Hi, I have quite a few A4983 boards but i’m having problems getting anything like enough current from them.
I have a VMOT voltage of 18v, I’m in full step mode and not moving the motors. When I monitor the current on my bench PSU I cant go above 300 - 500ma or the SMD chip gets red hot and eventually the thermal shutdown kicks in. Why is this if they are good for 2 amps?
Thanks if anyone can help.
Mike.
Hello Mike,
Have you tried turning the current-limit potentiometer?
- Ryan
Errr… Yep!! I start with the pot nice and low and gently ease it up watching the current on by bench PSU, Once I get to about 400ma the SMD a4983 chip starts to get far to hot and as a result the thermal shutdown will kick in.
Hope thats made things a little clearer.
It shouldn’t be overheating at that current. I’m skeptical of your current measurement. Do you have some other way to verify your PSU’s current display? What is the winding resistance of your stepper motors? You said you bought a few boards; do they all behave the way you described or does only one behave that way?
- Ryan
The coil resistance in the motors I am using in 2.5 ohms, They are 1.68 amp. I do not have any way of confirming the current display on my PSU but I have no reason to beleive its not accurate as other devices current draw is consistant with the analog dial display on the PSU. It may not be 100% correct but it is cerainly not 2 amps out, According to the datasheet, to set the current I should be in single step mode and not clocking the step signal, Then the measured current draw should be 1.4 times the current limit.
I have 9 of these drivers in total and all are the same, They were not even purchased in the same batch so I can rule out them being faulty.
Really cant understand what the problem is, Thanks for your time Ryan.
Mike.
Hello,
With an 18V supply and 2.5 ohm coil resistance, you’re asking the driver to supply over 7A. The driver is going to try to keep the average lower by turning the driver on and off, but something that would draw 7A is asking a lot of the driver. The duty cycle will also be quite low, which could make the RMS current quite a bit higher than what your power supply is showing if it is giving an average, and the RMS current is what is heating your chip.
Can you lower the motor voltage to something lower? Do you have it that high to get a high step rate?
- Jan
Hi,
I cant go lower than 15 volts really as I do require a reasonable RPM from the motor. I have used the same motors with a Purchased driver board (www.diycnc.co.uk) with multipul current settings. Its based on the a3977. When set to “1.5 amps” interestingly i’m still only drawing around 0.5 amps? this is with a Motor supply voltage of 19 Volts.
Thanks.
Mike.
The specs on the A4983 are similar to the A3977, so you should be able to get similar performance with the two. One difference is that the A4983 is physically smaller, so that will limit its thermal dissipation ability. If you’re close to something that works, you could try adding a heat sink to the chip. However, your motor might just be too big to be running at that voltage with this driver.
I’m not sure what settings you’re talking about setting to 1.5A, but as I mentioned earlier, a low duty cycle will give you very different results for RMS and straight average. To keep it simple, imagine a 10% duty cycle, completely square wave that has a peak of 10A (and 0 everywhere else). The average current is 1A, but power dissipated in the chips scales with current squared. During that 10% on time, I^2 is a hundred, for an average of 10 over the whole period. From a power dissipation perspective, therefore, the current looks more like a DC current equal to the square root of 10, or a bit over 3A. Since you’re describing a 3:1 ratio, something like this could be in play.
- Jan
That would make sense to me, I have today bonded a small heatsink to the top of the driver using some thermally conductive potting compound, ( to create a flat electrically insulated surface away from other components). This works well but as the driver is now encapsulated I cannot alter the current anymore!!!
Everything seems okay, The motors appear to have the same amount of torque as they did driven with my other drivers so i’m happy at that.
Thanks for your help and time.
Mike.
