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04-05-2012, 06:17 AM
Hello

I have a few Propellers on different breadboards. One thing I have been experimenting with is using different power batteries, such as 8 AA batteries in series (12v) or a 9 volt battery. I am also using the LM2937 3.3v voltage regulator in front of my Propeller chip. This works but the metal plate on the back of the voltage regulator gets extremely hot very quickly.

I notice on the Propeller Education Kit breadboard I have a 5 volt regulator feeding into a 3.3 volt regulator, and with the same 9 volt battery the regulators don't get hot. I suppose that is because there are two of them.

I am just wondering what is the standard practice for handling situations where you have for example a 3.3 volt microcontroller on a 12 volt circuit. One electronics book I have talks about voltage dividers... should I use something like that or just two voltage regulators like the Education Kit board??

Can I use only the 3.3 volt regulator if I attach a giant heat sink to the back?

Thanks
Matt

SRLM
04-05-2012, 07:03 AM
You never want to use voltage dividers to produce a constant voltage for your circuit for two reasons: 1) as the battery voltage drops, the divider output voltage drops and 2) as your circuit changes it's current consumption, the divider output voltage will change as well. Together this means that the output voltage is not constant (or even nearly constant).

The standard practice is to drop at most a few (3-4) volts with any one regulator. If you need to drop more (ie, 9 to 3.3v) you chain together regulators: 9->5->3.3. The amount of voltage drop that is acceptable depends on the current. If you have a low current (100mA approx) then you could (probably) go directly from 12v down to 3.3v. It all depends on how much power the linear regulator can dissipate, and that's in the datasheet and is a function of the heatsinking (if any).

My rule of thumb is to power 5v circuits with 7-8v to avoid dropping too much voltage. Note that if your circuit runs at 3.3v and consumes 100mA, and you are powering the circuit with 9.9v, your voltage drop is 6.6v at 100mA, or .66W. This power is dissipated as heat, regardless of the number of regulators. If you use multiple regulators the dissipation is simple spread across several units, not reduced. There are switching power supplies (buck converters) that don't have this waste, but for most circuits a LDO regulator is fine.

04-05-2012, 04:18 PM
Good info - I didn't think of the current consumption fluctuations with the voltage divider.

I will have to think about how I am going to proceed. My plan involves using my Propeller to light a very large light. I am thinking that the light could just have it's own batteries, but having two different sets of batteries doesn't seem like a very good design.

For anyone else who is learning like me, here is a good article I just found on switching power supplies: http://www.dimensionengineering.com/switchingregulators.htm

Duane Degn
04-05-2012, 04:55 PM
Larger voltage drops usually aren't a problem for switching regulators since they don't waste the power as heat.

One thing you need to watch out for with switching regulators is radio interference.

I'm surprised Dimension Engineering uses changing out the regulator in a RC transmitter (http://www.dimensionengineering.com/appnotes/spektrum_mod/spektrum_mod.htm)as a good application for their regulators. I've used one of their regulators with a Spektrum RC transmitter and it caused a lot of interference (the TX and RX had a hard time establishing a lock to each other). Their (DE) regulators also caused a problem when I attempted to use one in a GPS data logging project. The boost regulator on my LCD touchscreen also causes a lot of interference.

04-06-2012, 07:51 AM
When you get to that big of a drop you neeeeeeeeed to dissipate that heat.

91403

Or get a DC-DC converter. Waste not....

al1970
04-07-2012, 06:26 AM
Hi:

If you can get to the battery; tap the battery at 4 cells and feed that to the 3.3v voltage regulator .
Feed the 5 volt regulator with the 8 cells wire. This way you don't waste power. If you use cells that you can recharge, you have to keep an eye on the voltage at the 4 cells tap. Those are the cells that will discharge first.

Al

Hello

I have a few Propellers on different breadboards. One thing I have been experimenting with is using different power batteries, such as 8 AA batteries in series (12v) or a 9 volt battery. I am also using the LM2937 3.3v voltage regulator in front of my Propeller chip. This works but the metal plate on the back of the voltage regulator gets extremely hot very quickly.

I notice on the Propeller Education Kit breadboard I have a 5 volt regulator feeding into a 3.3 volt regulator, and with the same 9 volt battery the regulators don't get hot. I suppose that is because there are two of them.

I am just wondering what is the standard practice for handling situations where you have for example a 3.3 volt microcontroller on a 12 volt circuit. One electronics book I have talks about voltage dividers... should I use something like that or just two voltage regulators like the Education Kit board??

Can I use only the 3.3 volt regulator if I attach a giant heat sink to the back?

Thanks
Matt