Simple dual relay soft start

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de Ocampo

de Ocampo

Member
Joined 2010
#1
Hi guys,
I would like to ask your opinion about this dual relay soft start. I believe both schematic are based on Elektor.
I am doing a modified lay-out with ideas based on both schematic, but I have added parallel diodes on the relay coils. Will this be ok?

Thanks!
 

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#6
I've had this circuit (the one in the second picture) for many years in an amplifier with a 330 VA toroidal transformer. The R3 resistor burned at some point. This resistor limits inrush current at switch-on, whereafter C1 limits current (C1 should preferably be X2 rated or similar). I replaced it with a 2W resistor to be on the safe side. Otherwise the circuit has worked fine. I used a 24V relay (coil 1200 ohm).
 
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#8
de Ocampo said:
I would like to ask your opinion about this dual relay soft start. I believe both schematic are based on Elektor.
I am doing a modified lay-out with ideas based on both schematic, but I have added parallel diodes on the relay coils. Will this be ok?
Click to expand...
That's indeed Elektor's schematic style. I'd add flyback diodes across the relay coils as well. I'm a bit puzzled why the Elektor folks didn't. You can also use a diode with a zener in series if you want the relay to de-energize a bit faster. I'm generally not a fan of powering the relays directly from the mains, but I suppose it can be made to work. There are voltage regulators that can handle mains input now. I'd consider using one of those.

I'm not a fan of resistor-based soft starts, though. The peak power dissipated in the resistor during the inrush event is substantial, so if you do go that route you need to find a resistor that can handle this peak power (which can be in the kW range). I much prefer an NTC or inrush limiter designed for the purpose. I've written quite a bit about that. You can read my thoughts here: The Ultimate Guide to Soft Start Design (Neurochrome).

Tom
 
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#12
I have used something quite similar as inrush protection for a 4000 VDC power supply @ 1 Amps. When switched on, the power supply was NOT loaded, so the circuit only took the inrush current of the transformer and capacitors. The resistors I used were suited for high power…

Regards, Gerrit
 
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#13
tomchr said:
That's indeed Elektor's schematic style. I'd add flyback diodes across the relay coils as well. I'm a bit puzzled why the Elektor folks didn't. You can also use a diode with a zener in series if you want the relay to de-energize a bit faster. I'm generally not a fan of powering the relays directly from the mains, but I suppose it can be made to work. There are voltage regulators that can handle mains input now. I'd consider using one of those.

I'm not a fan of resistor-based soft starts, though. The peak power dissipated in the resistor during the inrush event is substantial, so if you do go that route you need to find a resistor that can handle this peak power (which can be in the kW range). I much prefer an NTC or inrush limiter designed for the purpose. I've written quite a bit about that. You can read my thoughts here: The Ultimate Guide to Soft Start Design (Neurochrome).

Tom
Click to expand...
You don't need any diode across relay coil because there isn't a transistor switch before the coil.Sutch a diode is for transistor protection against relay reverse current.
 
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