Problems with Easydriver current

[mvoltin]

I am having terrible problems with high currents and Easydriver and already fried two of them (with sparks and smoke coming out of the main IC and the chip getting a huge bump from the "explosion"). both are related to excessive current requirements but I thought EasyDriver limited the max current to 750mA irrespective of motor demands or PSU supply. Does it need any current restrictor and why (since other drivers do not need this)? If it does, any suggestion would be appreciated.

In both cases, I used 24V 1.6A PSU and motor was either 5 or 24V with around 1.2 amp per winding. They both worked fine without a load but, under a load (as it drew more current - I assume) the Easydriver blew up.

P.S. I am planning on getting another set of drivers but wanted to understand what is happening or what I am doing wrong since I plan on using ED for other projects. I have read some people restricting the current that goes to Easydriver but do not understand why this would be needed.

 

[alec_t]

What Vcc voltage were you using?
Did you disconnect/unplug a motor winding while Easydriver was powered up?

 

[mvoltin]

I was using Arduino for logic and Vcc should have been around or under 5v.

No, did not disconnect the motors. The chip blew up when I put a load on the motors (they were rated at more than 750ma) and probably drew higher current since it happened within 1 second on putting a load on the motors. otherwise, they were operating fine.

I was just surprised that this happens since the datasheet states: "The current in each of the two output H-bridges is regulated with fixed off time pulsewidth modulated (PWM) control circuitry. The H-bridge current at each step is set by the value of an external current sense resistor (RS), a reference voltage (VREF), and the DAC’s output voltage controlled by the output of the translator." and "Adjustable current control from 150mA/phase to 750mA/phase"

So, it should not have let more than 750mA through but it burned when the current demand increased.

 

[alec_t]

So, it should not have let more than 750mA through

I agree. Though IIRC when the Vref is 5V (trimmer at maximum) the limit would be 850mA?

 

[shortbus=]

Isn't this why you are always supposed to 'over volt' steppers? Running them at the name plate voltage is asking for disaster I was always told. But you also can't exceed the driver voltage either, but should use something close to that voltage limit on the driver. Check the Geckodrive web site for information on stepper voltages.

 

[alec_t]

According to the datasheet for the A3967 IC used in Easydriver:-

The load supply terminal, VBB, should be decoupled with an electrolytic capacitor (>47 µF is recommended) placed as close to the device as possible.

In the Easydriver schematic there is no such Vbb (V+) decoupling for that IC evident. The only 47uF decoupling cap shown is adjacent the LM317. Since one cap can't be physically 'as close as possible' to two ICs, and 47uF may not be enough to adequately decouple both, perhaps that is causing an instability and consequent overheating problem?

Edit: Looking at the Easydriver pcb layout, the 47uF cap is at the opposite end of the board from the LM317 and from pin 20 (V+) of the A3967

 

[mvoltin]

Thank you, alec,
I looked more closely to the ED and noticed some differences from the official version (I got mine from eBay for half price - made "somewhere"). the capacitor says "3cd (line break) 100 (line break) 35E" The last "E" is in superscript. I interpreted this as a three letter convention with the last digit being a multiplier (https://wiki.xtronics.com/index.php/Capacitor_Codes). Thus, this would be 10uF? Is this correct and could it cause the problem?

 

[alec_t]

I don't see how you get 10uF from the 3-digit convention. 10uF = 10 million pF, coded as 106. Those markings don't conform to conventions I've come across (albeit in a quick search only). My guess is it's 100uF 35V working. In its favour, that would be " > 47uF " as recommended for the A3967. Is it positioned very close to the supply pins of either IC as recommended?