MATH: My Achilles Tendon

Other Hardware General Electronics
1

Hello everyone,

OK, so I know Ohm's law (A=V/R) but I am having trouble figuring out what components to buy for a circuit I have in mind.

Using shift registers (the ones I have in mind are 74HC595), I want to control 16 transistors connected individually to 16 of these DC brushless pumps: http://www.amazon.com/temperature-approval-coffee-process-application/dp/B004HHW0FU/ref=cm_cr_pr_product_top

These pumps run at 300mA @ 6V. Only one pump will ever be running at one time. I plan on running them via their own power supply (obviously), but here is where I hit the mental roadblock.

3 problems:

  1. I don't know how much their external power supply should be (Just 6V? How many Watts?)
  2. I am TRYING to use ohm's law to figure out which type of resistors I'll need... but something doesn't seem right... I suck at math
  3. Capacitors/transistors. I am in the dark here. Will PN2222A transistors work? 25v 220uF Capacitor? Those are complete stabs in the dark.

For the last couple of hours I have been trying to figure this out on my own... so now I am reaching for help! Any answers to the above questions will be GREATLY appreciated!

2
  • 300mA@6V=1.8W, so any supply that can deliver 2 Watts or more should be fine (one pump at a time)
  • The only resistor you will need is to the base of the transistor. If your using the PN2222A then a 520 ohm resistor would do.
  • The PN2222A should be fine.
3

Thanks for the fast reply!

The news about the transistor/resistor is fantastic- I'm loading them into my cart right now then!

I have found a 6v DC 1.5A 9W power supply, and based on what you said it should work no problem then correct? Here it is: http://www.google.com/products/catalog?q=6V+DC+power+supply&hl=en&client=safari&rls=en-us&prmd=ivns&um=1&ie=UTF-8&cid=17299215671186240564

4

It should be more than enough.

5

Awesome, thanks again!

I would've worried hooking my pumps up to it would've fried them because of the extra amps/watts. Glad you were here :slight_smile:

6
  1. I don't know how much their external power supply should be (Just 6V? How many Watts?)

Power supplies are usually rated with volts and amps. Check the output-current rating for the power suppy. But FYI, power is calculated as Voltage x Amps. 300mA x 6V is 1800mW or 1.8 Watts.

Watch out! Sometimes the "wattage rating" applies to the power pulled from the wall, which is usually more than it puts-out (since power supplies are less than 100% efficient and the wall-power rating is for UL or other safety rating, and it might be over-stated).

With a little algebra, the power formula and Ohm's law can be combined to get Power = Voltage squared/resistance or Current squared x Resistance.

  1. I am TRYING to use ohm's law to figure out which type of resistors I'll need... but something doesn't seem right... I suck at math

The resistor in series with the transistor base, I assume?

Transistors typically have a current gain of around 100. But for a switching application, let's assume the gain is only 20 to make darn-sure the transistor switches all the way on. We can also assume/approximate that the whole 5V is dropped across the resistor (although it's more like 4.3, with 0.7V dropped across the transistor's base-emitter junction).

300mA/20 = 15mA base current.

5V/15mA = 333 Ohms Since we made the gain assumption of 20, anything in that ballpark will work.

You don't want to make the resistance too low, or you'll have too much current (from the Arduino, throught the resistor, and into the transistor base, not through the motor), and things could overheat & burn-up.

If you make the resistance too high, the current into the transistor base will be too low, making the current through the collector, emmiter, and motor too low, making the motor run slow, and possibly overheating the transistor by switching it half-on, where it will have voltage across it, and current through it at the same time (dissipating power).

  1. Capacitors/transistors. I am in the dark here. Will PN2222A transistors work?

I think so... I'm not sure. Check the current rating for the 2N2222. Don't worry about the transistor's voltage rating. Any transistor will be OK with 12V.

25v 220uF Capacitor?

I don't know... Do you need a capacitor? Where does it go?

7

Any transistor will be OK with 12V.

Most transistors will be OK with 12v. Many RF transistors are below that.

8

Watch out! Sometimes the "wattage rating" applies to the power pulled from the wall

... The power supply I found (I linked to it above) says it has 9W... this won't be a problem will it?

The resistor in series with the transistor base, I assume?

Yes. In between the shift register output and the transistor base.

Do you need a capacitor? Where does it go?

Haha I don't even know! I have a "starter kit" for my Uno (the seeed ARDX) and it has a schematic setup for a small DC motor that shows a capacitor like the one I mentioned plugged in between the power and ground coming from the Arduino. It lists it on the schematic as "optional" so I have no idea, I just "stabbed in the dark" lol

9

I would've worried hooking my

Usually, the voltage is "constant", and the current & power depend on the load (in your case, the motor). You wouldn't want to use a 12V power supply with your 6V pumps, but a 6V/10A supply would be fine (assuming the 6V is regulated/constant).

For example, here in the U.S., most of our household power outlets are rated at 120V/15A. When nothing is plugged-in, the 120V is still there but there is zero current. If you plug-in a 100W light bulb, you get a little less than 1 Amp. If you plug-in a 1000W light bulb, the voltage might drop a bit (because of the resistance in the wires) and you'll "pull" a little less than 10 Amps. If you plug-in a 5000W bulb, you'll get more than 15A for a short period of time 'till the curcuit breaker blows. :smiley: At that point, the voltage drops to zero (and of course the current drops to zero too).

10

... The power supply I found (I linked to it above) says it has 9W... this won't be a problem will it?

No problem! 6V x 1.5Amps = 9W. Perfect!

Sorry about the confusion. I know that was a lot of information... I was trying to say, that a power supply could be marked 9W for the input (power from the wall), but you might not be able to get the get 9W out of it (into your motor). The regulatory agencies usually require anything that's plugged into the wall to be marked with the maximum power it takes from the wall-outlet.

If it says 6V, 1.5A, that's the output, and that's what you want to know. They will almost always give you the output voltage & current, so you can can always calculate the output power (Watts) if you need to.

If the specs say "9W power supply", that's the output too (and you can always check the calculations). But, the markings on it could have a higher number that indicates the wall-power.

11

Hey thanks guys!

I really appreciate it. I love this forum- I have only hit it twice now with my vexing (to me at least) questions, and so far I have been met with enthusiastic gentlemen like yourselves, ready to help.

12

dhenry:

Any transistor will be OK with 12V.

Most transistors will be OK with 12v. Many RF transistors are below that.

Indeed, but I think they are exotic/expensive gallium-arsenide devices rather than silicon?