LMH/ Texas Instruments Linear Voltage Regulators LMH/ Voltage Regulator datasheet, inventory, & pricing. The LM/LMC is a voltage regulator designed prima- rily for series regulator applications. By itself, it will supply output currents up to mA; but external. The LM is a monolithic integrated programmable voltage regulator, assembled in. lead dual in-line plastic package. The circuit provides internal current.

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I’m trying to troubleshoot a delinquent power supply I have. Most recently one of the transistors Q1 actually exploded on it. This has led me into investigating the schematic, but try as I might, I can’t make heads nor tails of what’s going on. I’m an electrical engineer but optics and will admit being a complete noob with electronics, but I’m trying to change that. I’ve attached the circuit diagram. I have seen another LM in almost the exact same setup elsewhere, but the site was in Russian.

I have also spent about half a day playing around with the various configurations on the datasheet to try and get a feel for how it works. However, I don’t understand:. I understand that it is responsible for varying the voltage and regulating ithow is it accomplishing this? I’m quite lost after spending some time monkeying with this, so any direction is greatly appreciated.

I checked theand it has ohms from emitter to base i. I think that my pinout is correct used http: If you need to repair this circuit with original components then it will be possible to do so. Easier still, you can get LM’s that provide up to lk723h 1. You can parallel LM’s – ideally add a small output resistor between Lm7723h and feedback resistor network to achieve current sharing.

Put the sense resistor divider on the Vout side of the resistors. Be sure Vin does not exceed 35V!. With 35V in you can get about 1.

See data sheet for dissipation ratings which set max Vdrop x Iout you can use. LM was the bees knees voltage regulator controller and maybe also the only one readily available ‘way back when’. You mention 2N in comments but not on diagram or in question.

LMH Datasheet(PDF) – National Semiconductor (TI)

LM data sheet here. The error amplifier 4 5 inputs is used to compare a reference voltage with the voltage to be controlled.

Q2 is a current limiter. When the load voltage across R10 exceeds about 0. For 2A current limit R10 is about 0. Vref appears on pin 6. This will be compared to a sample of the Vout voltage.

National Semiconductor

Vin is divided down and stood on top of as “pedestal’ made from Vref scaled down by Vr and R5. The LM internal error amp has 6. It acts to produce 6. Vpot can be shown to vary from 7. Call ratio of R6: Above formula gives this graph.

The circuit is MUCH easier to understand if you draw it with the internals shown and arrange it around the in logical fashion. Above I named the ratio of R6: If you change k in steps of 2. The negative Vout will of course not happen with a ground referenced positive input supply, but would be possible if Vin was referenced against some negative value. The common zero point of 6. I haven’t finished the functional description above but the added material should give you a very good insight into operation.


So how do the transistors function? I have a basic grasp on their operation, getting more familiar with small signal characteristics, etc, but besides knowing the is the pass transistor and power transistor, that’s about it. What went into the design of that portion, especially with the feedback transistor? The Model T of the power transistor world. BUT the sinks current to ground as it turns on although you can invert the error amp sense and use a pullup resistor to get over this and-but the only provides mA max.

If you use a resistor to turn the on and the to turn it off, the will need to be sinkig most of the current when idle and so must be used near its current rating often. Q1 can provide as much base drive as you want in fact, as there is no series resistor between Q1 and the base – this is, if not a design “error”, then at least a design weakness.

If things go wrong and Vout is not as high as it should be due to a failure somewhere then Q1 tries to tear the arms off the ie apply too much base voltage – if the is tougher than Q1 then Q1 may lose instead. As Q1 is a TO92 package with dissipation well under 1 Watt this is probably why it exploded – summat went aglae, the tried to turn on to cause Vout to rise by turning on Q1 to turn on Q3, the cct did not comply so it kept turning Q1 on and it gave up dur to the ‘s base current. It should be at least 2W rated and I’d use a 5W as they are about as cheap and much safer.

If it is a 1 Watt it might last a long lm7233h. The ,m723h limit is crude in that it has no “foldback” action – it limits datasheef AT 2A if you try to draw too much. A will probably survive that if the heatsink is good enough. The current limit works by dropping voltage across R The design followed relatiovely logically from what was available at that time. The 2N was the logical choice of pass transistor, and as it was NPN you needed Q1 to invert the drive polarity and give current gain.

If you wanted current limit the arrangement used was simple obvious and standard fare. If you wanted to go to zero Vout you needed some cunning due to the non zero reference voltage so the upside down pot polarity system was used.

I’m not overly familiar with such things but I suspect it was probably standard practice in that day. IF you want to abandon this cct but keep it working If you can tolerate 1.


But, if you’d done that before working out how it worked you’d have learned far less: The LM is one of the first variable voltage linear regulator ICs produced. It was first introduced in the early ‘s. It is still available today and used in some designs, though there are better choices now available, depending on the specific application. The LM’s primary intended function is to “regulate” a DC voltage. These terms seem generic and innocent enough, but have a special meaning within the context of voltage regulators.

Voltage Regulator

It’s the terminology that often confuses beginners. Then study the example application circuits given ratasheet it. You may find there are several data sheets available from different manufacturers.

Get each one as they usually contain the same information presented in different ways. This can be helpful if you are a beginner trying to decipher the terminology. There are two aspects to this. First, the LM has a maximum allowable input voltage of 40 volts.

Go above this and kaput! Second, the input voltage must be at least 3. If you break this 3. All understood to mean “within the legitimate bounds of the LM’s capabilities”. When you study the data sheets, you will see that you can use the LM “on its own”, though with a few necessary external resistors and capacitors. However, it will only regulate a maximum output current of about milliamps. To get more output voltage you need to use “booster” transistors.

The latter being a very curious designation. Does it imply there are actually two transistors connected in parallel? With booster transistors you can increase the LM’s useful output current range to several amps. But again, all with certain limitations. Most transistors “explode” in experimental situations because they are connected wrong.

In non-experimental contexts they explode because a reverse voltage is inadvertantly applied across its terminals, or a very large current passes thru its terminals. I don’t see a “zener diode” in the schematic.

This is the conventional schematic symbol for a voltmeter. If you are only trying to dztasheet a volt 2 amp power supply, there are better ways to do it in The output of the ‘ is a pass transistor with the zener Vz pin 9 connected to ground, so that it pulls Vc pin 11 down in order to increase the output voltage.

If the zener is not grounded it won’t work properly. That increases the base current through Q1, which pulls up the base of the parallel? About 15 years ago I worked adtasheet a HV power supply company. A was used to regulate the HV output 15KV. The problem was traced to the The old one had 2 of the pins open while which were being used for voltage feedback sensing. The new replacement had these two pins shorted internally. Just wanted to mention this for something to think about while troubleshooting.