I am a hobbyist who is trying to build a DIY like car robot controlled via 2.4GHz radio and decided to use ULN2803A which was unused on my electronics equipment and has 8 outputs 4 for each motor and 2Ampers until 50Volts in DC if I'm not wrong. I'm good in programming so the problem is with electronics and understanding well the datasheets. I think ULNs are good for high voltages with DC and work with low current but they don't dissipate so much power (correct me if I'm wrong) so is a better option than L298N. The problem is that I can not change the polarity on monophasic motors (2-wire). Then I decided to use 4 relays to switch the poles of the motors with Arduino and digital pins. (LOW and HIGH, forward and backward).

The capacitor of the ULN2803A is an electrolytic one of 100 uF which I don't know if it's a problem. And the capacitor also electrolytic for the gyroscope is 1000 uF to prevent EMI of the motors. Also the motors have 100 nF ceramic capacitors 50V on each pole to the ground, to prevent EMI. (I read on Pololu's website).

I can draw the circuit for the relays but at the moment I'm showing you the circuit of the driver that is what I've drawn but it's easy to imagine what I've done with the relays (a typical board of 4 relays able for Arduino 3 holes with screws per relay): each wire of each motor is on the middle hole of the relay and as the relay switches between right and left connection the middle wire, in the left is positive and in the right negative. At first everything works fine until I start to switch, at first a motor stop to run and then after switching fast backward forward the ULN stop working completely.

So, the question is: Is possible to change polarities of the motors by this installation?

Sorry about my bad English and sorry about the complexity of the problem (I can imagine, I'm just a hobbyist).

Thanks in advance, I wish you a good day.

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    \$\begingroup\$ How about a really simple version of this focusing on the questions and not the back story? \$\endgroup\$ – Andy aka Feb 14 '17 at 14:31
  • \$\begingroup\$ Thanks for the comment but I think the doubt is clear. Everything worked without relays now with relays is failing with high pulses and I can't remember if used 3S or 4S LiPo when tested without relays for bidirectional. Sorry for the back story I just enjoy my hobbies...hope you can understand. In short words I did my best, I haven't studied electronics so I can not do it better, that's very confusing to me. Everything worked fine, then the ULNs started to stop working when mounted on the car and started to switch between forward and backward with high pulses. I can't do better.... \$\endgroup\$ – user54073 Feb 14 '17 at 15:19
  • \$\begingroup\$ I cleared my ideas and changed the question. Thanks again for your comment. But I had to find the problem, now I know how it becomes broken what I don't know is how to fix it. At first I was hearing bells but without knowing where... \$\endgroup\$ – user54073 Feb 14 '17 at 18:08
  • \$\begingroup\$ Your circuit description is vague and incomplete, can you show us a schematic? What is the purpose of the relays? \$\endgroup\$ – Bruce Abbott Feb 14 '17 at 18:11
  • \$\begingroup\$ I think that if you can not understand for what I'm using relays then you can not solve the problem. I know my explanation is vague, is the problem of not being an english. Is not incomplete, you didn't understand anything. Clue: read the title until you got the answer for your question ;) \$\endgroup\$ – user54073 Feb 14 '17 at 18:17

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