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Electrical Problem ( ReSet Problem)

 
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ahmad-al3omar



Joined: 25 May 2007
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Electrical Problem ( ReSet Problem)
PostPosted: Sun Mar 23, 2008 4:42 am     Reply with quote

hi all;
i have met several problems with reset of my system ( including PIC )
if i use a motor and any thing that need higher voltage or +5v only, when i turned it on my PIC system reset i dont know why, i separated the power supplies but the same problem occurs , can you help me to overcome this problem by any electrical advices.

thank you
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gjs_rsdi



Joined: 06 Feb 2006
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reset
PostPosted: Sun Mar 23, 2008 7:37 am     Reply with quote

you don't give enough information.
for example, is the PIC supply same as the motor?
the supply is from some power supply, or from battery?
best regards
joseph
ahmad-al3omar



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PostPosted: Sun Mar 23, 2008 9:24 am     Reply with quote

about the power supply, i tried to use the same one which i really want
and i tried to use different supplies for each motor and the PIC.
BUT the same problem occurs, the system reset.
i am using a power supply.
thank you
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Ttelmah
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PostPosted: Sun Mar 23, 2008 9:59 am     Reply with quote

Electrical advice:
1) Think about current paths. When the motor switches on, where will it's _ground_ current route?. Where is the PIC's ground connected relative to this?
2) Comment made here many times. The motor is an inductive load. As such there is significant energy stored in the coils. When the motor switches _off_, where is this going to go?. This energy _must_ go somewhere. If you use diodes, to route it to the power rails, what is going to happen to these rails?. Is there enough reservoir capacitance to keep the voltage rise within reasonable limits?.
3) The motor itself, (assuming it is a brushed design), will have intermittent contacts internally. As such, because of comment2, these _will_ want to arc. How are you going to kill this? (capacitive snubbers). If the motor is arcing, what are the implications (RF noise, and induced voltages on wires...).
4) DC motors, draw the most current, when you first apply power, with them stationary. For a tiny moment, the rate of current rise, will be limited only by the coil inductance, and the actual current peak will be limited only be the DC resistances present. Can your supplies deliver this? - think in terms of at _least_ 10* the normal running current. What will happen to the supply rails?. Can the switcher deliver this?. Might it be worth reducing this? (thermistor or similar device...).
5) Beware of multiple 'unexpected' ground paths. If (for instance), you connect a power circuit and a controller to the same supply, with a thin wire to the controller, and another to the power circuit. Then have a 'control' wire between, which itself has a ground connection, when current is drawn in the control circuit, it's ground level will rise, and current can flow through the ground side of the control wire. This is what '1' is about, and is the _biggest single problem_ when dealing with power control. Remember when when supplies are 'separate', unless they are _isolated_ supplies, they may have common ground paths...
6) Consider opto-coupling the drive....

I have PIC's controlling some H-Bridge systems, that handle over 10KW at 48v DC. They give no problems.

Best Wishes
luckyluke



Joined: 18 Apr 2006
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PostPosted: Wed Dec 20, 2017 2:38 am     Reply with quote

Quote:

2) Comment made here many times. The motor is an inductive load. As such there is significant energy stored in the coils. When the motor switches _off_, where is this going to go?. This energy _must_ go somewhere. If you use diodes, to route it to the power rails, what is going to happen to these rails?. Is there enough reservoir capacitance to keep the voltage rise within reasonable limits?.

hello
I have a question about reservoir capacitance. What should be the capacitor's voltage for a 12.5V battery?
Is 16v enough or should it be as twice of battery?
Thanks
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Wed Dec 20, 2017 3:58 am     Reply with quote

Depends massively on the actual capacitor specification (data sheet needed).

Now (for instance), a '12v' lead acid battery, has a peak voltage under charge of about 14.5v (when using fast charging).

Aluminium capacitors should be de-rated by at least 20%. Ceramic and tantalum by 50%. Now this in part is not a limitation of the capacitors themselves, but the 'likelihood' of really high frequency events that the capacitor may have to survive, and the nature of the capacitor. If you are choosing a capacitor capable of handling high frequencies, it suggests such frequencies may be present, and these will increase the margin that should be allowed.
There is a 'downside' to going too far, with things like electrolytic capacitors. On these if operating at low voltages relative to their maximum rating, there will be a decrease in performance. This is why the 20% figure is used for these, rather than the 50% for types that don't have this behaviour.
If you do choose a higher voltage, then you have to increase the actual rating used to allow for this drop.... :(

However if you buy good capacitors, you will find they have safety margin built into their data. While ones from less well known sources do not. So if you look at electrolytic capacitors from a manufacturer like TDK, the 16v 'rated' voltage, is the maximum continuous voltage, not the failure voltage. They actually have the 16v capacitor tested to withstand 1000hours at 25v. This is why it is often necessary to not buy the parts from cheaper sources. A 16v capacitor with such a rating is working close to it's ideal situation.

Now you then need to remember that an aluminium capacitor will slowly dry out. The types rated for higher temperature operation, will tend to dry slower. However even with these you will be talking something like 5% drop in capacitance over perhaps a year. This is why one of the standard 'service' operations on older kit when being rebuilt is to replace every such capacitor on the board, before doing anything else. If you are designing for a like of (say) 5 years, make sure that the new capacitor value exceeds what it needed by at least 25%.
temtronic



Joined: 01 Jul 2010
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PostPosted: Wed Dec 20, 2017 6:24 am     Reply with quote

other power supply comments..

1) old skool rule is 1,000 MFD per amp. So a 2.5amp PSU will need a 2,500 mfd cap. More is better. I use 10,000MFD/68V caps as I got a GREAT deal on them

2) motors draw a LOT of current, initally. can be 5 to 10X 'running' current. So you NEED a big power supply

3) EMI (noise) can be airborne NOT just 'through the wires'. Along with tidy PCB design,you need proper bypass caps 'all over the'. Critical points are PIC power pins, all 'off board' connections(to/from switches, LCD,etc)

4) caps aren't all that's required ! Depending on the EMI, you may also need inline chokes,MOVs, sparktubes,etc.

5)PCB design ! power supply--motor control--PIC. You want the 'motor control' parts(FETs, relays ,??) to be close to the power supply. That keeps the 'huge' currents from being near the PIC so less chance of resets.

There are literally libraries FULL of books dedicated to this subject and while a lot can be designed...there is a lot that is NOT in the books and only learned 'on the bench'. One example is kinda random 'crosstalk' from multiconductor cables...arrgh

Jay
luckyluke



Joined: 18 Apr 2006
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PostPosted: Wed Dec 20, 2017 8:40 am     Reply with quote

thank you for lecture like explanations
i am using 3s lipo battery and capacitor for h bridge driver.

problem is i dont have enough space for large capacitors.
i will try 1000 uf 16v firstly, if not i will use 470uf 25v
Ttelmah



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PostPosted: Wed Dec 20, 2017 9:05 am     Reply with quote

Though 'big is good', it is particularly the HF performance that is telling. Surface mount multi-layer ceramic capacitors may be better for the packing density available.
If the capacitor is smaller than ideal, you end up with much more ripple at the bridge. This may well be perfectly acceptable, but then means you need to think much more about how to prevent this affecting the PIC. Quite a small inductor/capacitor filter in the rail feeding this voltage to the PIC regulator can reject far more than a simple capacitor.
Then you have the question of heating. Heat is generated when ripple voltages change across capacitors. You need to look at the ripple rating of the capacitors, and their ESR. If you drive the capacitors too hard, they may fail prematurely as they overheat....
luckyluke



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PostPosted: Wed Dec 20, 2017 11:46 am     Reply with quote

I will find a space or make one.

Thanks for your replies.
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Wed Dec 20, 2017 11:56 am     Reply with quote

As a comment this is part of the balancing act of PWM.
Higher PWM frequencies mean more switching losses in the power transistors, but smaller capacitors can be used.
luckyluke



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PostPosted: Wed Dec 20, 2017 4:10 pm     Reply with quote

Thanks Ttelmah for detailed specific informations Shocked

I am following this forum for years sometimes i can't be sure about if you are a single person or a group of people from all electronics branches Very Happy

and all other helpers from forum thank you
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