78L33 regulator

[sundy]

Hey everybody. I'm new to the forum.

I have a electronic analog clock movement we manufacture. The motor drive we buy has started to not drive the second hand on initial startup. I measured 2.9 VDC out of the 3.3 regulator. I lowered the resistor from 1KΩ to 540Ω and now the hand starts as it should.

Finally my question: I now have 11 VAC driving the 3.3 regulator, if I am reading the data sheet (attached) correctly the regulator can handle 30 V input. Am I understanding the data sheet correctly?

Would the change from about 5 VAC to 11 VAC input significantly shorten the life of the regulator?

Thanks for your input.

 

[BobK]

It will not shorten the life of the regulator as long as it is not overheating. More voltage means more heat. Do you know how much current is drawn? Is there a heat sink on the regulator?

Bob

 

[(*steve*)]

Are you *really* applying AC to the input of a DC regulator?

That's a baaaad thing m'kay.

At the very least you need a diode in series if you're after a string of 3.3V pulsed at line frequency.

 

[Sunnysky]

report full schematic . 5Vac is better perhaps needs bigger C

anything get warm?

 

[sundy]

No the regulator is not getting warm. We have been running AC into this regulator for years and it does not seem to negatively affect the operation of the circuit. When I checked today the clocks are still running and the regulator is not hot after two days in a 90 degree building. Current draw is very low, not sure of a specific value.

 

[Sunnysky]

unless I see full schematic, I cannot advice further.

 

[(*steve*)]

We have been running AC into this regulator for years and it does not seem to negatively affect the operation of the circuit.

If you are knowingly operating the device outside of its absolute maximum ratings then any problems are your own.

 

[sundy]

unless I see full schematic, I cannot advice further.

This is a proprietary circuit that is part of my product catalog I make my living from. I hope you will understand if I only post a partial schematic. The risk is low but non-zero that someone could reverse engineer our design. I am playing with the resistor to get the input voltage down to approximately 5 V.

 

[Sunnysky]

Using 4 diodes or a diode bridge allows less ripple and lower input voltage than 24Vac

Normally one measures Vin dc and ac (p-p) or scopes Vin min to ensure dropout Voltage above Vout to regulator meets requirements.

There are many other solutions to this problem. But if you are happy with this solution ok.

 

[sundy]

Using 4 diodes or a diode bridge allows less ripple and lower input voltage than 24Vac

Normally one measures Vin dc and ac (p-p) or scopes Vin min to ensure dropout Voltage above Vout to regulator meets requirements.

There are many other solutions to this problem. But if you are happy with this solution ok.

The circuit board is actually laid out for a bridge rectifier but the previous owner/designer never used it. There are approximately 2000 units in the field with a very low failure rate.
I just measured the temp of the device and I have a 6 degree rise over ambient.
I think i will just set the resistor value so that the regulator holds 3.2 VDC out.

Thanks for the help everyone.

 

[Sunnysky]

If you test it with a relay power cycling every few seconds on / off for a few days with zero failures at highest ambient,then you can verify reliability.

 

[AnalogKid]

There is no resistor in either the datasheet or the schematic you posted. What resistor are you referring to in post #1?

ak

 

[BobK]

Look again. There is a resistor between the 24V supply and the diode.

Bob

 

[sundy]

Look again. There is a resistor between the 24V supply and the diode.

Bob

Indeed. I changes the resistance from 540Ω (2 270Ω 1/2w in series) to 750Ω (2 1.5k 1/4 w parallel) and the stalled second hand returned. I am beginning to think it is a current problem rather than a voltage problem. The 3.3 regulator puts out 3.2 VDC with either resistor setup.

FYI this is a synchronized clock system, the 24VAC is an industry standard so I cannot vary that. The current draw is very low, the second hand pulses once a second, the minute hand every 15 seconds, and the hour hand every 15 minutes. The only other draw is the microprocessor, well once the super cap charges that is. AC current would be easy to measure but nearly meaningless, most of the AC current is drawn by the first resistor in the circuit. I might have to lift some components and try to get a DC current draw.

 

[BobK]

You never mentioned a supercapacitor before. It is the most likely the cause of your motor not running on start up.

Bob

 

[(*steve*)]

Agreed. A series resistor on the supercap instead of on the input would possibly be a better option (but with few details it is hard to say)

And thank the electronic gods that AC is not being applied direct to the regulator.

 

[BobK]

Bah, the electronic gods would have smoked it if it was.

Bob