| View previous topic :: View next topic |
| Author |
Message |
will@reeve.org.uk
Joined: 17 Oct 2003
Posts: 1
|
| More current from a pin please! |
 Posted: Fri Oct 17, 2003 8:07 am |
 |
|
Guys,
I need more that 25mA from a 12F629 PIC pin! Running at 3.6volts (from a battery) I need to run a little RF receiver, that requires a 3.3volt input. Trouble is one pin will not supply enough current (approx 30mA) to run the receiver. Can I join two pins together (short) to source some more current to my receiver? I can’t use a transistor switch due to the voltage drop (0.7v). Any ideas much appreciated!
Will
P.S. Looking at the pin trying to supply the voltage, it give 3.6volts for about 5ms then sags to 2v!
 |
|
 |
TSchultz
Joined: 08 Sep 2003
Posts: 66
Location: Toronto, Canada
|
|
| Posted: Fri Oct 17, 2003 8:43 am |
|
|
| You can use a FET to switch the power, for a decent FET the voltage drop will be very low. |
|
|
SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
|
|
| Posted: Fri Oct 17, 2003 9:19 am |
|
|
You can parallel (short together) two or more pins, but using a MOSFET transistor switch is a better solution. A 2N7002 is my default choice for a small switching FET application such as yours. At 3V on the gate it has about 5 ohms or 150mV drop at 30mA. I am sure you can find better if you need to.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
|
|
dbotkin
Joined: 08 Sep 2003
Posts: 197
Location: Omaha NE USA
|
|
| Posted: Fri Oct 17, 2003 11:59 am |
|
|
| SherpaDoug wrote: | | At 3V on the gate it has about 5 ohms or 150mV drop at 30mA. |
Doug, where do you find that information -- or did you measure it? I can find all kinds of graphs in the 2N7000 / 2N7002 data sheet, but none seem to indicate to me Rds of 5 Ohms at Vgs = 3. I'm not disputing you, jsut asking where you found that -- I'm curious about performance of that and other MOSFETs at fairly low gate voltages.
Dale |
|
|
mcafzap
Joined: 07 Sep 2003
Posts: 46
Location: Manchester, UK
|
| Current from pins etc. |
| Posted: Sat Oct 18, 2003 9:46 am |
|
|
You can use a Zetex bipolar transistor. From memory they have some with Hfe of 900 and Vce about 14mV. They are also obtainable from Farnell. However, doubling-up on the outputs should be reasonably safe, cheaper and easier than any other solution.
Steve |
|
|
david smith
Guest
|
| driving higher currents |
| Posted: Tue Oct 21, 2003 9:47 pm |
|
|
I've used PICs to drive MOSFETs for several applications:
My frequent choice due to pkg/versatility is the
IRLD024 from I.R./Digikey. Ron <0.1ohm, 2.5A current, 4pin DIP, logic in
Other parts I've used, some with much higher currents. (Still ~$1)
mosfet IRLML2803 Nch 30V 0.25ohm SOT-23.pdf
MOSFET TN0604 N 4A 40V Logic Supertex.pdf
mosfet NTP45N06L N 45A 60V Logic.pdf
And, to drive loads connected to gnd, use a logic-level P-ch fet.
MOSFET VP3203 P -3V -30V .5A supertex.pdf
mosfet MTB30P06V P -30A 60V $0.94 Arrow.pdf
mosfet MTB50P03HDL P -50A 30V Logic D2PAK.pdf |
|
|
SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
|
|
| Posted: Wed Oct 22, 2003 1:13 pm |
|
|
| dbotkin wrote: | | SherpaDoug wrote: | | At 3V on the gate it has about 5 ohms or 150mV drop at 30mA. |
Doug, where do you find that information -- or did you measure it? I can find all kinds of graphs in the 2N7000 / 2N7002 data sheet, but none seem to indicate to me Rds of 5 Ohms at Vgs = 3. I'm not disputing you, jsut asking where you found that -- I'm curious about performance of that and other MOSFETs at fairly low gate voltages.
Dale |
The Diodes Incorporated datasheet available through the Digi-Key website has a graph which shows less than 5 Ohms at 3V. It was clearer on another data sheet I looked at but I don't remember which brand.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
|
|
dbotkin
Joined: 08 Sep 2003
Posts: 197
Location: Omaha NE USA
|
|
| Posted: Mon Oct 27, 2003 5:30 pm |
|
|
Ah-hah -- Thanks!! I hadn't thought of that, I was looking at the Fairchild data sheet. Should have looked at other manufacturers.
By the way, anyone want 250 2N7002's, SOT-23? I ordered them by mistake, meant to get 2N7000's (TO-92). First offer of eleven bucks including shipping gets 'em, I just received them from Mouser. Offer stands until I get an RMA number... :oops:
Dale |
|
|
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
