Can I run two regulator circuits off one transformer secondary?

[RDL2004]

Is it possible to run two regulator circuits off of the same transformer secondary? One would be a LM317 and one would be a 7805 type. Everything about the regulator circuits would be separate,  from the rectifier diodes to the output. The only thing in common would be the secondary of the transformer.

Thanks.

 

[audioguru2]

Hi RDL,
No Problem.
If the regulators share the same ground, they can also share the rectifier and main filter cap. Just make sure that the transformer, rectifier and filter cap can provide the total current.

 

[RDL2004]

The reason I want to try this is because I need to power a digital panel meter (19.99 volts) for my variable supply, it is a "common ground" type. It requires 5 volts, so I can't power it from the variable output of the supply because at times the output voltage could be too low to run the regulator.

I tried powering the meter from a 7805 tapped in to the unregulated input to the variable power supply, which worked fine unless the power supply's voltage was adjusted above about 11.5 volts, then the panel meter's reading would begin to start jumping around and became unstable, and even when the variable output was adjusted to maximum (19.9 volts) none of the meter's readings were over 13 volts. It seems maybe common ground mode is not too happy with too much differential voltage between the meter's supply and the voltage being measured ?

The meter works perfectly reading the power supply's output voltage if it is powered from a separate 5 volt bench supply, all the way up to 19.9 volts the readings are correct and stable. Because of this I thought to put in a separate supply for the panel meter, but there is no room for another transformer. The meter uses a maximum of 75 ma, so I could build it a tiny regulated supply of its own powered from the same transformer, and maybe get enough isolation to get rid of the unstable voltage readings.

 

[Ldanielrosa1]

Ouch!  Those meters suffer from a design shortcut.  Neither input pin is common to either supply pin, and it may be difficult to stabilize if there is an external current path.  They're great for a battery powered panel meter, but inconvenient in some other applications.

The current is a bit high to try capacitive isolation, unless you build a driver for higher frequencies (low kHz)- but that adds a bit to circuit complexity.

I'm sorry I couldn't contribute anything useful.  I'll be watching to see what comes of this as I am curious.

 

[trigger]

You have to build a rectifier circuit before the two regulator circuits.

Also be caution on the current drawn from each regulators and the circuit behind them.

 

[RDL2004]

Yes, as I said in the original post, it would be two totally separate regulator circuits, by circuit I mean the whole thing, rectifier, filter caps etc., not just the regulator chip.

Tonight I breadboarded up a 5 volt regulator circuit and connected to the transformer secondary of the variable power supply and it did work fine.

Unfortunately, before I could test whether the meter gave stable readings with this set up, a wire somehow came loose and shorted the 5 volt supply. It didn't hurt the regulator but sending 5 volts to the meters ground connection seems to have broken it. Now I will have to order a new one. Oh well.

 

[Dido]

Hi RDL2004,

If you have shorten the 5 V supply then the meter shouldn`t have blown up, maybe there is wrong something else, try to investigate all the circuits maybe something has unhooked.

 

[trigger]

unless you are using analogue meter.......digital meter will be able to take the 5V at its negative probe.....

 

[Dido]

The reason I want to try this is because I need to power a digital panel meter (19.99 volts) for my variable supply, it is a "common ground" type.....

Hi Trigger,

as you can see he is talking about a digital meter, and the meter won`t blow if you short circuit it`s supply so this is not the problem.

BTW: there are analogue meters which can measure both positive and negative

 

[RDL2004]

The meter won't blow if you short its supply? But I am pretty sure that is what happened.

The negative power wire came loose and touched the positive connection. Meter will now only read -1 on display. Maybe it also touched something else or something else went wrong. Whatever, meter is now not working.

I will look again and see if I can find some other problem though. I may have had some other connection wrong also, as I was doing this late at night and was pretty tired.

New meter is on the way and will be here next week, will just have to play with something else until then.

 

[japroach]

The meter won't blow if you short its supply? But I am pretty sure that is what happened.

The negative power wire came loose and touched the positive connection. Meter will now only read -1 on display. Maybe it also touched something else or something else went wrong. Whatever, meter is now not working.

I will look again and see if I can find some other problem though. I may have had some other connection wrong also, as I was doing this late at night and was pretty tired.

New meter is on the way and will be here next week, will just have to play with something else until then.

Did you check if theres a fuse inside that you can replace?

 

[RDL2004]

I put a fuse in the transformer's primary, but nowhere else. There is not much room for the manufacturer to have put a fuse in the meter, good idea though.

meter



When the wire came loose there were sparks and popping and clicking noises. The wire could easily have hit other places on the back of the meter while it was bouncing around as the back of the meter is open and not enclosed.

The 7107 chip is in a socket, and I have tried replacing it as I have several of those chips, but it did not help.

The good news is that I may have found out why the meter did not work correctly in the first place. The new meter should arrive sometime next week, I will find out for certain then.


- Rick