Controller Board Getting Very Hot / Allegro A3977

[MikeVA]

Hi - I have a problem with setting up my first CNC project. Its a conversion of a sieg x2 type mill (purchased from Chester Uk).

I'm using the download (unlicensed version) of Mach 3 and have got to the point of being able to control the motors from Mach3. The problem is that when I reset the system (power to the motors) the chips on the controller board get very hot, very quickly - being too hot to touch within 10 - 15 seconds. Surely this is not right!

The board is from DIY CNC (http://www.diycnc.co.uk/) and is the System3 board (http://www.diycnc.co.uk/html/system3.html), with Nema 23 stepper motors. The board is currently configured for minimum current delivery, and 1/8 micro stepping.

I'm concerned that something is seriously wrong and that so much heat production will result in the destruction of the controller board.

I've been advised that I should connect up the motors as follows:

__________________________________

To use the motors in series

1A pin to blue/white
1B pin to red
Join red/white to blue & keep separate

2A pin to green/white
2B pin to Black
Join green to black/white & keep separate

__________________________________


Any help or advice would be much appreciated,

Mike.

[Lee Roberts]

Hi, I’ve been in contact with Roy over at diycnc.co.uk about these drivers and this is what I got back:

Thanks,
Roy
P.S The Heat sink stuck to the top of the chip is not effective. The heat should conduct down the legs into the copper on the PCB. That’s why I use 2oz copper on my boards. Allegro do not mention using a heat sink stuck to the top in their data sheets.

Where he makes reference to the heat sink, that’s with regards to the drivers that routoutcnc make, they use heat sinks on their units. I personally think and in my experience of 12 years building computers have never seen a PC that doesn’t have a heat sink on the CPU. Granted there is a difference between the 2, I ask the question of: “do we want heat going back into the PCB?”, and I answer with NO WE Definitely DONT!

Why would you ever push heat back into the PCB when there is an option to pull heat away from the unit via heat sinks, how is it logical to go about things in that way. Your post actually falls in nicely with emails going back and forth from Me, Mike (routoutcnc) and Roy over at (diy cnc) as I’ve asked them both a few questions about each of their units.

I personally think that a heat sink would only but help, why Roy chose’s to not use one I don’t know, the fact that mike dose use them on his units can only be a positive thing. I am currently looking at building my own drivers using the Allegro A3977 chip, so I will be asking the “Heat sink” question to someone I know who has also worked with the chip allot, and I know for a FACT that his site suggests the use of a Heat sink by a member of his site and user of his drivers, I think the best option for you is to get hold of some old heat sink way back from yester-year and mount it to your drivers and see if that helps.

I’m going to ask Mike & Roy to read this thread and give us their inputs as I think they will have better understanding as to why they have both done things differently.

I will also ask the person who has worked with the Allegro A3977 to join us as well.

[Lee Roberts]

The chip dissipates the heat via the ground pins, center 3 pins on each side. A heat sink is a good idea for currents over 1.0a.

Picture is of an old 486 heatsink, in open air running at 2.5a it gets quite hot and so a cooling fan could be used additionally.

I think its going to come down to the users individual preference, i myself like to keep things COOOOL !

[irving2008]

I dont know this chip, but I do know systems thermal design. The spec reads:
"The A3977 is supplied in a choice of two power packages, a 44-pin plastic PLCC with 3 internally-fused pins on each of four sides (suffix ED), and a thin (<1.2 mm), 28-pin TSSOP with an exposed thermal pad (suffix LP). Both packages are lead (Pb) free, with 100% matte tin leadframe plating."

Both packages are intended to be cooled by contact with the PCB, either through the mounting tabs (ED) or via the underside pad (LP)

For the ED package the relevant thermal info is here and for the LP device here. It is essential to cool them via sufficient PCB ground area for which the thermal resistance (junction to PCB) is 6degC/W (ED) or 2degC/W (LP). If you want to stick a heatsink on it then stick it to the ground areas.

So irrespective of whether you want/like heat going into the PCB or not CheekyMonkies is correct (other than for the ED pack its all 4 tabs that conduct heat and all 4 must be used), that is the way the chip is intended to be used.

For such a device I would expect the case to get too hot to touch as what you are touching is the back of the heatpipe (the chip is actually under the metal support). Adding a stick-on heatsink here will make you feel better but will have minimal imact on internal junction temperatures as the thermal resistance from junction to case is very high.

Anyway, the chip has thermal shutdown.. if it were unhappy it would shut down and since it seems thats not the case I wouldnt worry...

BTW if that picture is of a board using the chip in question then my gut feel is that it would appear not to have enough copper heatsink area but its hard to tell without measuring it...

[Lee Roberts]

Another thing that needs to be said is that Mike (routoutcnc.co.uk) doesn’t use the Allegro A3977 chip, that’s from the horse's mouth.

[irving2008]

Another thing that needs to be said is that Mike (routoutcnc.co.uk) doesn’t use the Allegro A3977 chip, that’s from the horse's mouth.

So what are we discussing or are we all talkiung at X-purposes? what chip does he use?

[Lee Roberts]

I'm not sure ill have to ask him again, but this should be about Roy (diycnc.co.uk)'s drivers.

[irving2008]

I'm not sure ill have to ask him again, but this should be about Roy (diycnc.co.uk)'s drivers.

looks like the A3977 to me... if you zoom in to this pic...

[Lee Roberts]

Thats Roy's driver from diycnc.co.uk

[irving2008]

Thats Roy's driver from diycnc.co.uk

yep, got the pic off Roy's site... if you zoom to 400&#37; u can read the chip IDs :)

but doesnt look like enough copper there to me....

what I would do is solder some copper sheet vertically to the heatsink leads, then cut fingers in the top edge and twist to increase surface area