sudden voltage drop when the relay coil energises might be a factor here...
Right - back to real life
Cheers,
Mark
