PDA

View Full Version : Current limit on Vdd



agentile
02-15-2005, 10:24 AM
I am using the BS2 on a BOE powered by a 9V battery.· I am using Vdd to power several TTL logic gates on a protoboard.· What is the maximum current I can draw from Vdd before I damage the BS2?

thanks,
AG

achilles03
02-15-2005, 11:01 AM
I'm not sure "damage" is the right word... The regulator is a LM2940:

http://www.national.com/ds/LM/LM2940.pdf

Looks like so long as you have atleast 5.5V on it, it can source up to 1Amp. It's a function of the supply voltage AS WELL AS the supply's capacity. If you have an alkaline power supply, and are hoping to get 1A out of the regulator, you're going to be hard-pressed... not because of the regulator's capacity, but because of the battieres drop in voltage at high drains coupled with the regulator's limitations.

Dave

agentile
02-15-2005, 11:52 PM
Dave, I figured that the regulator could supply plenty of current, more than I need.··My supply may turn out to be 2 or 3 9V batteries tied in parallel.· So, I will have available current, I'm just concerned that my stamp may not be able to handle it.· The power going to my protoboard is not coming directly from the regulator, it is coming from Vdd, which means the current must flow through the stamp.··Do you know how much can I run through that pin without damaging the·stamp?· I would guess that although 1A wouldn't hurt the regulator,·it might heat up the stamp to point of no return.· Do you know of the limits on Vdd?



thanks,

AG

allanlane5
02-16-2005, 12:10 AM
The problem is not Current, it is Voltage.

The BS2 is designed so you can put 5.5 to 16 volts on Vin, NOT Vdd. If you put more than 5.5 volts on Vdd, you will kill the BS2.

So, you put 9 volts on Vin. Now, the BS2 will draw as much current as it needs to do its work -- typically under 50 mA. If you put two 9-volts in parallel, the BS2 will still draw the same current (but it can now draw that current for a longer time). If you put two 9-volts in series -- 18 volts -- the BS2 will STILL only draw 50 mA.

The BS2 has an on-module regulator, which gets its input from Vin, and supplies the rest of the circuits on the BS2. It can be bypassed by putting 5 volts on Vdd, but that 5 volts MUST come from some other regulator. Again, if you bypass all regulators, and put more than 5.5 volts on Vdd, the BS2 will be killed.

Oh, and the on-module BS2 regulator CAN supply other circuits.· I think it is limited to 50 mA TOTAL.· To do this, you wire your input voltage across Vin and Vss.· Then you wire your other TTL circuits across Vdd and Vss.· I'm not sure what the normal operating current is on the BS2 -- 1 mA?· 5 mA?· This gives you some current to run your other circuits with -- but 50 mA is not a lot.

If you do 'over-draw' the on-module regulator, it will over-heat.· Most regulators have a thermal cut-out, so it will shut itself down if it does this, until it can cool again, but it shouldn't be destroyed.· This is NOT a normal operating scenario for a linear regulator, the vendor put that behavior in to give you, the developer, some time to step back and see what went wrong.· So try not to do this very often.· "Over-draw" here means putting too small a resistance on the Vdd pin -- such as trying to supply too many TTL gates.


Post Edited (allanlane5) : 2/15/2005 5:16:53 PM GMT

agentile
02-16-2005, 12:44 AM
Thanks for the info.· So if I keep my adjacent circuit at 50mA or below, I should be fine.



AG

achilles03
02-16-2005, 01:22 AM
agentile, maybe you need to read the specs on the BS2 again. Any single pin (0-15) can only source 20mA. The BS2 can source a total of 50mA out of all pins. You can get more off the Vdd pin because it is not going directly through the stamp, but you can't control the output of the Vdd (it's only a power supply, not an output). The stamp itself runs off power provided by the regulator (it needs a precisely controlled voltage to work well, i.e. 4.5 to 5.5V). The Vdd pin is also the voltage source for the BS2.

How much power do you think you're going to need? For anything under 100mA, 4 AA's or a 9V work ok.

Also, linking 9V batteries in parallel probably isn't a good idea. They'll have slightly different voltages, and when you hook them up they'll try to reach an equilibrium voltage quickly. Even though the voltage difference is small (.05V or so), it amounts to a significant amount of current considering there's almost no resistance between the cells. Some cells will have a reverse current running through them, while others may be experiencing a significant drain for a couple minutes. This is not good for rechargable batteries, and it's terrible for alkalines.

Secondly, 9V's can't source much anyway. Are you using alkalines? If so, each cell can only source about 100mA per battery before you start killing the chemistry. You CAN drain them up to 300mA, but you're only looking at about 6 minutes of use.

http://data.energizer.com/PDFs/522.pdf

Hope that helps,
Dave