Capacitor for servo motor MG996R with Linear Voltage Stabilizer L7805

Other Hardware General Electronics
1

Hi, first of all, sorry for my bad english.
Also I have economic education, so about electricity, I only know that "it is there somewhere...". Can solder what I need, but have little idea about how these electric things works.

We have selfmade electric wire cutter for tiny cables (0,25mm2) at work. Controlled by Arduino, stepper motor (with A4988 driver) is moving the wire and servo motor is cutting it.

Recently the servo - MG996R stopped working. Dont know why, but its cheap so I plan to replace it with all related parts. Board is powered by 12V DC, so there is Linear Voltage Stabilizer THT, 5V, 1,5A, TO220 L7805CV-DG to change it to 5V for the servo.

There were some capacitors around the L7805 (10uF on input and also on output), but I am not sure if this was right. Technical documentation for L7805 is saying that there should be 330nF on input and 100nF on output, so I bought some tantalum capacitors, namely CT 330n/35V RM2,5 20% CA42 and CT 100n/35V RM2,5 20% CA42 (datasheet for both) for this job.

Now point is if I should put another capacitor on L7805 input. There are ones for Arduino itself and for stepper motor (both are CE 100u/25VT HIT-EHR 6,3x11 RM2,5 - datasheet), so I thought I should put one also for servo (to stabilize power drain spikes in moment servo starts working - or something like this from my poor understanding).

Searched through web, but found advices from "nothing needed" to "one on input and one on output", mentioning anything from 10uF to 2200uF, so I am confused.

Can anyone help me clarify if there is need to put some capacitors on L7805 input for the servo (aside those 330nF and 100nF) and how big capacity is should have?
Also I (mostly) understand Farads and Voltage on the capacitors, but have no idea what are those series (EHR, EMR, EXR.. etc). Should I be concerned about these too or it doesnt matter what I pick up?

Thanks for any help and sorry to bother you with this, but its like magic for me :face_with_spiral_eyes:

2

Sounds like a good opportunity get rid of that piece of 1970s technology as well and replace it with an appropriate buck DC-DC step down converter.

3

Use a different voltage regulator, capable of the 3 Amperes required by the MG996R, and a power supply to match.

4

Let us see if I can help. Consider a bulk capacitor (a few uF or greater a bank), smaller ones as petty cash. The ones shown with the 7805 are petty cash and you can get it quickly. They filter the high frequency noise (fast) to keep the regulator stable. The bigger ones are needed because the voltage will go to zero from the AC rectifiers(diodes) so the Bulk capacitors make up for this by drawing from the bank. So when the voltage is rising and above the capacitor's terminal voltage they are charging (putting money in the bank), this gives you a reasonable cash flow available to drive your motors. You should add some bulk capacitors totaling about 1000uF on the input such as two 470uF at 25V (not sure as I do not have the circuit). Size is dependent on total load , rectifier design, and supply voltage. On the output (servo side) something in the range of 50-100uF would be great. This will help the servo start smoothly. Wondering why two bulk capacitors on the input, reason, they take the bulk of the abuse and if one fails the system keeps on working.

6

I suppose you mean something like MEAN WELL SKM15A-05?
That number of pins scares me little bit and cant find info about any needed capacitors around it. Also I think even if L7805 is old, its still sold and used, so if its working, why not use it, not to mention its 10% of price of the dc convertor.
But if I will not be able to make the cutter work with the linear stabilizer, I will try the dc converter, thanks a lot for advice, didnt even know such thing exist.

7

Datasheet for MG996R mentioned 500-900mA so I thought 1,5A would be enough, didnt know what that stall current is, so overlooked it. Also the machine was able to work for one year, so I never thought It can be problem.
If I change it to THT, 5V, 3A, TO220 LM1085IT (datasheet) or THT, 5V, 5A, TO220 LM1084IT (datasheet), will it help?

8

Thanks a lot for your explanation.

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  1. Low ESR CE 470u/25VIT HIT-EXR 10x16 RM5 (datasheet)
  2. Low ESR CE 470u/25VIT HIT-EXR 10x16 RM5 (datasheet)
  3. CT 330n/35V RM2,5 20% CA42 (datasheet)
  4. THT, 5V, 5A, TO220 LM1084IT (datasheet)
  5. CT 100n/35V RM2,5 20% CA42 (datasheet)
  6. Low ESR CE 68u/25VIT HIT-EXR 6,3x11 RM2,5 (datasheet)

So with all what was mentioned above, it should be like this?

9

If you don't know a term, it is a good idea to look it up. First hit on Google for "MG996R stall current" https://www.piborg.org/motors-mounts-and-wheels-1140/metal_gear_180_degree_servo

The servo briefly draws the stall current of 2.5 Amperes every time it starts moving, and of course, continuously when it is stalled.

Capture

10

Hm, checked datasheet for new linear stabilizer and looks like it needs different tantalum capacitors. Ahh, need to still figure out which ones before I put it together.

11

We stopped using high-current linear regulators like that about 30-40 years ago.
Move on, get a buck converter.
Then you can drop the heatsink and use a 12volt supply with lower current.
Leo..

12

I dont use heatsink, there is 40mm fan installed (because of A4988) for cooling and I am already using 12V pow sup. Thats why I have the linear stabilizer, to change it from 12V to 5V for the servo.

13

Do I understand it well from the datasheet posted above (for the 5A version of the linear stabilizer), that this one uses 10uF tantalum capacitors for filtering (instead of 330/100nF for the 1,5A version)?

14

Maybe if we keep repeating this, the message will get through at some point.

This kind of module, although there's about a 1000 variants that do the same:

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There's only 3 pins you need to connect, so it's not any more daunting than a linear regulator.
Also no need to worry too much about capacitance anymore since the neccessary filtering and buffering already happens on the module.
They also cost roughly the same as a beefy linear regulator and the necessary caps combined. It's just less work, more efficient and overall a much better solution.

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