View Full Version : Power problems

09-15-2008, 09:43 PM
Hey guys,

What would happen if I try to connect an additional LM7805 5V+ regulator to the Vdd pin of the basic stamp?

Basically, I need some additional power for a line follower robot with·an a regulator to fix the problem associated with battery voltage drop. Since the basic stamp already has a 5V regulator will an external regulator and the basic stamp's 5v regulator work together?

I've been reading online that the one with the lowest voltage will actually try to source all the current and the other will do nothing, so if that were to happen I would exceed the power draw of the on board regulator of my basic stamp.

Thanks for your help,


Mike Green
09-15-2008, 10:00 PM
The on-board regulator will turn itself off if you try to draw too much current through it (as a protective mechanism).

Normally, you divide up the items that need +5V regulated current. Some of them get connected to the output of one regulator and the others get connected to the output of the other regulator. The outputs of the regulators are not directly connected together. The inputs to the regulators are connected together.

It is possible to connect several regulators together, but you have to follow the manufacturer's recommendations. These may vary from one type of regulator to another, whether there's just a requirement for a minimum output capacitance or you need small series resistors for current sharing.

09-15-2008, 10:15 PM
A "Linear Regulator" like the 7805 needs a certain voltage drop across itself in order to regulate. I believe the 7805 requires 1 volt -- so you need at least 6 volts across it to supply 5 volts at its output.

So, if you connect it to Vdd, you'll get nothing. If you connect it to Vin, you should be able to get another 1.5 amps out, assuming the wall-wart or transformer driving it with 6 volts can supply that much current.

09-15-2008, 11:51 PM
I think you may be looking in the wrong direction for a solution to your problem. If you're trying to fix a short term (ms) power drop caused by servo's starting - then look into installing a large electrolytic capactior (10000 uF) in the circuit that power's the servo's. If you're just trying to get more running time from your robot, then you'll probably need higher current batteries (ie NiMH) or more of them.

You'll probably get better replies if you move this topic to the robotics section.

09-16-2008, 03:54 AM
Thanks Mike, I just wanted to know how I should hookup the regulators since I need to add another to power some ADCs, digital resistors, 555s, h-bridges, and LEDs. What I'm gonna do is connect their inputs but not their outputs.


Adrian Schneider
09-17-2008, 05:24 PM
You can add a second regulator to work together with the stamp's regulator. You have to connect the extra regulator to Vin
not Vdd. See diagram below.

Vin--+--[ BS2-VoltageReg ]----Vdd
| | adj
| ---
+--[ Ext.7805 ]----Vext
| adj

Chris Savage
09-18-2008, 03:44 AM
Of course, if you’re going to use a larger regulator it might remove some complication to just power the BASIC Stamp from the output of that regulator as well. This is done on the Super Carrier Board. This way you don’t confuse two separate 5V supplies.

Chris Savage
Parallax Engineering