Voltage drop across JFETs? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 14th July 2010, 01:56 PM   #1
diyAudio Member
Join Date: Mar 2009
Default Voltage drop across JFETs?

Trying to incorporate the circuit from the schematic hereThe Rat Bypass in to my projects and just trying to work out the value of the dropping resistor for the LED on an individual basis for different colours etc (using this calculator, as it happens Calculate Resistors for LEDs).

Just wondering if anyone knows if there's a drop, how much it would be etc I know the diodes knock off 0.7V each anyway. Any help on this would be greatly appreciated, I tried googling "JFET voltage drop" and can't seem to find a definitive simple answer
  Reply With Quote
Old 14th July 2010, 02:14 PM   #2
diyAudio Member
BobEllis's Avatar
Join Date: Sep 2003
Location: Upstate NY
The jfet in that circuit is used as a switch, notice that when the switch is in position 1, the gate is tied to the drain, turning it on fully. When on it acts as a resistance. Look in the data sheet of your chosen jfet for Rds(on). It will probably be fairly insignificant compared to the 2K2 drop.
  Reply With Quote
Old 14th July 2010, 05:40 PM   #3
diyAudio Member
Join Date: Mar 2009
Yeah I know it's a switch, it's being used to activate the LED but I'm not sure what voltage is coming from the Source of the JFET as I don't know if there's a voltage drop through the component like a diode. I found a datasheet here but it doesn't seem to mention 'Rds'.
I am specifically asking about effects on the voltage to the LED: is it just 9 volts, minus the voltage drop of two intervening diodes therefore equalling 7.6 volts, going through the component (i.e. what voltage would be measured at the anode of the LED)? Sorry, confused!
  Reply With Quote
Old 14th July 2010, 06:23 PM   #4
diyAudio Member
BobEllis's Avatar
Join Date: Sep 2003
Location: Upstate NY
Substitute a J111, which lists Rds(on) at 50R. You have 1.2V across the LED, 1.4 V across the diodes, leaving you with 7.4V across 2200 R + 50R. This gives you 3.3 mA, meaning that you lose .16V in the JFET. It's not like a diode drop, it depends on the current.

If you ignore the JFET Rds(on), you'd come up with 3.4 mA through the LED, a 3% difference. The higher the voltage used to drive the LED circuit, the less significant Rds(on) will be.

You can also look at the Vds curve on page 3 of the data sheet. Which is really what you are asking. Look at around 3 mA and in that circuit when on your Vgs will equal Vds. Notice how much steeper the curves get with more positive Vgs., I'd guess that at 3 mA, you'd be around .5V or less. Again, a small fraction of the total drop and would lead to fractions of a mA difference in LED current. It usually takes a few mA to make a noticeable difference in LED brightness.

The long and short of it is that you can calculate the voltage drop, but as a practical matter in that circuit it doesn't matter much. If you are going to run your LED from a 15V rail, it matters even less, since the current limiting resistor will drop even more.

Looking at the data sheets, I think that the reason the 2N5457 was chosen was it's lower pinch off voltage requirement. The J111 should work, as long as the volume pot is a reasonable value (not megohms).
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
rectifier voltage drop dsavitsk Power Supplies 21 25th November 2015 05:01 PM
Voltage drop across JFETs? Test Electrix Construction Tips 0 12th July 2010 03:29 PM
How to drop voltage and current giralfino Power Supplies 9 5th May 2010 04:18 PM
voltage drop across a CCS... pedroskova Tubes / Valves 4 8th August 2005 07:11 PM
voltage drop ThSpeakerDude88 Chip Amps 30 19th July 2005 09:00 PM

New To Site? Need Help?

All times are GMT. The time now is 04:08 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2