Let's Make Robots!

Mr. Basic Motor Driver Kit Problem

Hi, LMR!

I need some help with troubleshooting Mr. Basic Motor Driver board. Basically, I couldn't make it work. I use 4xAA Alkaline batteries [5.8V actual voltage] as power supply. MC Picaxe 18X. Relays are working fine, but Motors are not. With my little experience in electronics I guess BD681 does not open for some reason. But not sue, I might be wrong...

My setup:


Troubleshooting done:

First I checked setup with only VCC and GND connected to motor driver board. I have VCC and GND potentials on respective board outputs with motors disconnected. Is this ok?? Well, as long as there is no current between them... But I'm not realy sure... Then I have checked picaxe actually sets out pin high. and connected PWM input of the board. At this point with PWM high, I have 5.8 (VCC) and 4.6v on motor outputs (motors disconnected.) And no current drawn by motor when connected..

Anyone could help???

Am I doing something wrong ??

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It isn't a bad batch because I have been using the same transistors for another project and they work fine, it was only a small bag of about 50 so I don't believe that every one I've used (about 20 or more) has been ok and every one sent out is bad.

It sounds as though you are putting too much heat on the transistor during soldering. You should not take more than a second or two to solder one leg. Allow some time for the transistor to cool before soldering the next leg. Having the heatsink mounted on the transistor before you solder will also make a big difference. If you need to hold the soldering iron on the leg for longer than a second then you may not have a hot enough iron.

Hmm. Do you think the circuit would work (at least well enough to get a motor running for testing) with a TIP120 Darlington transistor? I have a couple on hand I could test the circuit board with.
The TIP120 is a similar transistor but in a larger case and capable of higher currents. I have not used it before but the datasheet looks good. Be carefull of the pin orientation. It apears to be reverse to that of the BD681.
Trying it now.

OK. I successfully tested the TIP120 Darlington in place of the BD681 and it worked fine for a short test under no load. I guess I must have overheated the BD681 as OddBot suggested. This time I was careful to keep the soldering iron on for just a few seconds, and I had the heat sink attached BEFORE soldering. I still have to replace the other BD681.

Thanks for the advice, Oddbot.  Do you think the TIP120 is an OK long-term replacement for the original transistor, or do I need to order a couple of BD681s?

Update: Got both TIP120s installed. I really messed up the board a bit desoldering, but I manged to get all the contacts working OK. What a night.

From what I have read of the datasheets, the TIP120 should be as good or better than the BD681 as it can handle more power and has a higher minimum hfe. If you have the TIP120 working then they should do the job nicely.

First, thank you Oddbot for your 'customer support' and thanks ignoblegnome for the idea of using another transistor.

My housemate was desoldering some computer power supplies and he found some transistors; most of them where in pairs =) We checked their datasheets and decided to use a FQP45N03L to replace the BD681. It didn't worked...but in my blog post, robologist mentioned that the transistor was soldered wrong. So we fixed it and after cracking our heads for a while, we also discovered the cables going to the Arduinos were inverted. After hitting all those walls, the motor controller worked! Now I would like to know if this transistor would be fine for long term use. (The link is the datasheet.)


Originally I did not use FETs because most need at least 4.5V on the gate (Vgs(th)) to turn on and this motor controller was designed to work on voltages as low as 3.6V

The FETs you have found have a Vgs(th) of 1-2V so they are actually a better choice. If the price is not too high then I may change our design to use them in the future. As a bonus the pins are in the right configuration :D

Yep, those new transistors will be fine, as long as you don't crank the supply voltage above 30V. The FQP45N03L has built-in diode protection which is nice, and it can handle up to a massive 45A drain current.
Thanks for answering my question...but I have another one. The first one I used is working fine, but the second one is having problems. I think the metal ring in the board 'disappeared' and there's a poor contact between the transitor leg and the board. How could I fix that?