ECN Forum Forums Technical Discussion Electrical Theory and Applications 7800 Series Voltage Regualtor

In the photo above, I have a 7824 Regulator. It's max voltage is 40V (or something like that.)

Here's my theory: There's 340V coming in, but only 40V is dropped across the IC, and 300V across the zener diodes.

Am I correct here? Is it okay to assume that the IC would only "see" 40V?

Thanks,
Josh

Yes but ....

If you are using the standard TO-220 uninsulated package, the bare metal back is connected to the Com terminal. Which means if you are using a heatsink, it will be at + 300 VDC with respect to the bottom of your diode string. Since you are dropping approximately 16 Volts across the regulator, a heatsink will be most likely required.

IMHO, it is not most reliable design to operate the regulator up against it highest voltage limit. I personally would use a zenier diode string and dropping resistor to get the voltage to 330V and then use your circuit with a LM7806 to regulate it to 324 Volts. That will give you a little more protection against high voltage input.

Larry C

I concur with Larry, and I have some other concerns.

If there's any capacitive load at all, then if the input voltage rises sharply, the 78xx will see the full voltage momentarily. Also, without other protection diodes, if the input voltage suddenly drops to zero (e.g., accidental input short), it sees high reverse voltage.

If the output is momentarily shorted, you get the full differential across the 78xx.

If the zener voltage is on the low end of its tolerance and the input voltage is high, you exceed the 78xx rating.

In any of these cases, you have a dead chip.

With a little more info, I'm sure there are quite a few folks here who could suggest a better alternative. (Min/max input voltage, min/max load current, load type?)

And still more concerns...
Regulators like these give you a spec for maximum input/output differential voltage. The 7824 is a little higher than the rsst of the 78XX series. But here, you have to think more along the lines of the Vceo rating of the output transistor stage. If you are charging load capacitance, there would be close to 350 volts across the output stage. Another thing to remember is that diodes are noisy creatures that should be bypassed when establishing a reference. also, the "com" or "adj" pins of regulators tend to be high impedance inputs. Consider using a biasing resistor between 1 & 2 to set the current for the best regulation. Those 3 zeners are probably really 6 diodes internally and I doubt that you will be that close to 300 volts unless you have many diodes to choose from.

I guess I need to clarify. Did you mean a "7824" or are you using a newer "78024"? Keep in mind the old trick of using a resistor between the common and output to establish a fixed reference current. A resistance is then placed between common and the negative supply rail to drop the desired additional voltage. Bypass capacitance is required here just as it would be with your zeners. Then you still need to add the protective diodes.

Just as John mentioned, it would be interesting to know exactly what you need with this supply. I was playing with a PWM supply concept using a 555 driving a darlington opto-coupler in the biasing circuit of the series pass transistor. I was going to see if I could get decent regulation just working with the control voltage. It wasn't something that I really needed though, so it got back burnered.
Joe

OK, thought about it some more. Add a high wattage zener, cathode to the left, between input & output. Choose 1 that is greater than the steady state voltage drop across the regulator when things stabilize but less than the max voltage difference that the regulator can handle. Add a protection diode between common and output, cathode up, to discharge your reference filtering capacitance. Use at least a 1N4004 or equiv. Add a biasing resistor between your input and zener string and capacitance across the zeners. If your source is hi-z, you might be fine. If lo-z, consider a series resistance before the regulator that gets bypassed once your output gets close to the desired value.
Joe

[This message has been edited by JoeTestingEngr (edited 10-02-2006).]

As far as what I need the circuit for, I'm building a tube amp. The power amp section is staying the same, but the preamp section is being changed out, and it needs 325V instead of the 260V provided. I don't know enough about RC and LC filters to redo the power supply like that, and I figured it's easier to use a filter cap on a clipped sine wave than a reduced sine wave.

What I have shown here is just an example to make things easier. Also, the 78024 should be 7824. I'm using the TO-216 package. It's all plastic with a mounting hole for a heat sink, so the sink will be at 0V.

I was either gonna go it this way, or use an NPN transistor, Collector as input, emitter as output, resistor going from C to B, and zener diodes from B/resistor to 0V. The only problem is I forget how to make the feedback amplifier and what values to use for it.

It would be so much easier if they made a regualtor from the 7800 series that goes for high voltage, but this was the closest I could come up with.

OK Josh, is this just a line level pre-amp or more of a serious driver? How much of a resistive load do you think it will look like at min. and max. drive conditions? Is it OK for it to build up over a few seconds or will that hurt your other biasing needs? Is your 340 volt DC source from a full wave bridge and a filter cap unloaded? Or lightly loaded?
Joe

[This message has been edited by JoeTestingEngr (edited 10-04-2006).]

Do you even need a fully regulated supply for this application?

If this is a run-of-the-mill pre-amp the tubes will all be biased class A, so the average load will remain pretty constant anyway.

Joe and Paul - I'm not really sure, I downloaded some schematics and I'm copying them over. The preamp is for guitar to line level. The voltage is specified on the schematic. As far as how well regulated it needs to be, I'm not sure, but like I said before, I'm horrible at figuring out RC and LC power supplies like the ones in tube amps, that may have 4 or 5 voltages.

All I know is that the preamp calls for 325V. I actually have 450V coming in, but I was either gonna cascade a few of these so that the voltage drop + output voltage = less than maximum voltage for the device. The more I think about it, I'm better off with a bipolar transistor/resistor/zerenr ladder described in my last post.
like this:
http://en.wikipedia.org/wiki/Linear_regulator

Maybe you want to try this:

http://www.geocities.com/tjacodesign/300vsup/300vsup.html

(found by looking thru http://www.discovercircuits.com/P/pwr-hivolt.htm )

Just make sure the FET of your choice can stand the voltage and the power dissipation...

In my experience, tube circuits are very tolerant of voltage swings, and if you use a big enough (47+ uF) electrolytic condensator for the hi voltage supply, the small current drawn by the preamp will be regulated enough.
One similar, simple, tried-and-tested solution is two 47uF, 350V caps between + and ground, with an 1 kOhm in series between them. The resistor will limit the voltage swings from the rectified AC flowing into the second cap.
Oh, and when testing these circuits: Do not forget to add a load ("bleeder resistor") at the output or you might be fully awake in a spilt second ;-)

edited to fix link

[This message has been edited by StarTrek (edited 10-05-2006).]

Josh, I don't think you need to regulate at all. The class-A preamp stage will draw a fairly constant current, and the voltage is not especially critical. Just select a series resistor to drop the right amount of voltage and add an electrolytic filter cap after it for hum reduction, similar to StarTrek's suggestion.

The electrolytic should be rated at the full supply voltage, however, because there won't be any voltage drop until the preamp tube warms up and starts drawing current. Alternatively, lower the values of the voltage-drop and bleeder resistors to form a "stiffer" voltage divider that will limit the cap voltage during warm-up. Watch the power ratings on those resistors.

Note that the circuit referenced by StarTrek is not regulated... It just exhibits a lower output impedance than a simple voltage divider, and is variable.

Glass audio is a great hobby for the Winter months!

[edit: Oops, wrong UBB code]

[This message has been edited by John Crighton (edited 10-05-2006).]