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| How do make a current limiting knob using LM723 for Linear Reg power supplies? |
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| oldway:
Ok, i will try to explain the problem: that's all about phase control and transformers. To vary the secondary voltage by varying primary voltage with phase control triacs or scr's, that's not recomended. Why ? because a very little dyssimetry of primary voltage will produce a dc voltage, and this dc voltage will let a hight dc current flowing in the low resistance of the primary. The transformer get saturated and a very high ac current flows in the primary....this will blows up the fuses and can even damage the transformer. But you will answer:the scr's are in the secondary, then there can not have any problems.... But is this really a secondary phase control ? |
| MagicSmoker:
--- Quote from: oldway on May 19, 2017, 05:18:16 pm ---... Why ? because a very little dyssimetry of primary voltage will produce a dc voltage, and this dc voltage will let a hight dc current flowing in the low resistance of the primary. --- End quote --- Ah, yeah. Also the inevitable asymmetry in the SCR conduction times or their forward voltages can set up a DC bias in the transformer, too. I was so hung up on the current integration inductor not being in the right place I hadn't thought of any other problems. |
| oldway:
But indeed the inductor is not at the right place ....It can't be in the primary because any asymetry of the secundary current will apply a dyssimetrical voltage on the primary of the transformer....And asymetrical current occur during transients, when load vary rapidly.... This is why this power supply is basically unstable and the pre-regulator has a tendency to oscillate like a hell, saturate the transformer, consume a primary current of 30 to 50A and blow up the fuses and circuit breakers. With the years, it gets worse because the capacitor C27 loses its value and then there is no way to prevent it from oscillating Wat would be the solution of this problem ? There are two solutions: - to place the inductance not in serie with primary of the transformer, but in serie with the secundary of the transformer. - no need of inductance if you use a transformer with high short circuit impedance like MOT. --- Quote ---R108 is a little fussy, if you trim with the output unloaded it starts dropping the output voltage well before you reach zero current. The practical lower limit for R2 will be a few tens of mA. --- End quote --- Current limitation of this power supply is very basic and not intended to work as a precision current source. It only use a single diferential amplifier and gain is very low. But it is very stable for this reason and can be very fast .... The greatest problem of this power supply (0-20V and 0-7V) are the shunt resistors: they are made with thin wires and prone to failure. |
| Gurumurthy:
Dear oldway, I requested yesterday for nice reference book so that i can learn things and design. so please give me some advice to design circuit. |
| oldway:
http://www.ebay.com/itm/Power-Supply-Board-Kit-PCB-Based-on-LM317-amp-LM337-IC-/260484989112 http://www.circuitstoday.com/few-lm317-voltage-regulator-circuits |
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