Regulating Voltage with LM78XX

You can go up, but not down in voltage with that regulator.

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Member of DAV #85.

Michael A. Terrell
Central Florida

Hi, Don. As Mr. Terrell says, you can only increase the output of a

78XX by using the voltage divider trick. You'll have to just use a 7812 and be done with it.

| _____ | + | | + | o---o---| 7812|---o-----o | | |_____| +| | Vin --- | --- +12V Out | 18VDC --- | ---10uF | |.33uF | | | o---o------o------o-----o | - - | (created by AACircuit v1.28.6 beta 04/19/05

As you know from programming, elegant and simple usually isn't easy. ;-) For someone without electrical or electronics experience, it's not safe and doesn't make sense to try doing this without transformer isolation.

Good luck Chris

Chris; Thanks for the reply I'll get a transformer and a 7812 Thanks again; Don

Chris wrote:

If you must use that regulator, the only way to get 12 volt from the 15 volt regulator is by further dropping the output with some series diodes. So 15 - 12 leaves 3 volts, and each diode drops .6 to .7 volt so four series diodes will drop 2.8 volts leaving you an output of 12.2 volt. If your circuit has a stable current draw you can simply insert a resistor in place of the diodes to drop the extra 3 volt. So say if the circuit you're feeding draws .5 amp to drop 3 volts you will divide 3 by the .5 to get you a 1.5 ohm resistor which consumes 3 * .5 = 1.5 watt so you would use at least a 2 watt resistor there.

Since you have to get a transformer, just get a power supply. A transformer alone won't do it - you need a transformer, rectifier bridge & filter cap, plus an enclosure, and your

7812 circuit (2 caps and the 7812).

As an example, MPJA catalog #15548 at $6.95 is a 12 volt regulated supply capable of 2 amps. You need the line cord, too, catalog #15447 at $1.25.

Ed

He said that he IS a programmer, so at least calculate for him Ohm's Law. No need for computer, just pull out your slide rule. Dropping 3V at 1/2A gives (on mine 30cm Darmstadt) 6 Ohms, the power numbers are correct.

(Note: Since when relays started beying ssssoooo sensitive to supply?)

Have fun

Stanislaw

As you've now learned, if you want 12V then get a 7812. However, it puzzles me what led you toward the circuit you originally posted. It looks like a constant-CURRENT drive for some load represented by RL.

To answer your questions: Io is the current delivered by this current source. You can think of Iq as a "leakage" current from pin 2 of the regulator. Iq, along with the current through R1, contributes to Io. The spec sheet probably has a typical and maximum values for Iq (I think it's around 5 mA for the 7805 , for example), and you might want to measure it if you wanted some degree of precision in your current ... IF you were wanting to build a current source, that is, which you're not.

Regards,

Mark

Yep, i mixed up the watts. hehe. Thanks for pointing that out. So easy to make dum mistakes and its good to have many eyes looking it over.

I missed that part.

But the regulator may be there more for isolation (though it may not be the right way to isolate a relay).

Remember, in the old days it was rare to see regulators. They'd be in test equipment, and lab type power supplies, but rare in consumer electroncis. At the very most, you'd regulate the plate of the oscillator tube in that receiver. Most of the time, any regulation was taken care of by a VR tube, no pass element needed.

When solid state came along, suddenly regulators came into their own. The early magazines would show diodes feeding massive capacitors, because they were seeking that really low output impedance. Regulators took away some of that burden. Then when ttl came along, you'd want to regulate not because they needed precise voltage (unlike those receiver oscillators that would shift frequency when the plate voltage shifted), so regulators became mandatory. At that point, it became easy since the introduction of three terminal regulators coincided with that ttl boom.

But those three terminal regulators were originally called "card regulators", the intent being that you'd have a big transformer, diodes and capacitors in one place, and feed that higher voltage to all the boards. And each board would have a card regulator, now that they were small and cheap. The regulation was not in one central point, but distributed over the equipment. COmpared to previous regulator ICs, like the 723 needing all those external components and the pass transistor if you needed more current, the 3 termainl regulators needed nothing more bypass capacitors.

THose 3 terminal regulators got even cheaper. There was little point in not using them, they likely did provide some isolation between stages if you put in more than one. They beat the previous cheap arrangement, zener diodes that you'd have to calculate the dropping resistor, and recalculate that resistor if the current demand changed.

No, there's little need to regulator voltage to a relay. But it may isolate the spike when it clunks in from the rest of the circuit. Though again, there may be better means of providing that isolation.

I was given a used slide rule around the time I got my first three terminal regulator, which was around the time they first were introduced. Around the same time, I saw a pocket calculator for the first time, an HP-35 that a friend had paid big bucks for (though less than others since he was involved in a group buy at work). Wasn't long before that slide rule became obsolete, indeed before I had reason to get good at using it.

Michael

The method shown can raise the output voltage above the 15 volt normal output of a 7815, but it cannot lower it.

The concept is based on the property of the regulator that the output voltage is regulated to be 15 volts more positive than the voltage on pin 2 (the reference pin). You can use current from the output voltage to produce a drop in resistor R1 and that drop is applied to the reference pin. The regulator keeps raising its output voltage (raising the voltage applied to its own reference pin, till the voltage of the output is 15 volts more positive than that on the reference pin, and there it stops.

The formula tells you that the voltage drop across R1 is the sum of the current that passes through R1 (15 volts/R1, for the 7815) plus the small and fairly constant bias current that passes out of the reference pin, Iq. Normally you make R1 low enough so that its current is many times Iq, so that small variations in Iq do not much change the output voltage.

You could easily use this method to jack the output of a

7805, 7806, 7808, 7810 or LM317 (a 1.2 volt regulator designed specifically for this method, with an especially low and stable Iq), but not to lower the output voltage of any of them.

(snip)

You will have to be more pedantic in describing the available signals and what you want to do with them, for me to help with this.

John; Thanks for the information on increasing voltage with the LM78XX, I'll file it away for reference. RE:, "pedantic", I had to look that one up, but I got the drift from the sentence context. Thanks; Don

I meant the "emphasizing minutia" aspect of "pedantic".

So, tell us, in detail, what you are trying to accomplish and what you have to work with. There may be a simple solution that is completely different than what you are picturing.

Oddly enough, *my* "30cm" Darmstadt, that I've had for nearly 50 years (Castell 1/54) has a 0-10 *inch* scale on one edge, but no metric at all.

I can confirm your figures :-)

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Your point about relays is well made. From time to time you see people unaccountably straining to provide precise voltage for them. Still, he may have a circuit that *is* sensitive to the supply that drives the relay. He mentioned something about the circuit switching the relay after 2 hours.

Ed

Isn't it "minutiae", Mr. Popelish? I believe you implied the plural. ;-)

Cheers Chris

Yes, his diagram is confusing and confused. What he is calling RL isn't RL. I think the confusion comes from all his labeling and adding of lines to indicate current. The key is that the variable R that he mislabelled as RL goes to ground. I think what he has is this: ------ 18 in ---+---|LM78XX|---+------+---> +Vout | ------ | | [.33uF] | [R1] [.1uF] | | | | | +------+ | | | | | P | | O Gnd

The pot and R1 form a voltage divider that raises the gnd pin on the 78XX to some specific voltage above ground, and the 78XX keeps its Vout pin at XX volts above its ground pin. RL would be the load that gets connected from Vout to gnd, and is not shown on his diagram - he has mis-labeled the adjustment pot as RL.

Ed

Could be. My spell checker was happy, so I was happy. I'm just glad it wasn't happy with the entirely wrong word.

Hi, Mr. Popelish. Your spell checker was happy because "minutia" is singular, and "minutiae" is plural. Talk about minutiae! ;-)

Getting the relevant information from the OP seems to be like pulling teeth. I'll bet that when he's done, his best solution would be a simple Time Delay Relay (TDR). Power it up with DC or AC, and you get a two hour timed relay contact closure. The OP might want to check out SSAC for some good inexpensive ones that should do the job:

But the education is in the journey, not just the destination. You seem to enjoy this stuff more than just about anyone in the newsgroups. I'll bet you would have made a heck of a science teacher in another life, Mr. Popelish.

Cheers Chris