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Registered Member #579
Joined: Mon Mar 12 2007, 09:45AM
Location: Burntwood, Staffs, England
Posts: 46
I want about 15 Amps out of the supply 20A would be better I have read about the various Higher amperage regulators and about putting regulators in parallel but I really wanted to make this circuit
There are several similar ( PNP) circuits on the net and they all have the same values
I don't understand how the LM317 t controls the voltage of the by-pass transistors The input of the transistors is Supply voltage ( 29.6) The out put is straight to the out put terminal and the base is at 29V and 296mA
Anyway! I can not get the circuit below ( or the six other I have built) to vary . The LM317 side works OK but as soon as I add the 2955's the output goes to 32.6V
I have added the circuit below - I don't seem to be able to put it in here
If you are planning on building a liner power supply like I was originally you will probably run into the same problem I did with the transistors having to disappate to much energy at lower output voltage (still waiting for a reply on that by the way) if my new design works out you can feel free to copy it (high current 0-40v v-reg and PWM power supply)
Note sure if those designs are based on the "High Current Adjustable Regulator" from National Semiconductor's LM317 datasheet or not. I have attached the application circuit in question.
The LM317 would increase/decrease its output (and current passing through it as a result) as a respond to the load. The extra transistors "kick in and help out" when the LM317 is passing current beyond a certain point set by R1.
There is a fine print at the bottom that say "Minimum load = 30mA" at the unmarked resistor connected across the output. That's probably the same problem as your circuit namely that the transistors branch have current leakage. Without any loads, the output voltage would go beyond regulation. May be your circuit is missing that or the resistance or just not drawing enough current when there is no load.
As is right now, there are no over temperature protection for the transistors. You could at least mount the LM317 to the same heatsink with the transistors. May be you'll get luck and the on-chip over temperature protection circuit can kick in a tiny bit sooner when the heatsink is getting hot extending some minor protection to those transistors. A current fold back would help out a lot.
I have a funny gut feel about that 470uF cap as it will affect the feedback circuit and might even cause it to oscillate under some conditions. The fuses introduce extra series resistance and they are after the negative feedback so the voltage regulation would be affected. You are also missing a dummy load as I have pointed out previously.
My advice is to simulate the circuit in LTspice (or other favors of Spice) and see what it does. Why? It is easier to measure things in Spice like current, power dissipation, doing frequency sweeps, fft, efficiency than real life.
The gotcha is that it is only as good as the model you have. Most of the built-in behavior models (even some of the ones on vendors sites) are too simplistic.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
If you put two silicon rectifier diodes (e.g. 2x 1N4004 etc.) in series across the 100R resistor they would limit at about 1.5V which in turn would give output current limit for the 2955s. As it is, the only output protection is the fuse, which is annoying to replace frequently. With this modification you do not need the two fuses.
Registered Member #579
Joined: Mon Mar 12 2007, 09:45AM
Location: Burntwood, Staffs, England
Posts: 46
Thanks Neet Studio - I don't have a spice apps I have tried some free ones but never been able to work out how to use them.. I think te answer is a dummy load Iwill try that next Sulaiman Thank you also, I think you may be correct I am going to reduce the 100R to a50 -22 R so that the voltage drop is less and there for the Neg driev for the 2955 is less .. I am going to do it now I have finished answering my e-mail. Thank you everyone
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I heard that these booster circuits were designed for old, slow power transistors, and can oscillate when used with modern ones. I think it is mentioned in the Bob Pease troubleshooting book.
The fundamental way it works is that the LM317 tries to drive the load itself. However, the same current that comes out of the LM317's output pin must go into its input pin. (Otherwise it would get overfull of electrons after a while ) This current from the input pin is used as base current to drive PNP power transistors, which provide a collector current in proportion to it, helping the LM317 out.
The Nat Semi application circuit above goes one step further, using a Sziklai pair instead of a single power transistor. The extra current gain is unnecessary, so they probably did it because they wanted to sell more LM195s, and these only come in NPN.
The LM195 also has built-in protection circuitry which solves the current limiting problems inherent to these kinds of circuits.
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[moved] Help Required PSU hi amp - LM317 T and pnp bypass Trs
) This current from the input pin is used as base current to drive PNP power transistors, which provide a collector current in proportion to it, helping the LM317 out.