Am I missing any potential pitfals with this soft start / power sequence design?

Thread Starter

Metatronic Mods

Joined Aug 17, 2016
18
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).
 

mvas

Joined Jun 19, 2017
537
At what voltage does Relay #1 actually turn-on?

You say F2 will blow, if Relay #1 fails to close?
How would you size F2 to do that?

Is Relay #1 designed to energize before Relay #2 energizes?
How?
Maybe, use a N.O. Contact from Relay #1 to delay the voltage to the circuit for the the coil of Relay #2 ?

Is 10K equiv ( R1 - R4 ) way too high?

How many amps will you draw from the 120 Volt AC Line at Full Load?
 
Last edited:

Thread Starter

Metatronic Mods

Joined Aug 17, 2016
18
At what voltage does Relay #1 actually turn-on?
~14V. (The RC networks are placeholders for now, likely to be replaced by a comparator ckt)

You say F2 will blow, if Relay #1 fails to close?
How would you size F2 to do that?
No sorry, I meant in the event of an overload condition or short AND RL1 failing to close (ie. if the full load current or greater were allowed to pass through the ballast resistors).

Is 10K equiv ( R1 - R4 ) way too high?
Possibly, but I think it's within an order of magnitude of something reasonable given the transformer size and capacitance. My initial concern was that it would prevent RL1 from kicking on due to the voltage divider created with the xformer primary, but it's a ways off from causing that in my test ckts.
 
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