How to activate 12V/2A electromagnet with 5V (USB power) each 60 minutes?

[Alan2017]

once again, thank you all very much for kind support (especially calculations).
I agree..will need to experiment with capacitors.
May I please ask three more questions:

1. where to place currency limiter resistor (in front of step-up converter, or after converter)?
2. If I place it as resistor R2 on attached picture, (after converter), it will slow down charging of capacitor?
3. Should I charge capacitor to higher voltage and to use step-down converter for the solenoid?

Thank you,
Alan

 

[MikeMl]

once again, thank you all very much for kind support (especially calculations).
I agree..will need to experiment with capacitors.
May I please ask three more questions:

1. where to place currency limiter resistor (in front of step-up converter, or after converter)?
2. If I place it as resistor R2 on attached picture, (after converter), it will slow down charging of capacitor? View attachment 104942
3. Should I charge capacitor to higher voltage and to use step-down converter for the solenoid? View attachment 104943

Thank you,
Alan

1. input to step up. (ps, I changed my mind, see post #24)
2. who cares? You have an hour to charge it...
3. I think you will find that a capacitor charged to 25 to 35V will work best. No need for a current limiter or step-down converter; just the NFET as shown before. The capacitor will begin re-charging after the timer turns the NFET off.

 

[Tony Stewart]

12V*2A*200ms = 4.8J (watt-seconds) where you want the voltage to remain constant within say 5% for this duration.

If the load current was constant 2A a droop of 5% voltage from ESR implies cap or battery ESR must be 5% of load or 5% * 12V/2A=300 mΩ or RC value must be <10% of T=60% decay thus RC<10% of 100ms or C=10ms / 6Ω >= 1.67mF

I would use this criteria for defining the worst case storage ESR & C values but target for a much better values.

Size may not be an indicator of cost but rather lower ESR*C product results. e.g. $1 1"D 10mF https://www.digikey.com/product-detail/en/epcos-tdk/B41231B5109M/495-6158-ND/3493557

Current limiting must be considered in the boost converter so as not to fault the USB port voltage, which can be achieved in many ways including a large series R considering the length of time between discharges.

 

[MikeMl]

Here is what I would do:

Use one of these to boost USB 5V to 28V.

Put a 2.2K 1/2W resistor between the converter and the 10,000uF 35V capacitor.

Use a NFET to switch the solenoid. Vds > 50V. Id >5A, Vth<=2V (logic level).

Any Si rectifier as a snubber.

This shows that the capacitor re-charges in about 100s. Only 12.5mA is drawn from the boost converter. That means the USB 5V current is less than 100mA.

Here is the solenoid current:

 

[Pommie]

All good advice. All I'll add is to make sure the boost converter is adjustable so you can fine tune the voltage.

Mike