# ZX-DCM2 - Maximum Amps

Does anyone know what the max amperage rating of the aforementioned device, the ZX-DMC2, is? I just tried to run it with 8 AA batteries in series and got a face full of smoke. I thought that the device was supposed to work with a potential difference of 24 volts?

Nowhere can I find out what the max amp of the thing is.

On another note, since my next step was to connect the Fez to those 8 AA batteries (in series)… What is the max amp rating of the Fez Domino?

amp = current not voltage!

FEZ can run 6V to 12V
A battery is 1.5 volts so 8x1.5=12V

About the motor driver board. It looks like it is using L293. Here is the datasheet

So, the chip can handle 2A and can handle up to 36V so the smoke you saw was due to wrong connections?

Just looked over the pictures I took of it…

Yep; I connected it backwards. I’m an idiot.

So then can the Fez Handle 2Amps at 12V then?

The FEZ will never draw 2 amps, nor can it drive any other device with 2A of current.

I’m not exactly sure what information your trying to get at with the way you asked the question. What are you trying to do?

I’m worried about hooking up 8 AA batteries in series to power Fez. From what I can tell, Fez’s high resistance would prevent it from drawing that much power and blowing itself (provided of course that it’s corrected the right way round).

Would those 8 AA batteries at 1.5V each in series generate a 12 volt potential difference or a 24 volt potential difference, and it does generate 24 volts potential difference would Fez care?

1.5 x 8 = 12v. The 12V goes into a 5V regulator which must disapate the extra 7V as heat. 12V is the maximum voltage allowed but it very high. The device will get very hot.

Why not run 5 batteries? 5 rechargeable batteries would be better @ 1.25V each.

At 24 volts the unit will briefly give off some smoke and maybe flame.

I had also the same problem with some “heat” when I use 12V.

@ Gus:

It is definitly too high the input voltage as I said in my post(see above)
Better to fix the documentation

OK, I understand where you are coming from now. An electronic device like a FEZ will only use the amount of power that it designed to. In the case of just the FEX board itself that might be 50mA or so at 5 Volts or 250mW (guessing hi I’m sure). No matter how large (in capacity) a battery pack you hook to it, the FEZ board will only use ‘its’ amount of power.

Power = voltage applied * the current flowing in the circuit. (Think of this like your car engine where power is the combination or torque and RPM). So as in our example above: 50mA * 5V = 250milliWatts. Think of this as the amount of power the FEZ needs to operate.

The FEZ boards use linear voltage regulators, which regulate the output voltage to what the device requires from a given range of input voltages (like the 6-12V rating of the FEX power input). Linear regulators work great but have the quirk that while they regulate the output voltage to what is needed, the current passing through them is the same as what the device (FEZ) is drawing. So we have so ‘left over’ voltage that is being ‘dropped’ across the input of the voltage regulator. Just like when we figured out how much power the FEZ was using we can figure out how much power is being used by the regulator itself if we apply 12 volts to it (12V applied - 5V that FEZ needs = 5V left over).

P = 50mA * 7V = 350mW (more power being wasted as heat than what it takes to run the FEZ)

If we use six volts though it looks more like this

P = 50mA * 1V = 50mW (much cooler operation)

@ Kurtnelle

Your motor driver needs 12v (I assume) but the FEZ does not. You are connecting the AA cells in series, may be you can tap out the wire after the 4th cell to get a 6v potential (1.5*4=6) between the +ve terminal of the 4th cell and GND.

Cypher, the documentation is correct. 6V to 12V is fine on all boards except FEZ Cobra where you should only use 6V to 8V

Really?

I wonder , because both boards uses the same power circuits.

12V -> LM1117-5MP -> LM1117-3.3MP ->processor

While the chips may be the same the heat disipation capabilites may be different.

The chips are capable of 30V I think so the voltage is not the problem. The problem is that Cobra draw a lot more current, especially when you add the color display…
more current => more power => more heat on the regulators.

So if you connect cobra to 12V, it will be fine for the first few minutes but then the regulators will heat too much to the point that they will melt off the board or shut down if they have internal thermal protection.

Hmm ,

but both voltage regulator are the same and there were no heatsink.

The cobra board is much bigger than the FEZ Domino so if the board was the heatsink the Cobra must be much cooler. But as I said , the Cobra board runs very well with 7-8V but not with 12V

So can you explain the better heat dissapation of the Domino board?

Ahh ok,

that is the reason much more current , much more heat

I understand better now. Anything left over will be converted to heat by the regulators. So its possible to blow the regulator; that makes sense. What I have is a battery pack that takes 8 AA batteries. I have another pack that takes 4 but when I tried that the Fez’s power lights lit up, however it wasn’t sending out any signals (I had the ZX_Servo16 connected. There is a yellow light on it that indicates activity on the serial line) At that point I just assumed that there wasn’t enough power to the system.

So what I’ll do is: I’ll use a 9V battery to get away from a high amperage as well as getting a lower voltage and use that to power the Fez! The on board regulator should be able to handle that no problem!

As for the center tap Idea: I had thought about doing that just prior to my smoke incident; so I think its a good idea.

I’ll try it and see how it all works out.

Why not use high capacity rechargeable batteries like this one?http://www.sparkfun.com/commerce/product_info.php?products_id=9703

This is rechargeable and should get your FEZ running for long hours

Rechargable 7.4v Polymer Li batteries: Thanks gus! This would also be a cheaper long term solution as well.

I can write the c# to run the Fez in 1 afternoon but the hardware stuff just takes months for me to figure out. Good thing I have these forums.