25j 182

Tricky Nekro

I'm looking for an good transistor or preferably a fet to enable the power of the servos by the stamp (bs2). The output of the stamp is about 25mA...They have said me that the 25J 182 is a good choise...

Will the current of the stamp·be enought to enable the output of the fet·and power up the 2 servos...
Is any schematic available? (I searched the Google and I didn't find free datasheets)...
Is the 25J 182 good for me...

The input will be ~5.05V...
The servos absorb almost 1.000mA and more...

What can I do...

Thank you...
Provas, Greece

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-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

allanlane5

A servo already supplies its own driver electronics -- that's what is so nice about a servo.

So, a Servo has three wires -- Supply, Signal, and Ground. The Supply wire should be supplied by a 'high-current' power supply at 5 to 6 volts. The 'Signal' wire can be driven directly by a BS2 -- it's not taking much current at all. The 'Ground' wire MUST be connected both to the ground wire of the 'high-current' power supply, AS WELL AS the ground of the BS2.

The 'high-current' power supply can be 4 Ni-Cad batteries (which can produce LOTS of current -- 5 amps or so for short periods of time), or even the output of the BOE Linear Regulator. Note a Linear Regulator is limited to about 1.0 Amp, so if you are using high-current servo's, you may need two regulators, one to supply current to each servo.

Personally, I've never seen a servo pull that much current, but I don't know your application. Typically one Linear Regulator can drive 2 servo's no problem.

If you have a BOE, you can connect the servo connectors to the Servo plugs (those three-prong things) on the board. Then connect the servo voltage jumper to 'Vdd', and you'll be powering the servo's from the Linear Regulator. You CAN connect the servo voltage jumper to 'Vin', but that is intended to be used when running from 4 batteries.

IF you connect your servo's to Vin, then connect a wall-wart power supply to the BOE, you'll probably be driving the servo's with too much voltage. This can shorten their life at 7.5 volts, or destroy them at 12 volts (depends on the voltage of your wall-wart).

Tricky Nekro

I'm using a 8Ni-MH·battery pack·to power up the 7805 regulator circuit, I have..

The input is measured by a 555 circuit...

I want to automaticly swich off the output (via stamp)·from the servos·when the battery is running low or recharging...

Thankfully, Provas, Greece

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

allanlane5

Ah. So, in addition to controlling the 'Signal' pin with the BS2, you ALSO want to be able to 'disconnect' (so to speak) the Supply line from the power supply to the Servo.

Well, first of all I don't think you have to. If you simply quit commanding the Servo, the current used should drop quite a lot. Otherwise, I'm sure others can give you some indication of what you can do. I'd be tempted to use a latching relay, myself.

Tricky Nekro

Yes...

I·want no current to be absorbed during the charging progress...(special battery for sevos with a 7805 regulator)

I need something like a relay but·as a·MosFet like the 25J182 I have...But I have no datasheets...



Thankfully, Provas, Greece

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

Tracy Allen

How about a low dropout voltage regulator with a shutdown pin, in place of the 7805? For example, the LT1529 draws only 15 microamps in shutdown, but it can supply up to 3 amps of output with only 0.6 volts of headroom. All the Stamp needs to do is pull the shutdown pin high for ON and low for OFF.

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Tracy Allen
www.emesystems.com

Tricky Nekro

I already have a 7805 power board to my bot can I use this chip you told at the output of my current regulator...Of course, I want to make it as good as possible and with minimum noise...

A schematic will be very well acceptable...

Thank you for your replies...

Provas, Greece

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
-Rule your Destiny-
--Be Good. Be Bad. Be Provas--

Tracy Allen

Okay, the circuit is quite simple. On the left is the circuit using the fixed 5 volt version of the regulator. It would work with an input of 5 volts, but it would be much better to power it directly from your 12 volt Bot supply. That way, there won't be so much load and noise on your main 7805, and this auxiliary regulator for the servos can have ample headroom, so it won't be operating in dropout. The Stamp controls it ON OFF with one pin.



The diagram on the right is the adjustable version of the same regulator. Use that if you want the servos to operate at 6 volts instead of 5, say.


The LT1529 is just one example of a low dropout regulator with a very effective shutdown pin. There are many such regulators available with the power rating you need. Another one I have used is the LT1963A, which is an especially low noise fast response regulator for telecom circuits.

There is another anti brownout cirrcuit posted in this article
www.emesys.com/BS2power.htm#Brownout
...a circuit to avoid completely discharging sealed Pb-acit batteries in a solar powered system.

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Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 11/3/2005 7:32:18 PM GMT