×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

How can I eliminate the voltage sinking?

How can I eliminate the voltage sinking?

How can I eliminate the voltage sinking?

(OP)
I build a pcb.
When it plug in the socket of the back-board,the vlotage will sink to 4.35V for 20us.  
But I use the +5V power. So the boards that is sharing the power with it will reset due to the power sinking.
How can I deal with it?

Thanks.

RE: How can I eliminate the voltage sinking?

It sounds you are building a hot swap PCB and found the PCB will reset. Also, you PCB has a micro processor inside.

I suggest you check the peak current of your PCB. It seem when you put the board to the slot, the capacitor draw a lot of current make the voltage drop. The way you can do is limited the current go into your PCB and reset the CPU after the voltage supply stable.

Remember, you must reset you microprocessor after you supply voltage. It can avoid your processor working wrong.

I hope it help.


RE: How can I eliminate the voltage sinking?

Suggestions:
1. The 5 V power supply internal impedance is too high causing the voltage dip to be too low. Try some 5 V power supply with lower internal impedance.
2.  Also, try to connect two 5 V in parallel to find out the difference.

RE: How can I eliminate the voltage sinking?

I disagree with jbartos:
1.) The when you connect your board which has decoupling caps on it, its impedance = 0 i.e. pulls too much current.
Reducing the source impedance will INCREASE the current spike.
2.) Paralleling two 5 V-s possible only is they are exactly
the same value ELSE one will load the other

I suggest adding a series inductor BEFORE any decoupling cap
 on the new board -- this will reduce the peak current and slow down the current spike. May want to add parallel
diode or resistor to prevent sparking on disconnect.

<nbucska@pcperipherals.com>

RE: How can I eliminate the voltage sinking?

How many microF decoupling does the new board have ?

<nbucska@pcperipherals.com>

RE: How can I eliminate the voltage sinking?

Suggestion: My posting is based on the following:
1. Consider a voltage divider circuit applicable to the posted problem, namely:
E4= 4.35V = 5V x Z4 / (Z4 + Z5).....Eq1
where
Z4 is impedance of the board that exhibits 4.35V across its terminals
Z5 is internal impedance of 5V source
E4 is given terminal voltage that sinks.
Now, from Eq1
Z5/Z4 = (5 - 4.35)/4.35..........Eq2
Next, by paralleling the 5V source Zsourceinternal is
Z5 x Z5 / (Z5 + Z5) = Z5 / 2 ....Eq3
Then, Eq1 becomes
E = 5V x (Z4)/(Z4 + Z5 / 2) =
  = (10)/(2 + Z5/Z4)..............Eq4
Substitute Eq3 into Eq4
E4' = 4.65V (93% of 5V) that is higher than E4 = 4.35V (87% of 5V)
2. Now, when it comes to transients over capacitor C and resistor Rc in series, the following holds:
Io = E4/Rc.......................Eq5
I4 = Io x e**(-t/CRc)............Eq6
Ec = E4 [1 - e**(-t/CRc)]........Eq7
Therefore, even if the current I4 is spiking more across the capacitor since
E4' > E4 or 4.65V > 4.35V
The capacitor voltage is greater for E4' than for E4 since
Eq7 holds, and voltage E4 or E4' governs the transient process related to Rc and C in the series connection.

Obviously, if the Zsourceinternal < Z5 /2 then the E4' will be even higher, which I implied in my previous posting.

Question remains, if the recommended inductor addition, as posted in other posting, is feasible.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources