All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum

Notices

The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 10-27-2010, 06:34 PM
Tate Tate is offline
Junior Member
 
Join Date: Mar 2010
Location: San Antonio TX
Posts: 11
Default XR-8038 square wave generator

I am wanting to use a XR-8038 as a signal generator to control an IGBT transistor. I need a positive bias to turn the transistor on and a negative bias to turn the resistor off. I need to do this at 20KHz, 28KHz and 40KHz.

I have the XR-8038 wired to a +/-12V power supply, and get very nice square wave output. The output is 0V to+12V when the ground of the oscilloscope probe is grounded to the common, and -12V to 0V when the probe is grounded to the negative power supply. The circuit is wired according to the Generalized Test Circuit on the datasheet.

http://users.ece.utexas.edu/~grady/EE362L_Spec_Inverter_triangle_wave_generator.pdf

The question is, how do I get a square wave that is +12V to -12V, or shift the wave to +6V to -6V?

Thank you for your time.

JT
Reply With Quote
  #2  
Old 10-27-2010, 07:15 PM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,038
Default

The attached will get you close for the +12v/-12v. It's basically an emitter follower configuration.
Attached Images
File Type: png 12v to 24v level converter.png (65.3 KB, 47 views)
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
Reply With Quote
  #3  
Old 10-28-2010, 03:50 AM
DonQ DonQ is offline
Senior Member
 
Join Date: May 2009
Posts: 320
Default

Quote:
Originally Posted by Tate View Post
The output is 0V to+12V when the ground of the oscilloscope probe is grounded to the common, and -12V to 0V when the probe is grounded to the negative power supply.
Something is not right here. If your scope is referenced to the -12, and you get a -12 reading, you are getting -24V from somewhere. I think you are just having a problem interpreting your measurements. (For example: If you put the probe itself on the -12 with the ground clip on the output, it would not be called "grounded to the negative power supply", but this, along with an inadequate pull-up, would explain your readings.)

If you have the 8038 powered between the -12 and +12, and a proper pull-up on the output transistor tied to +12, your output should have the resistor pulling up to +12 rail when the transistor is open, and when the transistor is conducting, it will pull to the -12 rail.

If you measure this relative to 0V (the center point between your supplies), your output will be +12/-12. I think this is what you are wanting. End of story...

If you measure relative to the -12V rail (scope ground clip on the -12, probe center on the output), you should get a square wave from +24V to about 0V. If you have a very large valued pullup (or none at all), you may not get all of the 24V to show.

Start with about a 10K for RL and try again.
Reply With Quote
The Following User Says Thank You to DonQ For This Useful Post:
SgtWookie (10-28-2010)
  #4  
Old 10-28-2010, 03:33 PM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,038
Default

Thanks for your post DonQ; you made me re-visit the datasheet.

Figure 1 shows the output transistors' emitter grounded. However, there is no "ground" pin on the IC! This is cause for some confusion/ambiguity, as "ground" is normally defined as the 0v reference. Since there is no "ground" as an input to the IC, it is an unknown whether they are creating an "artificial ground" by taking the average of Vcc and Vcc and using that point as an artificial ground, or if the ground point in the diagram actually means Vee.

I actually have an NTE-labeled version of the 8038 kicking around here somewhere, if I can only find the darn thing. It's an unknown whether it is an ICL8038 or an XR-8038, but they should be equivalent.

Anyway, if RL is of too low a value (<12k for +12v/-12v) even with no load, the saturation voltage of the square wave output can exceed 0.5v from whatever they're calling ground. The output will be very sensitive to additional loading. It will not drive a capacitive load well without aid.

I've attached a schematic of what the output transistor configuration might be, along with a voltage follower/buffer made from a couple of transistors. It's not ideal, but it's quite simple.

[eta]
Here's the NTE864 datasheet, which is the IC that I have:
http://www.nteinc.com/specs/800to899/pdf/nte864.pdf

They show pin 11 as GND instead of Vee, which clears up that whole ground/virtual ground issue.

So, the schematic I've attached should work just fine, unless Qout has been unduly stressed due to RL being of too low of a value.
Attached Images
File Type: png XR-8038 Output Buffer.PNG (67.0 KB, 22 views)
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.

Last edited by SgtWookie; 10-28-2010 at 04:33 PM.
Reply With Quote
  #5  
Old 10-29-2010, 02:32 AM
DonQ DonQ is offline
Senior Member
 
Join Date: May 2009
Posts: 320
Default

Yeah, it's like a 24V supply for the 8038, with a built-in center-tap on the power supply.

I guess my point was that if he is looking for +12 to -12 from the center-tap, he should be able to get that without any extra transistors. That is as long as he doesn't need to draw much current. With current, the transistor in the 8038 takes the place of your Qout and RL is the external resistor already there. Only Q1 and Q2 are added for the current.
Reply With Quote
  #6  
Old 10-29-2010, 04:48 AM
Tate Tate is offline
Junior Member
 
Join Date: Mar 2010
Location: San Antonio TX
Posts: 11
Default

Thank you genglemen for your assistance. Yes, I trying to get +12 to -12 from the center tap. I just looked at my power supply that I will be using when I take this project off the bread board and the supply will be +6 to -6 so I may have to make some changes.

I am using an RL of 10Kohms and it is working well at this point. I plan on amplifying the signal before putting it into the IGBT.

SgtWookie, I will be trying your solution in the morning, I will post the results. I am still very green behind the ear when it comes to electronics. Thank you for your responces.
Reply With Quote
  #7  
Old 10-29-2010, 01:07 PM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,038
Default

The XR-8038 is specified to operate on a single supply as low as 10v, and dual rail supply as low as +5v/-5v, so you should be OK with that.

Keep in mind that the 8038 square wave output has a very low current sink capability; try to sink more than ~2mA and the output saturation voltage starts getting too high, and may burn out the square wave output due to power dissipation.

For +6v/-6v rails, or +12v and GND across Vee, use 6k or higher for RL.

The transistor emitter follower configuration I posted will multiply the 2mA output by the gain of the transistors.

You might consider using a bjt (transistorized) 555 timer instead of the XR-8038. The XR-8038's are scarce and expensive, as they are out of production, along with having meager source/sink capability on the square wave output. You would be better off to save it for a signal generator project.

555 timers are cheap, plentiful, and can source/sink up to 200mA on pin 3, and require very few external components to set up a PWM output. They will operate just fine with a single-ended supply from 5v up to 16v; some are rated for up to 18v.
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
Reply With Quote
  #8  
Old 10-29-2010, 09:38 PM
Tate Tate is offline
Junior Member
 
Join Date: Mar 2010
Location: San Antonio TX
Posts: 11
Default

Since I am having trouble getting the square wave to be cross zero volts, I am seeing that a 555 and the emitter follower circuit you posted earlier in post #2 should get me everything I need. I have already built the 555 signal generator but I did not know how to get a +6 to -6 signal with it. Any other suggestions would be greatly appreciated.

Again, I want to say thank you for all of your help.
Reply With Quote
  #9  
Old 10-29-2010, 10:23 PM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,038
Default

I ran across my NTE864's, which are equivalent to the x8038's - but I've been occupied today fiddling around with car problems (finally got my EGR problem resolved - yay!)

You wouldn't need an emitter follower with a bjt (bipolar junction transistorized) 555.

Have to run for a bit.
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
Reply With Quote
  #10  
Old 10-30-2010, 02:59 AM
SgtWookie's Avatar
SgtWookie SgtWookie is offline
Expert Member
 
Join Date: Jul 2007
Location: In the vast midwest of the USA; CST
Posts: 22,038
Default

I'm really too tired to do this thread justice this evening. Sorry about that, but it's the truth.
__________________
General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.
Reply With Quote
Reply   Post New Thread

Tags
, , ,


Related Site Pages
Section Title
Textbook Audio oscillator : Discrete Semiconductor Circuits


Similar Threads
Thread Thread Starter Forum Replies Last Post
Square Wave Generator marims The Projects Forum 3 03-08-2010 10:40 PM
Low frequency hi output square wave generator?? magnetman12003 General Electronics Chat 1 12-09-2009 07:30 PM
Making a 1Ampere Square Wave Current Generator From 18V/18A Power Suppky richeek.arya The Projects Forum 11 10-22-2009 07:33 AM
Signal Generator Distortion (square wave distorting at high freq) - HP/Agilent 33120A donatosd General Electronics Chat 9 04-01-2009 10:53 PM
Coupled Pulse & square wave generator for electron gun beamcircuit The Projects Forum 17 04-28-2008 02:14 AM

Thread Tools
Display Modes

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


All times are GMT. The time now is 01:43 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.