CS and Shoot-thru question for the18V25CS

Hello, I am using the 18V25CS to forward and reverse drive a Ford winshield wiper motor driving a single wiper blade. Using a static car battery @ 12.9 volts I have a no load CS reading of 2.31 volts. Running the wiper I read a constant + 2.75 volts. With my multimeter in series with the battery it shows a max draw of around 4 amps and a nominal 2.8 amps with its sample rate. Is the CS formula " 66MV PER Amp ? " If so, the CS reading is much higher at app. 6.6 amps. Why do I see a static 2.31 volt CS centering voltage instead of 2.5 ?
My second question is about " shoot-thru". I am controlling the driver from a PIC 684 micro and have inserted a Shoot-thru delay in my program. The wiper works great but I feel I could shorten the delay time between forward and reverse. My question would be, is an external " Shoot-thru" delay neccessary when using the 18V25CS driver, or dose the board do that internally for you? Thanks for any help!


How are you powering the current sensor? Can you measure VCS with your multimeter (make sure you don’t short VCS to GND with your probes!) and let me know what voltage you see? In general, I’m skeptical of your getting decent measurements with your multimeter of what sounds like a very unsteady current. Do you have access to an oscilloscope that you could use to look at the CS output? If not, can you think of a way to generate a static test case (e.g. do you have a motor you can stall at a lower voltage, and then read that stall current with your multimeter along with the CS output voltage)?

You should not have to worry about shoot-through. The driver automatically inserts delays before turning on various parts of the H-bridge to prevent shoot-through.

- Ben

Thanks Ben, I am using the boards + 5 volts jumpered to VCS. I just checked it and it read 4.93 volts unconnected. I put that problem ( if it was one) aside, disconnected the jumper to VCS and fed a + 5 volts from the micro. The micro’s voltage was 5.02 unconnected and fell to 4.54 connected to VCS. CS unloaded now read 2.44 vs the original 2.31. I unconnected everything to check for solder bridges ect. everything looked good. I read the resistance between the +5 volt pin and GND and read 1 ohm with continuity. Not good. I reconnected the 12 volts and now there is no output from + 5 volts ( 25 mv). ( I watched the mA’s ready to pull the plug)

All of my solder joints are very good, and I use a ground strap when handling circuits. I assume the 4.93 vlots accounted for my initial low centering voltage. Unloaded on a 12.9 volt supply the circuit is pulling 26mA. I would like to still use the driver if possible and not loose $60.00. This just happened so it would seem resonable that something was failing.

I originally bought this to run an electric power steering motor but thought this was a good test. I do not need the CS for this application. What are my options? Can I isolate the 5 volt source and cut a trace or resistor. Or, can I continue to to use it with the fault and tolerate the 26 mA draw. Ramping the voltage to 14.0 volts the 26 mA stays the same. Any advice is appreciated. Dale

I’m a bit confused. From the symptoms you describe, it sounds like your motor driver is now dead, which can happen if you ever accidentally short the 5V output pin to any other voltage or try to draw more than a few milliamps from it. (Is there any chance you brushed it with the wrong probe or wire while trying to connect it to VCS or look at it with your multimeter?) However, you are also making it sound like the motor outputs are still working. Are you still able to drive a motor with the driver?

I will continue to look into why you might have been getting such strange current sense readings, but in the meantime, if your driver is actually dead, please contact me directly at ben at pololu dot com and I can at least get you a substantial discount on a replacement unit.

- Ben

Hi Ben, Yes you are correct. No 5 volts no control. I was typing faster than thinking. I am careful but I could never say for sure that I did not short the pins. If the regulator blew can I isolate it by cutting the in and out trace. Then check to see if the short is in the remaining traces. If it is game over. If not I could externally supply the 5 volts. It would be worth a try if possible.

Questions: . Is there not any diodes ( zener ect ) for protection? 2. I will try one more time and purchase the 18v15. Can you tell me what the “on” state mA current draw should be so that I can check it before and after operation? I have to admit that I am a little leary as I was always careful with MOS devices. Is there any chance that back EMF from a wiper motor could affect the circuit? I have built BJT bridges and because I always work with DIP chips, I find these small surface mount bridges current carrying ability to be amazing . If all goes well I will get another 18/25 for my power steering application.

Ben, thank you for your time in this matter. It is appreciated. Thanks,Dale

The board does not have an external regulator. It is an integrated part of the MOSFET driver IC, so at this point the entire MOSFET driver would need to be replaced. We would typically offer to have you send the board back for us to look at and repair if possible, but a big discount on a new board in this case is likely more convenient for everyone in terms of time and shipping costs.

In my experience, the MOSFET driver is somewhat reliably easy to damage through misuse of the 5V out pin, where misuse includes trying to draw more than a few milliamps. There are no external protections on this line that would prevent the user from trying to draw more current than it can source.

This question is so general that it makes me wonder if you really meant to ask something else (or maybe I’m not understanding you). Yes, if you spin your motor fast enough (specifically, faster than it would spin on its own at full power), it will generate more voltage than you are supplying and it will try to put that voltage on your power rail, which could affect the rest of your circuit.

I don’t know the quiescent current of the driver off-hand, but I will get one out of stock later today and run some tests on it. I’ll let you know what I find out.

- Ben