MechMate CNC Router Forum

Go Back   MechMate CNC Router Forum > Electrical & Electronic > 70. Control Systems
Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Reply
 
Thread Tools
  #1  
Old Mon 28 September 2009, 09:19
javeria
Just call me: Irfan #33
 
Bangalore
India
Driving a relay using 2n3904

Hello folks

I need to drive a relay using one of the BoB's output - the relay is the one similar to the one's we use on the proximity circuits (pcb mounted)

the resistance on the relay is like 30K, I have 2n3904's lying with me which I want to use to drive the relay

the schematic of the BoB is here http://www.sunrom.com/files/1117-schematic.pdf

I needed to know what value of current limiting resistor I should use on the base of the transistor.

also kindly post any formula to be used

RGDS
IRfan
Reply With Quote
  #2  
Old Mon 28 September 2009, 10:46
javeria
Just call me: Irfan #33
 
Bangalore
India
some things I downloaded

here is some of the stuff i downloaded on the relay stuff
Attached Files
File Type: pdf transistor relay.pdf (431.5 KB, 72 views)
Reply With Quote
  #3  
Old Mon 28 September 2009, 11:07
jhiggins7
Just call me: John #26
 
Hebron, Ohio
United States of America
Irfan,

I used this information to construct a circuit to control relays from the PMDX-122 BOB. Works fine.

Last edited by jhiggins7; Mon 28 September 2009 at 18:36.. Reason: Fixed link that was erroneously pointing to my hard drive
Reply With Quote
  #4  
Old Mon 28 September 2009, 13:56
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
John,
Looks like your link is referenced to your e: drive on your local machine.
Reply With Quote
  #5  
Old Mon 28 September 2009, 18:39
jhiggins7
Just call me: John #26
 
Hebron, Ohio
United States of America
Thanks Heath. I fixed it, I hope.
Reply With Quote
  #6  
Old Mon 28 September 2009, 21:28
javeria
Just call me: Irfan #33
 
Bangalore
India
Thanks Folks!
Reply With Quote
  #7  
Old Tue 29 September 2009, 09:01
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
One of the problems with the 2N3904 transistor is that it is an NPN device . The base on an NPN transistor requires a positive voltage to turn on the transistor. A PNP transistor has a base that turns on the transistor when the voltage goes to ground. Many TTL outputs are ACTIVE LOW, meaning that they work best with PNP transistors. If the TTL output is ACTIVE HIGH, then it is best suited for use with NPN transistors.
Reply With Quote
  #8  
Old Tue 29 September 2009, 20:43
javeria
Just call me: Irfan #33
 
Bangalore
India
Mike if you have seen the Bob layout - do you think it can drive the 3904
Reply With Quote
  #9  
Old Tue 29 September 2009, 21:18
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ifran,

Most small general purpose transistors can easily handle a beta of 10X. That means if you want 30mA between the collector and emitter on the transistor, you have to have 3mA at the base. The circuit in the BOB layout shows that the collector of the output is 470 ohms, so you have roughly 10mA available to drive the base. Without an additional resistor the 3904 would saturate, meaning only that turn-off times would be a little longer than if an additional resistor were used to limit the current to the base.

Put a 1k resistor in series with the input to the base and see what happens. If the transistor always stays on, then replace the 1k resistor with a 2k resistor. When I'm playing with low cost, general purpose components, I rarely go to the trouble of calculating the ideal resistance, instead, I just substitute values until I get the expects results. It's much faster. The cost is minimal. If you blow out a $0.20 transistor, just put another transistor in the circuit and keep trying.

A very highly recommended designer showed me that starting with a reasonable guess and then using a "clicker box" (a box that had lots of internal resistors that could be selected, one at a time, by turning the "clicker switch") was much more cost effective than spending a lot of time computing the ideal resistance. Every transistor varies a little from every other transistor. Find a value that works with several transistors and you'll probably be just fine. Just use a "plug board" so that you can plug in the components, one after the other, without soldering.
Reply With Quote
Reply

Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Thread Tools

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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Solid State Relay (SSR) for switching Router under control of Mach3 Gerald_D 70. Control Systems 77 Tue 07 January 2020 18:43
PMDX-122 relay output voltage? chopper 70. Control Systems 10 Thu 04 December 2008 07:00
Programmable Relay to Watch Dog the Limit/Home sequence smreish 70. Control Systems 13 Mon 07 January 2008 05:28
Newby question re driving one side fieldknot 10. Base Table 1 Wed 05 September 2007 22:55
Selflatching relay/contactor Mike Richards 70. Control Systems 0 Sun 25 February 2007 15:20


All times are GMT -6. The time now is 10:41.


Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.