100ms 1A Supply

[mshalash]

Hi all

I need advise on this one I have 5-24v dc-dc connected to 15kuF capacitor via a transistor switch from the supply side and to a solenoid 29ohm 24v at the load side again using a switch in between these switchs are a transister with PIC controlled.

It is required to have 1A 24v output to the solenoid for 100ms every (at leaste ) 60 seconds.
The PIC program is no problem but the electronics configration for the capacitor is in question?

Is that possible?

Thanks in advance

 

[carbonzit]

15kµF? What's that? Do you really mean 15,000µF?

Details matter.

 

[mshalash]

Yes 15000uf

 

[mshalash]

I should have said 0.015F

 

[Reloadron]

You may want to think about using a logic level MOSFET, something along these lines which will allow easy interface between your PIC and the solenoid. That assumes the PIC output is in fact a 5 volt pulse. Additionally if you have not done so you want a diode across your solenoid coil to snub the back EMF when the coil field collapses. There are other methods like using an opto coupler in the scheme but a simple MOSFET is likely the simplest method.

Ron

 

[mshalash]

Thanks for the reply but,
I donot have a problem with PIC interface and I do have a diod across the soleniod,the issue is to do wit the supply of 1A for 100ms using a capacitor charged from dc dc convertor and before activating the solenoid I disconnect the capacitor from the supply and connect to the solenoid.

 

[Reloadron]

OK, I think I better understand what you are doing. You are using the DC supply to charge the cap and the cap needs to supply at least 1 amp to the solenoid coil for a duration of 100 mSec on average every 60 seconds. When this happens you want to unload the supply to the cap. This could likely be done by having the PIC drive a small relay either directly or indirectly depending on the relay coil current and let the relay switch between the cap and solenoid. The relay would be a SPDT type with a 5 volt coil. The switch time of the relay might be a consideration. I could make you a rough drawing later if time allows at work or late this afternoon (US EDT Time). What is the available high out current on your PIC?

Ron

 

[mshalash]

Hi Reloadron,
and thanks for the reply

You got it right, I can disconnect the Cap via transistor (that is no problem) and using TIP21 with a diod to switch on the solenoid for 100ms, the question is
will I get 1Amp from this confiuration for 100ms/60sec considering the charging is 1w at 24v for the .015F cap.
Thanks

 

[4pyros]

Leave the cap charging all the time.

 

[mshalash]

No,I don't want to risk the dc-dc convertor..

 

[mshalash]

Because the dc-dc convertor is only 1W 24V..

 

[4pyros]

I am sorry and how much power do you think it takes to charge a dead 15,000µF cap anyway?
You will have to current limit your DC to DC converter anyways so just leave it hooked up and switch the output to the solenoid.

 

[mshalash]

ok
thanks

 

[Reloadron]

Thanks 4pyros, I fully agree.

You have a 24 VDC coil that draws 1 amp so we can it p-resents a DC resistance of about 24 ohms. The instant the coil is powered by the cap the cap will begin to discharge. With your setup it will take 360 mSec for the cap to discharge 63% or down to 15.12 volts. This is also not a linear change. Personally I would leave the power to the cap connected and let the cap do its thing for the solenoid coil. That should work for you. Worst case you could parallel another 15,000 uF cap and double the RC time constant. That would bring it out to 720 mSec for the voltage to drop 63% which with the power on the cap it won't do. The solenoid on time is only 100 mSec.

Ron

 

[mshalash]

Thanks that is good analysis, what if I replace the cap with another of .022Fbecause it is cheaper than adding anothe rone?

Thanks a lot

 

[alec_t]

will I get 1Amp from this configuration for 100ms/60sec considering the charging is 1w at 24v

Yes. The long term average power dissipated is 24V*1A*0.1 / 60 = .04W; so the 1W capability of the charger is plenty.
As pointed out above, you will need to limit the charge current during the initial cap charge from 0V.

 

[mshalash]

Thanks all
This was very intuitive.

 

[mshalash]

Hi All
Done th eprototype and it is working fine, I couldn't have done it without your help specially limitting the cap echarging current .

The device is Bluetooth Cash Drawer opener used in my Android POS system.
Thanks all a lot