UNdervolted Linear REgulator

[dknguyen]

Hello. Everyone here knows that if a the input voltage to a zener+resistor voltage clamp goes below the breakdown voltage of the zener, then it just passes the voltage across.

I was wondering what happens with a linear regulator. Does it just pass the input voltage to the output (with some diode-ish voltage drops)? Or does it shut off?

 

[kchriste]

It depends on the regulator. The 7805 will do the diode-ish voltage drop down to a certain point and then shut off once there is not enough input to turn on the main pass transistor.

 

[RadioRon]

I've been working with the Micrel MIC29152 in an application where the device works out of regulation about 50% of the time, in other words the input voltage is too low for regulation. In this case, the regulator drops about 0.2 to 0.3 volts at full operating current. It is a lot like you would expect a resistor of low value to behave. When you want to know how another type of regulator will behave under these circumstances, try looking closely at the curves on the data sheet. For the MIC29152, the voltage drop vs current below regulation voltage is clearly shown (see data sheet link below) on page 6, top right corner, "MIC29150-5.0 Dropout Characteristics". Notice how the output voltage in this 5V example is at 3.7 volts when the input is at 4.0 volts while passing 1.5 amps. When the current is reduced to 10 mA, the voltage drop is negligable.

https://www.electro-tech-online.com/custompdfs/2007/12/mic29150.pdf

Don't assume that this voltage drop applies to other regulators, you need to look at each one specifically. This particular regulator is a low-drop-out type.

 

[Hero999]

The dropout varies a lot depending on the load current, a typical LM317 varies between 1.5V to 2.5V so it isn't very reliable - you're better off with a zenner or even a rectifier diode.

 

[dknguyen]

I see. I was curious because I had always wondered seeing as how I seem to have trouble finding zeners of sufficient current sometimes.

 

[Hero999]

You can connect a diode to a large NPN transistor to increase its power rating and if you're worried about reverse current then connect a rectifier in reverse parallel.

 

[dknguyen]

And if I wanted to provide a voltage that was ~15V whose upper reference was the supply voltage (rather than ~15V whose lower reference is the supply ground) then this would be the correct variation right?
(The white triangle would be the "floating" ground of the supply, ~15 below the supply voltage.)

 

[Hero999]

You've confused me, where does the zener's cathode go?

This is what I'm talking about, the equivalent zener voltage will be Vbe greater than the zener - in this case it'll be 4.7 + 0.6V

You don't need to wory about a base resistor, the transistor will take as much as it needs to sustain the collector current and no more.

 

[dknguyen]

For my circuit:
THe zener's cathode goes to the positive supply AND is also the positive voltage terminal of the load. (I'm trying to make almost a negative supply, I guess, that's referenced to the +V of the source).

If I short the resistor...won't that short the zener to ground? If I just open the resistor, where does the base current go?

For your schematic:
Huh, that's different. I was working off of this one presented here:
**broken link removed**

But I am trying to make the load voltage equal to that of the zener's breakdown. It seems that your circuit is dropping the the zener voltage across the NPN and the extra source voltage is being dropped across the across the load, isn't it? I need a regulated voltage that is connected to the high-side of the voltage supply- I think a PNP is needed for that. THe +V of the load is equal to the source's postive voltage, and the load's ground is sitting 15V below the +V of the source., with the extra source voltage being dropped somewhere between the load's negative terminal and the source's ground.

Doesn't your circuit also effectively form a series circuit with the NPN's BE diode, the zener diode, and the load, destroying the zener? I'm thinking the the NPN circuit model here. Or am I forgetting something where the collector current can also control the base current? My understanding right now is that the base current controls the collector current and it does not work the other way around.

 

[Hero999]

I thought you just wanted a high power zener diode?

Or am I forgetting something where the collector current can also control the base current?

Yes you have, the collector current does effect the base current in this circuit because the transistor actually diverts the current away from the base when the it turns on. If the transistor has an HFE of 10 and the current through RL is 1A, Ic will be 900mA and Ib will be 100mA.

Also, if you ignore RL for the moment, and just look at D1 and Q1 it doesn't matter whether you put RL on the high or low side providing the current is limited to a safe level.

 

[dknguyen]

I guess I did steer away from the topic at hand to my actual circuit at some point. THanks.