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If the reset capacitor was polarised (directional) what way
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If the reset capacitor was polarised (directional) what way

by josheeg on Mon Oct 13, 2008 9:38 am

If the reset capacitor was polarised (directional) what way should it point if it would work?
Could it cause probelms if it was not non-polorised?
What kind of resistance could be in series with it and still have it work properly?
josheeg
 
Posts: 213
Joined: Wed Jan 16, 2008 8:38 am

Re: If the reset capacitor was polarised (directional) what way

by lou on Mon Oct 13, 2008 12:01 pm

josheeg wrote:If the reset capacitor was polarised (directional) what way should it point if it would work? Could it cause probelms if it was not non-polorised?


A polarised capacitor needs to always have its + terminal at a voltage that is positive relative to its - terminal. Non-polarised capacitors work just fine, providing the needed value is available. Not knowing precisely what reset circuit you're talking about, it's hard to be any more specific. The failure mode of polarised capacitors connected backward isn't fully specified, but I've seen circuits that assume that they act like a diode. Circuits that make a non-polarised capacitor out of two polarised ones connected end-to-end are depending on that.

If you size your resistor properly, you should be able to use a small enough capacitor value that it is available in ceramic or another non-polarised tech.

josheeg wrote:What kind of resistance could be in series with it and still have it work properly


It depends on the value of your capacitor, threshold voltages, and how long you want the reset pulse to be. A huge resistance value would probably work, but it would take awhile for your device to come out of reset. Too small of a resistance requires a large capacitance or yields possibly too short of a reset pulse for it to be handled properly. In general, the time in seconds to charge the capacitor 2/3 of the way is the resistance in ohms multiplied by the capacitance in farads.
lou
 
Posts: 91
Joined: Fri Jul 11, 2008 9:39 am

Re: If the reset capacitor was polarised (directional) what way

by josheeg on Mon Oct 13, 2008 1:03 pm

I was refering to the cap. that was added to make the Arduino Decima reset using the RTS line from the virtual com port.
But I think your info will help but if you have any added info that would be useful.

lou wrote:
josheeg wrote:If the reset capacitor was polarised (directional) what way should it point if it would work? Could it cause probelms if it was not non-polorised?


A polarised capacitor needs to always have its + terminal at a voltage that is positive relative to its - terminal. Non-polarised capacitors work just fine, providing the needed value is available. Not knowing precisely what reset circuit you're talking about, it's hard to be any more specific. The failure mode of polarised capacitors connected backward isn't fully specified, but I've seen circuits that assume that they act like a diode. Circuits that make a non-polarised capacitor out of two polarised ones connected end-to-end are depending on that.

If you size your resistor properly, you should be able to use a small enough capacitor value that it is available in ceramic or another non-polarised tech.

josheeg wrote:What kind of resistance could be in series with it and still have it work properly


It depends on the value of your capacitor, threshold voltages, and how long you want the reset pulse to be. A huge resistance value would probably work, but it would take awhile for your device to come out of reset. Too small of a resistance requires a large capacitance or yields possibly too short of a reset pulse for it to be handled properly. In general, the time in seconds to charge the capacitor 2/3 of the way is the resistance in ohms multiplied by the capacitance in farads.


I was refering to the cap. that was added to make the Arduino Decima reset using the RTS line from the virtual com port.
But I think your info will help but if you have any added info that would be useful. :mrgreen:
josheeg
 
Posts: 213
Joined: Wed Jan 16, 2008 8:38 am

Re: If the reset capacitor was polarised (directional) what way

by lou on Mon Oct 13, 2008 1:48 pm

josheeg wrote:I was refering to the cap. that was added to make the Arduino Decima reset using the RTS line from the virtual com port.
But I think your info will help but if you have any added info that would be useful.


That design uses a non-polarised cap. 100nF is a common size in SMT ceramic caps, which is what they're using there. The problem with using a polarised one there is that the charge on the cap is opposite when resetting via the button or via the FTDI chip. Put a diode in parallel with the capacitor (mentally, not for real) and see if you can see why that might not work very well.

I thought it was DTR, rather than RTS.
lou
 
Posts: 91
Joined: Fri Jul 11, 2008 9:39 am

Re: If the reset capacitor was polarised (directional) what way

by josheeg on Mon Oct 13, 2008 1:52 pm

lou wrote:
josheeg wrote:I was refering to the cap. that was added to make the Arduino Decima reset using the RTS line from the virtual com port.
But I think your info will help but if you have any added info that would be useful.


That design uses a non-polarised cap. 100nF is a common size in SMT ceramic caps, which is what they're using there. The problem with using a polarised one there is that the charge on the cap is opposite when resetting via the button or via the FTDI chip. Put a diode in parallel with the capacitor (mentally, not for real) and see if you can see why that might not work very well.

I thought it was DTR, rather than RTS.


DTR your right my mistake I will try to replace it with a nonpolorised cap. my design was made from the boarduino but the caps were turned polorised to get the same make and similar Number but that cap is best non-polorised.
josheeg
 
Posts: 213
Joined: Wed Jan 16, 2008 8:38 am

Please be positive and constructive with your questions and comments.