3 pin VR falls out of reg. way too early

[YouAgain]

I'm stopped by a simple 3 pin adjustable dc voltage regulator that quickly falls out of regulation when a load is applied. The application is a filament voltage regulator set at 6.3 volts. It regulates perfectly without a load. This one is supplied with 7.32 VAC which falls to only 7.24 vac when applied with a regulated DC load of 800ma. The pre-regulated (full wave ss diode rectified) DC is 9.1V no load and 8V loaded. The regulator is LT1084 but I swapped with an LM317 and got essentially the same results. I have the voltage set with 125 ohms between the adj. leg and the output and 500 ohms between adj. and gnd. I've tested the electrolytics, lifted the protection diodes and checked the resistors values. I've put the AC primary on an adjustable auto-transformer and found that it will stay in regulation a little longer with a higher primary voltage but it still drops out way sooner than the data sheets for the regulators show that it would. The more load I put on the lower the voltage goes.

[Loudthud]

Do you have an oscilloscope or are you using a DMM? You probably have too much ripple on the incoming supply. As the ripple dips below the voltage needed by the regulator to maintain regulation, little dips occur in the output voltage. A DMM reads this change in average voltage. You need more filter capacitance and probably Schottky rectifiers to ever have a chance to make the regulator work properly. If you are rectifying 6.3V for the unregulated voltage, (sorry to be the bearer of bad news) it will never work. You need at least 9VAC.

[DigitalJunkie]

I second that. Probably too much ripple. You might squeeze by with more capacitance,and schottky rectifier diodes..If you have the parts on hand,you can give it a try before getting a new transformer.

[Elvee]

Originally Posted by YouAgain

I'm stopped by a simple 3 pin adjustable dc voltage regulator that quickly falls out of regulation when a load is applied. The application is a filament voltage regulator set at 6.3 volts. It regulates perfectly without a load. This one is supplied with 7.32 VAC which falls to only 7.24 vac when applied with a regulated DC load of 800ma. The pre-regulated (full wave ss diode rectified) DC is 9.1V no load and 8V loaded. The regulator is LT1084 but I swapped with an LM317 and got essentially the same results. I have the voltage set with 125 ohms between the adj. leg and the output and 500 ohms between adj. and gnd. I've tested the electrolytics, lifted the protection diodes and checked the resistors values. I've put the AC primary on an adjustable auto-transformer and found that it will stay in regulation a little longer with a higher primary voltage but it still drops out way sooner than the data sheets for the regulators show that it would. The more load I put on the lower the voltage goes.

Give your schematic, with capacitor values, etc

[AndrewT]

You can measure the ripple on the supply rail with your DMM set to Vac.

This is very approximate but multiplying the Vac by 3, gives the Vpp of the ripple.

7.3Vac for a regulated 6.3Vdc sounds far too low.

[KatieandDad]

My guess is the the main reservoir cap isn't doing its job properly or that you've got a faulty component or joint that isn't showing itself under light load.

You might need to go to a Full Wave rectifier to bring the unreg voltage up a bit into the regulator.

[QSerraTico_Tico]

You need at least 3V dropout voltage (voltage across the regulator)
Been there done that, even with so called low dropout regs like LT1086 and LT1033.

[KatieandDad]

Going Full-Wave may rescue the situation without having to buy a new transformer.

7.3 x 1.4142 = 10V. Even allowing for diode drops that may be enough to feed the regulator.

Half wave rectification will starve the regulator.

No major mods needed, you might need to mount the bridge rectifier between the transformer and the PCB and then feed DC to the board as opposed to feeding it AC.

[YouAgain]

You guys were right about the ripple and also about the 3 volt input to output difference.
This morning I looked at it on the scope and found 1.5 Volts p-p of ripple. I added more capacitance and then put it on with my Variac turned all the way up and I was able to get it to lock at 6.3 VDC output with a 1.6 A load. I then backed down the Variac voltage until the circuit dropped out of regulation. I found the minimum voltage before it would drop out and took measurements. It was good at 8.35 VAC input for 9.26 VDC (full wave bridge rectified with MUR410 diodes and 24,000 uF). At that point I measured 1 volt p-p of ripple on the raw supply. So I have decided to get another transformer with a higher secondary voltage and to get more capacitors. I've built dozens of three terminal adjustable regulators but never any that had to source more than a couple hundred milliamps of current. The ripple really goes up steeply with increased current and the ripple seems to lower the regulators drop out point.

Thanks to all who replied.

[Elvee]

Originally Posted by YouAgain

You guys were right about the ripple and also about the 3 volt input to output difference.
This morning I looked at it on the scope and found 1.5 Volts p-p of ripple. I added more capacitance and then put it on with my Variac turned all the way up and I was able to get it to lock at 6.3 VDC output with a 1.6 A load. I then backed down the Variac voltage until the circuit dropped out of regulation. I found the minimum voltage before it would drop out and took measurements. It was good at 8.35 VAC input for 9.26 VDC (full wave bridge rectified with MUR410 diodes and 24,000 uF). At that point I measured 1 volt p-p of ripple on the raw supply. So I have decided to get another transformer with a higher secondary voltage and to get more capacitors. I've built dozens of three terminal adjustable regulators but never any that had to source more than a couple hundred milliamps of current. The ripple really goes up steeply with increased current and the ripple seems to lower the regulators drop out point.

Thanks to all who replied.

If you don't want to change the transformer, first thing to do is to use a good LDO regulator, then if not sufficient replace the standard rectifiers by schottky's, and if it is still too short, turn to synchronous rectification:
6.3 filament from 5vac winding