ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
sparky 5
Recent Posts
Dimmable LED 2x4 lay in fixtures
by ghost307. 12/15/17 08:13 AM
2017 NEC Changes
by ghost307. 12/15/17 08:09 AM
Casino Wiring methods
by HotLine1. 12/09/17 10:44 AM
Ground wire Size to water Line.
by HotLine1. 12/05/17 12:11 PM
Flexible conduit to an outside Spa
by gfretwell. 12/05/17 12:28 AM
New in the Gallery:
Gallery Test
Popular Topics(Views)
243,057 Are you busy
179,497 Re: Forum
170,416 Need opinion
Who's Online Now
0 registered members (), 24 guests, and 6 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
Page 2 of 2 1 2
#218605 - 07/08/17 12:48 PM Re: Linear Power Supply Quits Under Load [Re: Potseal]  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
That should be a good linear supply to learn on. I should have had the datasheet in front of me when I responded. The current is sensed between pins Current Limit (2) and Current Sense (3). Check that you have <<0.6V between pins 2 and 3 to verify that you aren't in current limit. Then, use your DMM on DC, then AC Volts, with black on Pin 7 (V-). Pins 11 and 12 have to be stable and several volts above your final output voltage. If that's OK, the (typical) Reference Voltage at Pin 6, should be 7.15VDC. As with any Op Amp, the 723's main purpose in life, is to keep its inputs, pins 4 and 5, equal. VRef, or another stable reference voltage, is fed into one input, while a divided down sample of the output, is fed to the other. On supplies <7V, you would see the reference voltage divided down. It's all of the inputs and outputs available on the 723, that make it useful for both positive and negative supplies. Output current is limited only by your filtered source, and room for driver and output transistors, and emitter balancing resistors.
Of course, you don't want the 723 to work too hard if it's driving other transistors. So you would keep the PDIP well under its 150mA and 660mW maximum ratings. That's where the Alphas, or emitter current gains, of the driver and output transistors come in. A 5 amp supply with its 723 sourcing <= 50mA, would have to use transistors with a combined gain of at least 100. That's why you will see a driver transistor driving the bases of multiple, emitter balanced, output transistors.
Now, looking at the datasheet for the On Semi 2N3055, that I just downloaded from Digi-Key, the features say it all. The DC current gain ranges from 20 to 70 at 4 amps of collector current. You can see that a transistor with a gain (Beta) of 20, (Alpha) of 21, would need a base current of almost 230mA, to get an emitter current of your supply rating of 4.8A. That's more than the 723 has to offer, and why your supply will be using an external driver transistor. The 2N3055 isn't a common driver for such a small supply so I wonder if that third one might be part of the current limit or a crowbar circuit.
Good luck,

Last edited by JoeTestingEngr; 07/08/17 01:03 PM. Reason: typo

Tools for Electricians:

#218749 - 09/15/17 10:37 PM Re: Linear Power Supply Quits Under Load [Re: Potseal]  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
Did you ever get back to troubleshooting the linear supply?

Page 2 of 2 1 2

Member Spotlight
California ,Long Beach
Posts: 28
Joined: May 2007
Show All Member Profiles 

2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.016s Queries: 16 (0.004s) Memory: 0.7539 MB (Peak: 0.8717 MB) Zlib enabled. Server Time: 2017-12-15 14:00:00 UTC