EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jebnor on August 31, 2014, 05:57:57 pm
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I repaired an older "Power Max" brand PSU and decided to add a front panel fuse. The PSU is based on the LM723, an older voltage regulator. I had to replace the power pass transistor as it was blown. I also added a front panel fuse holder.
Also, I made a video of it.
(https://www.youtube.com/watch?v=JjCfx5-mXRU#)
Constructive criticism please, both technical and production. I'd like to improve my videos in future.
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If you look at the internal schematic of the 723 you'll notice that the current is sensed using a BE junction, basically a diode as far as temp coefficient is concerned (approx -2mV/C) which means that your current limit value decreases with temperature which is not necessarily a bad thing. It was probably set to 4.8A for that exact reason. Take a look at Figure 8. http://www.ti.com/lit/ds/symlink/lm723.pdf (http://www.ti.com/lit/ds/symlink/lm723.pdf)
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If you look at the internal schematic of the 723 you'll notice that the current is sensed using a BE junction, basically a diode as far as temp coefficient is concerned (approx -2mV/C) which means that your current limit value decreases with temperature which is not necessarily a bad thing. It was probably set to 4.8A for that exact reason. Take a look at Figure 8. http://www.ti.com/lit/ds/symlink/lm723.pdf (http://www.ti.com/lit/ds/symlink/lm723.pdf)
I had opened this before and put the two 0R27 in parallel. I can't remember what was in there before, for either the Transistor nor the Rsc resistor. I put the TIP122 in because that is what I had, same for the 0R27.
I hadn't thought about the tempco, I'll keep that in mind.
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You probably will also be best advised to put a 100R resistor in series with the top (pin2) lead of the current limiter, so that you do not blow out the current limit transistor into a short circuit. It will increase the short circuit current slightly, but the 723 was never a stellar performer in current limit any way.
You probably want a bigger heatsink on it, the flat plate is too small for a high current output for long periods. I took my clone and added a larger old CPU heatsink to it to get it to run cooler than the 120C it originally was running at.