"High Voltage" DC Power Supply

[Fanie]

I want to put a 300V digital DC meter on the solar panels, without load about 250V.
The digital meter has a SMD 7805 for the pic's supply, hence one would like a supply around 12V for it, the meter draws up to 30mA.

I have 12V, but to measure the 250VDC you have to connect the negative of the safe 12V to the floating 250VDC, which I don't want to do because the 250V DC then becomes dangerous.

I will either have to make or obtain a high voltage PSU, which I purchased before and it did not last, or make an isolated power supply off the 12VDC.

I think the latter could be the better solution, it is a free running transistor push-pull oscillator working off the 12V DC and the ferrite transformer outputs a 10 to 12V AC I can rectify with some fast diodes, ie Schottky.

Any better ideas or solution ?

[Stephen Moss]

Could you not essentially create 3 resistor potential divider such that you get a tenth of the 250/300V (2.5-3V) developed across the middle resistor and use one end of the resistor as the +ve input to a differential amplifier with the other end of the resistor as the -ve input, then use the output of the differential amplifier to drive the input of the DVM.

Would that not then effectively isolate the high voltage side from the 12V side used to power the op-amp and DVM so you won't have to connect the two?   

[See_Mos]

You say you have a 12VDC supply so something like the Traco TBA 1 1212 (Farnell 3259979)  isolated through hole DC/DC converters which will supply up to 85mA with 3KV isolation .  There are dozens of similar parts and we used many in industrial applications and never had a failure.

Another option would be to use a small SMPS that has 'universal' input powered from the solar panel.  Most will operate on DC just as well as AC input.

[See_Mos]

Of course  another way would be to run a PIC directly off of the solar panel to do the AtoD then opto isolate a serial signal to another PIC in the 'safe' area for the display

[Fanie]

The easiest would probably be to add a small solar panel sharing the 250VDC negative that will supply the meter with power and I can then measure the 250V that way.  I just remembered I have an old small solar panel someone gave me and it outputs around 18VDC and is thumb suck about 10W.

The voltmeter indicates what the solar panel(s) voltage is when under load.  Below around 160 to 180V the power becomes unusable for certain applications, but depends what it is.  One may not be able to power two or three 2000W elements then, but as power decrease late in the day, it can still power one 2000W or boil water in a 1000W kettle.  A 1000W kettle will still boil the water at 500W, it just takes twice as long, you lose however when the heat losses in the water becomes more than the power supplied.

I want to add an Amp meter as well, as soon as I can figure out how to make a 50A or more shunt that will have a few V drop over it and without generating heat.  I have a 50A shunt but I have doubts if it will make it,  the terminals are 4mm screws to start with which I think is too small and it has a 75mV drop at 50A (which one can amplify).

The Amp meter will then also be powered from the small solar panel.

Thank you all for the inputs.  The more ideas triggers different thinking.

[Fanie]

like the Traco TBA 1 1212 (Farnell 3259979)

The thing one must keep in mind if you run something off solar panels is that the power goes on very slowly and dies down very slowly.  Many electronic circuits cannot cope with this and is not designed to work when it is on the verge of on or off where the voltage may be sufficient but the current not, and will result in a continuous on and off switching.  Perhaps this one can, but I decided to go with the solar panel because I have it.  Thanks Charlie.

[JonW]

Why not use an El cheapo DVM powered via a diode sum into a nine-volt reg.  You could have a battery backup. Most DVM's are really low current. Thus, you could use one of the 400V linear Fet regulators available like the LR8.

[trastikata]

3$ buck-boost DC-DC convertor IC with wide input from 3v to about 70v converting it to 5v output and then a VLDO to 3.3v.

Then a small back-up LiPo battery with 1$ LiPo charging IC that will charge the battery to (3.7v not 4.2v) when power is available and if needed.

And another 3$ supervisory IC that will switch between the main (buck-boost) supply when available and the battery if the main supply is not powering.

The battery will last years before the capacity drops enough to not be able to sustain during the main power off time.

 

[Fanie]

Why not use an El cheapo DVM powered via a diode sum into a nine-volt reg.  You could have a battery backup. Most DVM's are really low current. Thus, you could use one of the 400V linear Fet regulators available like the LR8.

The diodes will give a large voltage drop and the meter will only power up at near full voltage.  Adding batteries, charging... will just complicate it unnecessary.

With this setup the meter(s) should already power up when the 250V reaches around 100V and the 18V panel reaches around 8V at sunup, and will start to die down when the solar reduces to around the same late afternoon.  The setup is robust too.

Btw, I make my own meters.  It was the first project I did when I got the PDS and still use the same.

[JonW]

I meant powering the DVM from a high voltage LR8 to 10V, then a diode sum to the battery of the DVM.

[Fanie]

I meant powering the DVM from a high voltage LR8 to 10V, then a diode sum to the battery of the DVM.

100%  It can also work.  I didn't know that Microchip had such a regulator, thank you.

I put the small panel up and the wires are in !  I still have to make the meters (V & A) fit, calibrate the V meter and wire them up.  The baby panel looks like the decimal point of a display 

I though I was very clever by making the 76mm tubing so it can slide on the large panels tubing.  It turned out to be quite a feat and although I got it done, it far exceeds the effort I anticipated.
And then to get it up there I had to climb the five meters up all the way...
Just kidding.  I used scaffolding.

[JonW]

The LR8 type of regulators is quite unique; there are other devices on the Chinese market that operate up to 450V.  Most are pre-regs for running the circuitry on the isolated flyback converters. It's pretty neat and cheap.

[See_Mos]

Seeing the mention of the LR8 I thought I had struck gold, just what I needed for a project, that was until I read the current is limited to 10mA. and the components to increase the current make it unviable for my purpose

[JonW]

There are some slightly higher current parts from China but are only 30mA.
They are cheap, you could diode sum them with a small resistor in the sum so they share current.

You could use a 80V buck to increase the current.

[ken_k]

Are the voltage and amp meters transmitting data to other devices or simply to look at?
Have you considered analog moving coil meters? No power supply required.
For the voltmeter a 100uA meter sounds about correct. For low current devices I'm a fan of 50uA and 100uA meters as they give me 0-5 and 0-10 scales. I see you have a few solar panels in series, could you simply tap off a lower voltage and use the LM7805. If single panel is over the LM7805's maximum input voltage a series zener may work ok.

 

[rick.curl]

Have you considered a universal input wall wart? I'll bet you have a few around the house already.

-Rick