quick advice on diode choice.

Other Hardware General Electronics
1

Hello,
i am trying to fix a pool cleaning robot.
I figured out maybe i should remove all the electronic and build a very simple circuit.
the robot looks like that

There is a 3 cable going from the robot to the home station delivering 24v.
There is also 3 DC motor in the robot.
So i plan to use 2 cable for each direction motor (controlled via a switch at the home station)
and 1 for ground.

The third motor will be activated when any of the 2 motor is activated.
So the diagram is very very simple

I wonder what would be the best diode to use?
The motor in the middle is for water aspiration and is pretty big but i can't find any number on it so do not have the specification.

Capture d’écran 2017-07-20 à 22.32.48.png
Capture d’écran 2017-07-20 à 22.32.48.png750×421 11 KB

2

Without specifications like voltage and current, everything is a guess. Can you measure anything? Supply voltage, motor current (big motor), motor current (small motors).

I am used to a definition where a cable is two or more wires bundled together, but I think that when you write "cable" you actually mean "insulated wire".

3

You need to tell us how much current is drawn by the motor circuit.

Once you've got this measurement, use a diode rated at least 5x the measured amount. Check the voltage rating of the diode. At 24V, I'd use a diode rated at least 100V, preferably more..

Hope this helps.

4

A T-220 (or bigger) schottky double diode (two in one) might work.
Plenty of them in old PC power supplies.
Leo..

5

vaj4088:
I am used to a definition where a cable is two or more wires bundled together, but I think that when you write "cable" you actually mean "insulated wire".

Yes sorry i'm not native english. So it is 3 insulated wires together.

From what i can mesure, the "home station" deliver 11v.
I belive there is something wrong with it because on the paper of this powersupply it is written 150w 30v DC.
Also the motors for the wheels are "82048075" rated 24v 3w

I can't get the specification of the third motor. It is a DC motor and it is 3 time bigger than the one for the wheels... I doubt it draws the 144w left...
I think the mosfet that controls this motor is a Z44N

6

i made a little change so that if in the future i revrse polarity on the powersupply, the robot goes backward but the pump no :slight_smile:

7

The way you have it drawn, the pump motor will be in SERIES with the opposite wheel motor, don't think that is what you want. :slight_smile:

8

edgemoron:
The way you have it drawn, the pump motor will be in SERIES with the opposite wheel motor, don't think that is what you want. :slight_smile:

true!

i guess this is better, but now the big motor is in serie with the small one ... that is not ideal i guess but should work, right?

I planto buy these SB5100 diode rated 100v, 5A...

9

No. Now the big motor doesn't run at all.

Look into "bridge rectifier" to get DC from any input.

Don't forget about the voltage drop through each diode. Your motor loses power and the diode heats up. Less diodes is better.

10

true!

i'm going to think about a way to fix it
i have 2 spdt switch so i draw the system i'll use to reverse polarity.

The center motor is not connected because i still can't figure a way to do it.
Maybe when going reverse, this motor will stop. The aspiration is quite strong and maybe the wheel will not be strong enough to go backward.

11

So what is the truth-table you actually want?

With Switch 3 in the "forward" position...

switch 1+2 off = all motors off
Switch 1 on = motor 1 forward + motor 3 forward
Switch 2 on = motor 2 forward + motor 3 forward
Both 1+2 on = all motors forward

With Switch 3 in the "reverse" position...

switch 1+2 off = all motors off
Switch 1 on = motor 1 reverse + motor 3 forward
Switch 2 on = motor 2 reverse + motor 3 forward
Both 1+2 on = motor 1+2 reverse + motor 3 forward

That's an unusual requirement, but not unexpected since you usually want to reduce the number of wires in an underwater cable.

Since I'm an Arduino guy, I would put an Arduino in the underwater unit. I would have two big fat power wires that are always powered. Then one or two small signal wires to send commands to the Arduino. If you do the calculations, on a 12V system the voltage drops in long wires can be crippling.

If you try to keep to the original schematic, then you're probably going to need a relay or two inside the underwater unit.

12

i started thinking that i want an arduino in there
look at this thread Help me hack my swimming pool robot - Jobs and Paid Consultancy - Arduino Forum.

But i have 2 problems:

  1. I am on low budget mainly for something i do not need.
  2. I do not want to spend lots of hours to figure out the best way to do it ( which surely is with a arduino nano ) when i probably can do it with 4 diodes and 4 switch...

To answer your first question.
I have 2 spdt switch so i have drawn them but it would be best to use one dpdt and they should be thought as such.
meaning, either S3 is up and S4 is up ( forward mode) or S3 is down and s4 is down ( backward mode)
S3 up and S4 down will do nothing and i know it...

Now the truth table would be
S3+S4 up :
S1 on + S2 off : M1 forward, M2 off, M3 forward
S1 on + S2 on : M1 forward, M2 forward, M3 forward
S1 off + S2 on : M1 off, M2 forward, M3 forward
S1 off + S2 off : M1 off, M2 off, M3 off

S3+S4 down :
S1 on + S2 off : M1 backward, M2 off, M3 forward
S1 on + S2 on : M1 backward, M2 backward, M3 forward
S1 off + S2 on : M1 off, M2 backward, M3 forward
S1 off + S2 off : M1 off, M2 off, M3 off
Note that with S3+S4 down M3 being off all the time is acceptable to me and i think it would bring me back to my first drawing.

i hope i am being clear!

[EDIT]
I made a better version of the diagram where you can see where the 3 wires goes underwater together

13

Cunbersome, but it works

Allan

mdrv1.pdf (12.8 KB)

14

thanks, looks perfect to me :slight_smile: