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| Am I missing any potential pitfals with this soft start / power sequence design? |
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| Metatronic_Mods:
I'm just wondering if there's any glaring problems with this design which I have overlooked. The basic idea is to use a relatively large resistance to limit the inrush current to the transformer and reservoir caps. Once the caps charge to a specified voltage, relay RL1 closes, bypassing the resistor array. Then at some slightly higher voltage RL2 closes, connecting the load to the caps. RL2 is necessary in order to ensure the supply voltages come up with a fast edge and reach their specified levels at roughly the same time, otherwise one of the micros can be pretty temperamental. I know without RL2 (ie. load always connected) the res caps would never be able to reach a sufficient voltage to close RL1 (not with such a large ballast resistance at least), and from my understanding this is one of the reasons inrush current limiting for the transformer and res caps is advised to be handled separately. The only other issue I can see (vs. existing valid designs) is overload protection in the event RL1 fails to close; which is overcome easily enough with the addition of F2. Is there anything else I should be worried about? (Never mind about particular component values, or the relay coil charge/hold circuitry - it may prove over simplistic as drawn, but I plan to cross that bridge after I select which relays to use). |
| Gyro:
The only real issue [EDIT: issues] I can see (assuming all your RC time constants work with the relay coil resistances), is that the relays will close relatively gently as their coil currents increase. Slow closure could possible lead to excessive burning of the contacts (or maybe not), probably more of an issue on the mains contact one. The whole thing would be a lot more predictable if you used a couple of comparators to monitor the capacitor bank voltage and actively switch the relays at the required voltage thresholds. Relay switching the supply to your "temperamental" micro's regulator could also cause problems due to contact bounce - A MOSFET might be better. |
| Metatronic_Mods:
--- Quote from: Gyro on April 09, 2019, 03:24:28 pm --- The whole thing would be a lot more predictable if you used a couple of comparators to monitor the capacitor bank voltage and actively switch the relays at the required voltage thresholds. --- End quote --- 100% agree. There's a few solutions I'm considering, dependent on PCB area budget. The RC networks were really just placeholders. --- Quote from: Gyro on April 09, 2019, 03:24:28 pm --- Relay switching the supply to your "temperamental" micro's regulator could also cause problems due to contact bounce - A MOSFET might be better. --- End quote --- My only concern would be the extra heat dissipation. But I don't expect bounce to be a problem. The problem with the micro is due to a Vref and Vdd connected to separate rails, so long as they stabilize at nominal levels within ~10ms of one another there's no issue. Still, something to thing about that I hadn't yet, thanks! |
| Gyro:
--- Quote ---My only concern would be the extra heat dissipation. --- End quote --- I really wouldn't worry about that. It's easy to get low voltage MOSFETs with on resistances in the milliohms region, no additional heatsinking required. |
| jmelson:
--- Quote from: Gyro on April 09, 2019, 03:24:28 pm ---The only real issue [EDIT: issues] I can see (assuming all your RC time constants work with the relay coil resistances), is that the relays will close relatively gently as their coil currents increase. --- End quote --- Not much of a problem. As the relay armature starts to move, the gap closes, and the magnetic field becomes stronger. The relay will close just about as sharply as if voltage is applied with a switch. Now, this circuit may have a slow OPENING of the contacts, but presumably that doesn't happen under load. One issue is if the relay ever fails to close, the resistors burn up. So, use flame-proof resistors there. Also, as the transformer needs magnetizing current to develop flux, if the resistors are too high in value, the filter caps may never charge. You might have to decrease the resistor value quite a bit. Jon |
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