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Hi all,

I am working on a personal project but I am a beginner in electronic, from now I'd like to push my project a little bit further by adding bypass capacitor to keep a clean signal as I use long wire (I have heard that was useful in that case).

Here is a quick explanation of my project :

The Arduino is linked to another board through a long 2 meters jack cable, on the other board there is a solenoid valve with her own 12v DC supply. The Arduino control a TIP transistor (on that board) to open or close 12v power to the solenoid valve.

My questions are :

- Is it really useful to put a bypass capacitor on the other board just before the TIP transistor gate to ensure that the gate always receive a clean power ?

- Does bypass capacitor are standard capacitor ? I mean I can use a normal ceramic capacitor ? Because I have seen some website selling specially named "bypass capacitor".

- To produce drops with the solenoid valve I have to put it on for short amount of time (like 50-60ms), then wait desired delay between drops and so on .. Would a capacitor apply lag to this process has it have to charge himself ? While it's charging the power won't reach the transistor gate, right ?

- And the last one, how do I calculate the size of a bypass capacitor ? I am by example thinking to add one on the 12v DC of the solenoid as it is provided by a very old power supply.

Thank you for your help.
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Post a schematic!
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Leon Heller
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Post a schematic!

I don't have at the moment. But what about my 3 lasts questions, they are more theorical ?
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My questions are :

- Is it really useful to put a bypass capacitor on the other board just before the TIP transistor gate to ensure that the gate always receive a clean power ?

Doesn't hurt...

Quote
- Does bypass capacitor are standard capacitor ? I mean I can use a normal ceramic capacitor ? Because I have seen some website selling specially named "bypass capacitor".

I am not an expert by any means in electronics, but I never heard of a capacitor that was just for bypassing..

Quote
- To produce drops with the solenoid valve I have to put it on for short amount of time (like 50-60ms), then wait desired delay between drops and so on .. Would a capacitor apply lag to this process has it have to charge himself ? While it's charging the power won't reach the transistor gate, right ?

It shouldn't..

Quote
- And the last one, how do I calculate the size of a bypass capacitor ? I am by example thinking to add one on the 12v DC of the solenoid as it is provided by a very old power supply.

Usually go for .01uF to 10 uF is a good number to start off.  Maybe for higher voltages, 100uF...
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//LiNK

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More important is a diode across the solenoid, and some surge protection.
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Leon Heller
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More important is a diode across the solenoid, and some surge protection.

Yep, it already have one.
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The solenoid's response time is perhaps 5 to 20ms.  No need to worry till the cable is 50 miles long!  There simply isn't any chance of picking up enough low frequency noise than can affect a solenoid or its driver transistor in that length of cable.  (Actually a 50 mile cable would have too much resistance, as it happens)

And a long cable itself acts as capacitor.  The TIP transistors base-emitter junction and base-collector junctions will also have capacitance come to that.

For logic signals (where the rise time is perhaps 3ns) then there will be all sorts of issues with crosstalk, noise injection, signal reflection, but an output to a slow mechanical device is automatically low-pass filtered by 6 orders of magnitude or more, so should be fine.
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Ok thank you for your help.

I will just go with a bypass capacitor just after the 12v DC so.
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