Topics Topics Edit Profile Profile Help/Instructions Help Member List Member List  
Search Last 1|3|7 Days Search Search Tree View Tree View  

Switchable constant current source

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 26 January, 2014 » Switchable constant current source « Previous Next »

  Thread Last Poster Posts Pages Last Post
  ClosedClosed: New threads not accepted on this page        

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

atferrari
Frequent Contributor
Username: atferrari

Post Number: 1464
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 12:16 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Switchable constant current source for a green LED.

Switching is commanded with logic signals +5V and 0V and constant current is around 15 to 20 mA.

Before I buy components, given the circuit below, could I go away if Vcc is just 5V, replacing the MOSFET with a 2N7000?

http://www.alldatasheet.com/datasheet-pdf/pdf/32720/TRSYS/2N7000.html


Agustín Tomás - Buenos Aires - Argentina
Top of pagePrevious messageNext messageBottom of page Link to this message

alexr
Frequent Contributor
Username: alexr

Post Number: 266
Registered: 02-2008

Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 04:34 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A 2N7000 might work with a 5V supply but equally it might not, it depends on how lucky you are and the characteristics of the individual device.
The 2N7000 is NOT a true logic level mosfet but if it is lightly loaded and the gate voltage is close to 5 volt then it might work.

You would be far better off getting a real logic level mosfet such as a 2SK1336.
Alex
Top of pagePrevious messageNext messageBottom of page Link to this message

cjay
Regular Contributor
Username: cjay

Post Number: 32
Registered: 10-2013

Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 10:00 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've used a variant of this circuit many times with success, the 0.82R resistor need not be anything like 2 watts and I've never needed the circuitry in grey to PWM the LED, just driven it directly from a PIC I/O pin to the gate of the MOSFET.

I usually use IRF521s because I have a few hundred of them from a component clearout at a manufacturer's stores and it works fine at 5V, as do the 2N7000 devices I've used when breadboarding it..
Top of pagePrevious messageNext messageBottom of page Link to this message

g0hjq
Regular Contributor
Username: g0hjq

Post Number: 44
Registered: 02-2010


Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 04:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Seems rather complicated just to light a 20ma LED. If the supply voltage is a fairly constant 5v, you could just use a resistor, with a single transistor for switching.

Driving an LED with a transistor

The transistor shouldn't be critical - just about any low power NPN would work. I've also had good results using a 2n7000.

By the way, the 0.82 ohm resistor in the original circuit seems very low - this would result in an LED current of around 700mA. 33 ohms would give aproximately 18ma.

Also, the 1Meg pull-up resistor plus the gate capacitance of the FET will cause a fairly slow turn-on time. Not a problem if you are just turning the LED on and off, but it may be an issue with PWM.


(Message edited by g0hjq on 20 January, 2014)
Top of pagePrevious messageNext messageBottom of page Link to this message

cjay
Regular Contributor
Username: cjay

Post Number: 34
Registered: 10-2013

Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 08:23 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Can't remember the exact values of the resistors in mine but I seem to remember R1 being in the region of 10K ajnd Rs around 47R for something under 20mA
Top of pagePrevious messageNext messageBottom of page Link to this message

atferrari
Frequent Contributor
Username: atferrari

Post Number: 1465
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 08:36 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I started to feel that I could be over designing this.

Just in case, for the moment, I will go with a common emitter NPN.

Gracias for the replies.
Agustín Tomás - Buenos Aires - Argentina
Top of pagePrevious messageNext messageBottom of page Link to this message

james
Frequent Contributor
Username: james

Post Number: 310
Registered: 02-2007

Rating: N/A
Votes: 0 (Vote!)

Posted on Monday, 20 January, 2014 - 10:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Agustin

Just in case you want to play with an alternative switchable constant source that works on a +5V supply....


Switchable Constant Current Source

Cheers

James

Administration Administration Log Out Log Out   Previous Page Previous Page Next Page Next Page