voltage questions

Badger

Hello

i have a motor here rated at 1.2 volts at 300 mh .. how do i cut the voltage down to run this motor on a home work board (i am very new to this) any help would be appreciated


Phil king

metron9

What voltage is your main supply you wish to drop to 1.2V and I think you mean 300 mA (milliamp)
Do you want battery power supply or wall wart that plugs in to mains?

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Badger

Hello

i am using the bs2 stamp i think it is either 5v or 3.3 volt i want to be able power this motor from the bs2 i guess is what i am trying to say


Badger

Mike Green

You can't power this motor from the Stamp itself. It takes too much current. The BS2 runs off a 5V regulator, either one on the development board (which can usually supply 0.5 to 1A) or a regulator on the Stamp module itself (which can only supply 50mA total). You still need to reduce this voltage to 1.2V. An LM317 adjustable regulator (available from RadioShack) will usually put out 1.2V if its adjustment pin is grounded. Check the datasheet for the LM317 which you can download over the internet. You'll need to switch this on and off which will require a small relay or power transistor. Look at the Nuts and Volts Column #6 which you can find through the Resources tab on Parallax's main web page. It discusses the use of transistors for switching motors, relays, etc. with a Stamp.

Badger

Hello

i am using the bs2 stamp i think it is either 5v or 3.3 volt i want to be able power this motor from the bs2 i guess is what i am trying to say


Badger

Badger

Hello


so the homework board can not supply this current in a native circuit with what comes with the kit..


Badger

metron9

The VIN on the homework board (Voltage In ) (I have the one that uses a 9Volt battery for the power supply) will be the voltage of the battery. You can connect as Mike has said using a voltage regulator to drop the voltage down to 1.2V. At 300mA using 1.2V your 9V battery may use 50 to 100 mA depending on the regulator.

Typically though you should have a separate power supply for your motors so you don't draw too much power from the stamps main supply, that can cause brownouts or dirty power supply that can cause problems with input pins. A typical rechargable battery 9V has only 120mAh and 50mA draw is a big percentage of its rated output maximum.

This motor is very small but given the rechargable 9V specs above you are pushing your luck using the VIN supply. You could use a rechargable AA battery directly and control the power using a transistor of fet as a rechargable AA battery is about 1.2V and 1200 millamp hours is typical so your motor could run for about 4 hours at 100% power on one battery. If you need more time you can use more batteries in parallel. Be sure to connect the two power sources ground lines so you have a common ground or your transistor or fet will not work.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike Green

The HomeWork board specifically cannot provide the current needed for this motor. The regulator on the HomeWork board is limited to about 50mA, the same as with the Stamp modules. You could run the motor off a separate 1.5V battery, again using a transistor as a switch to turn it on and off. RadioShack carries battery holders (A AA battery would be ok, but a C battery would last longer). You'd need to connect the (-) battery terminal to the Vss connection on the HomeWork board. You'd need something like a 2N2222A switching transistor. The built-in 220 Ohm resistors on the HomeWork board would work to limit the base current for the transistor. RadioShack should carry the 2N2222 or equivalent (like the MPS2222A). The Nuts and Volts Column I mentioned shows how to connect it all up.

Post Edited (Mike Green) : 8/6/2008 1:11:59 AM GMT

metron9

Am I in error here Mike? The VDD on my homework board is the regulated supply but the VIN is a direct connection to the positive side of the battery no? Assuming you had a 9V wall wart supply to it, you could use that, correct? You would still however need a regulator and a transistor or fet. With a rechargable AA you don't need a regulator and solve any potential noise problems as well.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

Mike Green

metron9,
Yes, you're correct.

Badger

thanks guys the information here is great


i just have one more question what is a fet LOL

Badger

i am going to be looking into this method i have a battery pack for 2 aa bateries

Mike Green

The Wikipedia is your friend. Do a websearch for "wiki fet" for more information.
FET = Field Effect Transistor

Badger

Mike

newbies cant live with them and cant grow with out them hehehe

i should have thought of that i am a disabled but was a network engineer for 15 years and such... i will do that..
i am starting back to college this fall to take electrical engineering classes. so i am trying to start early and learn what i can on my own .

thanks for your help very much

Philip King

Badger

Mike

but here is a good question... What size of fet do i get

Phil

Badger

Hey


if i dont use rechargable batteries to i still need a regulator or is the regulator the piece that keeps the current or voltage constant to my motor



phil

Badger

hey



sorry about the last question reread the post...


Phil

SRLM

For my current robot project, I bought some Ni-MH batteries on sale from a local RC car store. I plan on connecting all the grounds together, and giving the electronics 12V 2200 mAH, and the motors twice that. For me, I don't need an external regulator for the electronics since the BOE can regulate it to suit (and it looks like to HomeWork board has the same regulator...), but for my motor I'll need some regulation. I put them on two different circuits since the motor is a huge thing, and when it is turned on the voltage dips a little.

Hope this provides some perspective...

Mike Green

You really don't want to have to regulate your power supply to feed a motor since motors take a lot of current and don't require tightly controlled voltages and all of the "excess" voltage just gets turned into heat. If you've got a motor designed for 12V, use a 12V battery. If you've got a 6V motor, use a 6V battery. You can run your logic off 12V through a regulator, but keep in mind that, if you have a lot of sensors and LEDs and an LCD with a backlight, you will draw quite a bit of current through your 5V regulator and the excess voltage (above 5V) is getting turned into heat and your regulator may overheat. 12V-5V = 7V x 1A = 7 Watts! That's a lot of heat for a tiny heatsink (like on the Board of Education) to get rid of.

Mike Green

To choose an FET, you need to know how much current is being switched and how much voltage is being switched. Most power FETs are very happy with voltages well in excess of what you would be working with (like 40V). If you're going to switch something that draws 300mA, pick an FET rated for at least twice that (600mA). In your case, since you're going to be controlling the FET from an I/O pin, you want a logic level FET. Most FETs require more than 5V to fully switch on. Their datasheets will have graphs and tables showing the gate voltage necessary to provide a certain amount of current. Logic level FETs are guaranteed to switch a significant amount of current at 4-5V which is what you want.

For example, the IRF510 is a nice power FET sold at RadioShack, but it won't conduct much current at a 4-5V gate voltage. The IRL510 is a nearly identical power FET that fully switches on at 5V.

Badger

thanks all the info is very well needed and appreciated i will be looking into getting these parts soon


Badger

Badger

just got off the phone from my local radio shack.. they only have the IRF510 but dont have and cant fine the IRL510 in there computers. so any ideas. is there away to boost the voltage from the output pin via a voltage regulator or some sort of amplifire to make the IRF510 turn on when the pin goes high.

badger
newbie

Badger

or i guess i can order one off the internet someplace .. keep forgetting my main source of information

badger

sam_sam_sam

Here is one place that you can get it from

http://parts.digikey.com/1/parts/1019473-mosfet-n-ch-100v-14a-220-5-irc530pbf.html

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··Thanks for any··that you may have and all of your time finding them

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Sam