Super simple supply problem! Help! , RC Circuit not charging?

Discussion in 'The Projects Forum' started by fouadalnoor, Aug 31, 2010.

  1. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Hello guys,

    I just received my new pcb design from pcb-pool and soldered most of the components on.
    Though I am still waiting to solder the two chips I have as the pins are small and I want to check that the other parts of the circuit is working first.

    The first thing I did was to solder on my 9v battery clips to the vdd and gnd. The 9v battery is actually showing about 7v though when I connect it I get around 2v on the meter!?

    I tested the battery again by itself and I get around 7v but as soon as I put the clip on and test the same battery terminals I get 2v!?

    Can someone tell me what I'm doing wrong?
     
    Last edited by a moderator: Sep 1, 2010
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Have you tried a fresh 9V battery? - 7V no load is a bit low for a good 9V type.

    If that's not the problem there may be a soldering (e.g. bridge?) or mounting error / fault which is creating excessive current drain on the battery under load. For example a polarized capacitor incorrectly wired (reversed) and so forth.
     
    Last edited: Aug 31, 2010
  3. SgtWookie

    Expert

    Jul 17, 2007
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    A 9v "transistor" battery that measures 7v with no load on it is dead.

    Batteries should be measured with a load on them. For dry cells, a milliamp-meter in series with a 330 Ohm fixed resistor works well. A fresh 1.5v alkaline cell should measure ~4.0mA. A fresh 9v cell should measure ~25mA.
     
  4. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Ok, so I bought a brand new 9v battery now. It still says around 2v when I measure the voltage, but the decoupling capacitor is getting super hot!

    I'm just not sure what to do now.. also I heard a "pop" when I plugged in the 9v batt...does this mean one of the chips just blew?
     
  5. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    I have attached the pcb design in case anyone can see any obvious faults..
     
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  6. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    should I just take off the decoupling capacitor and just solder that little part together and then connect the battery directly?

    also should I take off the chips just in case?
     
  7. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    ok...more news... I have now taken off the capacitor and it seems to work better. I get 8.69v on the terminals.

    I then tested the 30k resistor as seen on the diagram and that gave me 8.69v as well which is as expected.

    Though when I check the other side of the 30k resistor I only get a steady 1.20v?!

    The aims was to get the 100uF cap to charge to 9v in 7 secs...

    any advice?
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    If the "decoupling capacitor" is polarized, it could be soldered in the wrong way. A super hot capacitor is bad sign. It could be broken also If you have short leads from the battery it will not make much difference if you do not use it. But it is not wrong to place capacitor like you have done in your circuit. But I may be wrong since I do not have schematic
    However I think you perhaps have got the use of decoupling/bypass capacitor somewhat wrong. As both your ICs do not have any decoupling/bypass capacitor. Here is a nice intro.
    http://www.seattlerobotics.org/encoder/jun97/basics.html
    I prefer using 0.1 uF my self. You may also use google for more info. By the way post your schematic also
     
  9. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Thanks for the reply.

    Well, I am not using a decoupling cap as there does not seem to be a strong need for it in the circuit.

    The problem is that the the other capacitor for the RC circuit is not charging.
    (It is not polarized) I seem to only get 1.20v between the 30k res and the capasitor. Any ideas why that is happening?

    Please look at the attached Multisim Diagram.
     
  10. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    also I do realise none of my IC's have a decoupling capacitor on them. I will add that later, though it should still work relatively well without them I think? (at this point I just want it to work as a simple prototype)
     
  11. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    by the way, the second picture is supposed to show what it SHOULD be...I am getting 1.2v instead of 8.787v...
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You're going to be better off adding the decoupling caps now or it's probably just going to give you fits.

    And I would also agree, bet the electrolytic was in backwards.
     
  13. t06afre

    AAC Fanatic!

    May 11, 2009
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    Always use a 0.1uF by-pass on components or manufacture recommended values in your design. Your schematic is also hard to follow. You should clean it up. For your own shake. As an example it is nothing wrong in placing several VDD and GND symbols in a schematic.
    Also another tip. On your prototype circuit board. Always place a pin header there cables are entering or leaving the board. Then you work with a prototype you will do a lot of soldering and desoldering and poking around with measuring pins. So you may very quickly damage your PCB. But with a pin header you can do as much rework as you want without damaging your board. I use this type. The cost is peanuts. And it can be cut in any length
    [​IMG]
     
  14. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Thanks for the response guys. The only problem now is I cant actually change my PCB as it was manufactured by a company and its all made now.

    I will definitely change the design to add those bypass caps, but at this point I want to see if I can make the circuit at least partially work.

    I took out the chips and at this point I am simply trying to get the RC part to work. Is it because the cap is the other way around that its not working?

    Also I will definitely change the schematic to make it clearer.

    Keep posting suggestions as I need them!
     
  15. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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  16. t06afre

    AAC Fanatic!

    May 11, 2009
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    Do not worry we have all been beginners and done dumb things. And most of us are not completely done doing such things either. You have to learn this as you walk.
    Edit: We have a good free e-book in this forum. The links are on top of page (Vol. II - AC Vol. III - Semiconductors Vol. IV etc) You should read this I think http://www.allaboutcircuits.com/vol_1/chpt_13/5.html
     
    Last edited: Sep 2, 2010
  17. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Ah thanks. Umm do you have any suggestions about what to do with the charging capacitor? I mean like something I can do now.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    The capacitor you linked to on Digikey is a polarized tantalum capacitor.
    Tantalum capacitors have a mark near the positive side lead, where aluminum electrolytic capacitors usually have their negative lead indicated by a stripe. If you install them backwards, you will kill them.

    ALL integrated circuits require a minimum of a 0.1uF ceramic or metal poly film bypass capacitors across their supply rails. If it is a dual-supply component, you will need two capacitors, one for each power rail to ground.

    Some IC's require more than a single 0.1uF cap per rail; a 555 timer is one such device that is extremely common. The 555 requires a minimum of a 0.1uF and a 1uF aluminum electrolytic across the supply pins.

    The leads of the capacitors should be as short as physically possible.

    If you do not use bypass capacitors, you WILL experience problems.

    Also, you are using 4000 series CMOS ICs, however you have left many inputs "floating", or without a path for current to Vdd or ground. This will cause very strange problems due to the IC's oscillating at high frequencies, which will result in high current usage.

    ALL UNUSED CMOS INPUTS MUST BE CONNECTED EITHER DIRECTLY OR VIA RESISTOR(S) TO VDD OR GROUND.

    Do NOT connect unused OUTPUTS to VDD/GND, or you will likely destroy the IC.

    Lastly, 4000 series CMOS IC's have very limited current source/sink capabilities. It appears that you are attempting to control a motor with the output. I predict that you will see smoke instead of what you expected. It appears that you have attempted to build a form of PWM control for the motor, however you do not have any reverse-EMF diodes, nor clamping diodes for the output. The first time you pass current through the motors' windings and then stop it, you will have a very large voltage spike destroy the output of the 4081 AND gate.
     
  19. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Gosh... Thanks a LOT for your reply. I guess a newbie like me cant expect to get all of this right the first time (I have never actually designed anything all by myself before)

    Let's see, I guess the PCB I now have is not worth much so I will stop trying to get that to work and go back to the design.

    At this point I will add a 0.1uF ceramic cap to the supply rails. (I actually have that already in my design but I must have (like you said) soldered in backwards.

    I will add two 0.1uF decoupling tantalum caps to the supply rails of the two IC's

    (as shown on the astable circuit here below)

    https://homepages.westminster.org.uk/electronics/555.htm

    I will also connect all FREE INPUTS to Ground and Leave the OUTPUTS that are not used OPEN.

    Once I finish this off I will post the new PCB Layout on here and i'm hoping you will check it out to see if I make any mistakes.

    Thanks SgtWookie, hope that you keep advising me along with this project as There aren't any workshops or anything so I'm lost without a little guidance!

    Thanks.
     
  20. fouadalnoor

    Thread Starter Active Member

    Oct 20, 2008
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    Ok, I have now connected the unused inputs to ground and left all unused outputs open.

    I have also added a 0.1uF Cap to both IC's, though can you guys please check that I did that correctly?

    If it that looks fine I will continue with the motor part by adding the diodes as you mentioned.

    (I like doing things one step at a time so please bear with me)

    Thanks.
     
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