3 digit panel meter to read -volts

[Greatvalue1]

I am trying to use a 3 digit panel meter that only reads +volts to read negative volts (It has the 3 wires in the back. 2 to supply and 1 to probe). I am currently using a tl082 op amp as an inverter; However... The power supply I'm using outputs more voltage than the input supply.

i.e. (3A 12.6 V Transformer supplying a duel output power supply using 337 & 317t regulators, giving me @16 volts out @1.5A).

So even though I've managed to invert the 337's output to +, it can only show 12.6 volts max at the meter since the +vcc and -vee on the tl082 is only 12.6V. I'm not asking the circuit for more power, just more volts; enough to power the tl082's +vcc.

(If this sounds weird, you can see the schematics at Jameco JE215 Adjustible Power Supply Kit).

How Can I increase the Voltage to the tl082 +vcc so the meter will read up to 16V?
Or... Is this even possible?

ps. I will consider purchasing a new panel meter only as the very last resort. My intention is to repurpose the stuff I already have.

 

[Greatvalue1]

Yeeaa

I wonder if I get a prize for stumping the forum?

 

[duke37]

You have stumped me since I do not understand what you have.

1. I assume that you have a dual power supply that can be adjusted between +/-5V and +/-16V.
2. You have a 3 digit panel meter with no information on power requirement or input range.
3. You have an op-amp to invert the signal.

Show us a circuit diagram of what you have. Are you trying to use the meter to measure the power supply voltage?

 

[john monks]

The transformer is a 25.2 volt transformer. That is 12.6 volts to the center tap.
The op-amp you use a SPST switch to go from inverting to noninverting with the output tied to your meter. Then select the right resistors to give the right reading. Examples of these are on many datasheets.

 

[Greatvalue1]

The transformer is a 12.6 volt total. Center tap is at 6.3v. If I were more computer savvy I would put a direct link, but the pdf is at jameco and the part # is 20626 (je215) adjustible dual power supply kit. I did not buy this particular kit, but it is built per spec.

The meters run on 4.5 - 28 volts, and they appear to use about 13mA.

I have connected the op amp's +vcc to the +12.6v side of the rectified transformer output, and the -vee side to the -12.6 side of the rectified output. (Before they enter the regulators.

I have no idea whatsoever as to how the regulated voltage is increased to 16 volts.

The regulators are the 317 and 337t voltage regs. with an output from 1.3 - 16 volts+-.

 

[duke37]

The Jameco transformer is 12.6VAC and is half wave rectified.

Reservoir voltage is 12.6*SQRT(2)-0.7 = 17V, this is then reduced by the regulators.

Show us a diagram.

 

[Greatvalue1]

+vcc to the op amp, and also the red wire to the meter is between the rec. diode and 317t reg.

Black wire of the meter connected to ground.

-vee of op amp connected between rec. diode and 337t reg.

yellow wire of meter connected to tl082 output.

I'm sorry for the poor illustration.

 

[CDRIVE]

I'm sorry for the poor illustration.

Was there an illustration? Maybe I missed it? I'm still lost. I have no clue as to what you're wanting to do. If you're wanting to increase the voltage range of the meter, a simple way is to use a voltage divider and divide by 10. But as I said, I'm confused.

 

[Greatvalue1]

I'm sorry, the illustration was the one I wrote out. I don't know how to post the drawing from the pdf I've listed in the previous post.

I've tried a voltage divider on the bread board, but I haven't been able to successfully get more than 12.6 volts from a (positive to negative) volt divider.

It seems that it only goes from -12.6 to 0 volts, or +12.6 to 0. never more than this.

Is there a way to get greater than 12.6 from a volt divider than I'm aware of?

 

[Greatvalue1]

I am trying to increase the output voltage from the op amp that the meter is sensing, so that the meter reflects the correct output voltage of the 337t negative voltage regulator.

And sense the op amp's output can only go as high as its input +vcc, it will not register at the meter when the output of the 337t is greater than 12.6 volts.

 

[(*steve*)]

I'm sorry, the illustration was the one I wrote out. I don't know how to post the drawing from the pdf I've listed in the previous post

That would be this pdf (from the post you've thoughtfully deleted)?

 

[Greatvalue1]

I am using the tl082 op amp as a unity gain inverter.

the output of the 337t is connected to the inverting input (pin 2) through a 1k resister.

The non-inverting input (pin 3) is connected to ground through a 1k resister.

The 1k gain setting resister is connected between (pins 1 and 2).

+vcc (pin 8) is connected to +12.6v, between the rectifying diode and the 317t's input.

-vee (pin 4) is connected to -12.6v, between the rectifying diode and the 337t's input.

The meters sensing wire (yellow) is connected to the tl082's output (pin 1).

 

[Greatvalue1]

Yes that is the pdf. Thank you for reposting it. I wasn't sure I had done it right.

 

[john monks]

Greatvalue1, your correct. The transformer is 12.6 volts from end to end. The easiest way I see to driver your meter is to use a transistor inside your op-amp feedback loop and use a voltage doubler from your 12.6 VAC to get about 34 volts DC to drive the inside transistor.
Maybe a 2N5209 will work. The TI-082 is limited to supplies of +/-18 volts.

 

[CDRIVE]

Post shematics of your circuit and a link to your meter. Your discriptions are frying my brain.

 

[CDRIVE]

That would be this pdf (from the post you've thoughtfully deleted)?

Thanks for posting this Steve, It clears up the PS part of this topic. Now I'm going to go back to post 1 and read it all over again.

 

[Greatvalue1]

Thank you John. I thought it might be like that, but I was hoping it would be something a little less invasive.

My original Idea was to put in a small, higher-voltage transformer w/ rectifier and its own caps just to power it, but nevertheless, I'll have to either stumble on to something like we've mentioned or find another meter somewhere in my garage.

 

[john monks]

Either that or juggle the supplies like use -5 volts and 30 volts for the op-amp..

 

[Greatvalue1]

How do you juggle the supplies?

 

[CDRIVE]

Geezzzzz this is one of those downright confusing topics. I think It's because some of your statements don't match the information extracted from the PDF. These are your quotes compiled from multiple post.

How Can I increase the Voltage to the tl082 +vcc so the meter will read up to 16V?

The pdf specs the output as +- 15V Max. So what's the point? The DC input to the regulator is going to be ~17.8V. Milking 16V out of the regulators leaves only 1.8 volts of differential between the input and the output with best case conditions and minimal load. Regulation will be poor to none.

I have connected the op amp's +vcc to the +12.6v side of the rectified transformer output, and the -vee side to the -12.6 side of the rectified output. (Before they enter the regulators.

Yeah, got that but the voltage at these points should be about +- 17.8V with no load on the PS. They could be lower do to slight loading from the regulators but I wouldn't expects to see less that 17V. Have you actually measured the inputs to the regs?

I have no idea whatsoever as to how the regulated voltage is increased to 16 volts.

Like I said, the PS pdf doesn't spec to 16V. +- 15V is what's stated.

The regulators are the 317 and 337t voltage regs. with an output from 1.3 - 16 volts+-.

No it's not.

+vcc (pin 8) is connected to +12.6v, between the rectifying diode and the 317t's input.
-vee (pin 4) is connected to -12.6v, between the rectifying diode and the 337t's input.

And you measure what at these points? It should be close to or > 17V

The meters sensing wire (yellow) is connected to the tl082's output (pin 1).
I am using the tl082 op amp as a unity gain inverter.

I haven't checked the specs on the TL082. Is it a rail to rail OpAmp?

Btw, if your line voltage (mains) is low the voltages I stated could be lower. Let's face it, you don't have a lot of head room there!

A link to more head room but this will make your regulators work harder and dissipate more heat.......
http://www.mpja.com/18V-2A-Center-Tapped-9-0-9-Transformer/productinfo/7843+TR/