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| awemawson:
Time will tell Russell, but I think so - the whole header tank gets all hot and bothered as soon as the inbuilt thermostat opens, as the engine holds a large volume of water which of course rushes there quite quickly. I expect that there will be some lag, and also the hysteresis in the system should prevent it flapping open and shut all the time. . . . but if it's not quite right I'll (try and!) sort it ! The other thing I need to sort out before I nail the lid on is arc suppression on the relay contacts. As the actuator is reversed by reversing it's polarity I cannot just put a quenching diode across it. So I suppose either a capacitor / resistor network across each contact pair (a pain physically) or maybe an MOV or varistor across the actuator itself As usual, all suggestions welcome ! |
| WeldingRod:
Are R/C type suppresor? Sent from my SAMSUNG-SM-G891A using Tapatalk |
| awemawson:
For a coil that is driven in only one polarity usually it suffices to put a (flywheel) diode across it in the reverse (ie non conducting) direction, so that as the relay contact opens the back emf of the coil is shorted. In this situation, this simple solution is not applicable as the coil can be driven in both polarities. Back to back zener diodes, an MOV or Varistor, or a simple capacitor in series with a low value resistor are possibilities. I think an MOV is the simplest but without robbing other equipment, I have non to experiment with ! |
| PekkaNF:
You prpbably know that MOV:s will take only limited amout of hit and then fail and short. You need a some sort of fuse on feeding line or limit the current safe way - unless it is intrinsically safe. Then again RC surpressor will fail eventually too. But I have used to see them in industry often on AC solenoid valves. This sort of thing is found to operate well. I think that since this is AC/DC this sot of stuff would work in switched polarity and would be a standard item. https://www.productsforautomation.com/murr-din-43650-formb-24V-mov-11021-p/7000-11021-2260150.htm Do you like that sort of component? Alternatively: If I had to build one, I would resort to ovekill it: I would put RC-snubber and the higher voltage MOV with series PTC-fuse tied to MOV. https://m.littelfuse.com/~/media/electronics/application_notes/reed_switches/littelfuse_magnetic_sensors_and_reed_switches_inductive_load_arc_suppression_application_note.pdf.pdf You know the standard rouge: MOV+PTC in series, but mechanically tied together with hitemp shrink tube or in same encapsulation (2-4 lead models). https://www.automotive-fuses.com/sale-11081692-thermal-protection-surge-suppressor-metal-voltage-dependent-resistor-mov-tmov25s-series.html Pekka |
| mc:
What about a couple Zener diodes in series back to back? However, I probably wouldn't be that concerned about arc suppression. I'm guessing most of those components will be rated for a good few amps at 240VAC, in which case a couple amps at 24VDC shouldn't be a major concern. If it was, I'd design the circuit so a couple pluggable relays were doing the actuator switching, so you could just change the relays if it did become an issue, however you're then into the possible area of the switch contacts oxidising due to lack of switching current. |
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