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I'm having trouble figuring out the name of this circuit.

It needs to delay turning "on" a 12V circuit and a 120VAC circuit, after a 5V circuit has been turned on.

Example:

  • Turn on 1x power switch, which turns on a microcontroller (5V), a DC motor circuit (12V) and a large AC motor (120VAC).

  • The sequence of switching "on" should be 5V, 12V, then 120VAC

  • Without that sequence the 12V motors and 120VAC motor turn on wildly (for ~1 sec) as the microcontroller is "powering on".

Ideally a passive circuit.

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2 Answers 2

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You can use two or three power-on reset chips with different delay times. Here is one example of what I am talking about: TI LP3470.

http://www.ti.com/lit/ds/symlink/lp3470.pdf

There are other vendors and other chips with different features. On first application of power, these hold their reset output low, and keep it that way until after some delay time has expired, then they release it.

If you use three of them with different delays, you can get your three sequencing signals. You just need to work out the part where the release of reset causes your power rail to turn on. Let me know if you need help with that part. I suggest that you use a PFET to turn on 12V, and use the 12V to energize the coil of a relay for 120V.

I have one warning for you. With sequenced power come all sorts of potential problems. If there is any IO connection between 5V and 12V stuff, you should try to make sure that IO is held low by the 5V system until after the 12V system is powered on. Otherwise high current may flow from 5V IO into inputs of the 12V system (through power rail clamp diodes). If the 12V system is motors only, then I don't think there will be any problem.

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A power supply sequencer or power rail sequencer. This could be done with a simple microcontroller for example, or you could use an analog comparator.

Essentially you want a circuit which has a transistor or other controllable switch for each power rail. When the first rail in the sequence reaches some threshold (e.g. detected with an op-amp or analog comparator), then the switch for the second rail would be turned on, and so on for each rail in turn.

Figure 1 here shows a very basic example of how to start one power rail after another has reached a certain threshold. There are also ICs available which will sequence multiple power rails.

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  • \$\begingroup\$ Can you get the microprocessor to sequence the rails? If you can cause an IO on the microprocessor to go high when you want to enable 12v, and then have a different IO go high when you want 120V to come on, you can solve this pretty easily with relays (might be able to use a FET for 12V, but I would definitely use a relay for 120VAC). \$\endgroup\$
    – user57037
    Jan 22, 2015 at 4:24
  • \$\begingroup\$ I will give more detail if you indicate that this might be workable. Tag me. \$\endgroup\$
    – user57037
    Jan 22, 2015 at 4:24
  • \$\begingroup\$ @mkeith thank you for getting back I'm worried the microprocessor strategy might not work because upon turning the microprocessor, the I/O pins "jerk" high or low randomly, which causes whatever relay it may be attached to to also jerk "close", which momentarily turns on the 120V. Is there no passive circuit that could delay the 120V and 12V circuits? thanks \$\endgroup\$
    – rcdriver23
    Jan 22, 2015 at 22:37
  • \$\begingroup\$ I have dealt with a variety of different processors over the last 15 years. On occasion, pins are ill-behaved during boot up. But very often, they are driven very weakly (either high or more often, low). An external 10k pulldown will very often keep a pin low throughout bootup until code can take control of the pin. You are right to be concerned, but the thing to do is check the pin before you count on it. Not give up on the idea altogether. \$\endgroup\$
    – user57037
    Jan 23, 2015 at 6:59
  • \$\begingroup\$ @mkeith, interesting, I have not heard of that solution before. Do I simply tie a 10k resistor, ground on one side, and I/O pin on the other side? thank you \$\endgroup\$
    – rcdriver23
    Jan 26, 2015 at 6:46

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