Sorry for writing so much, I tried to explain my findings.

1. The output voltage of your bridge is not the same as the voltage on your smoothing capacitor.

True but but that's what it is, I didn't make it, I bought it ready. I'll replace it with a 24V capacitor if that will be better. I'll also remove the series resistor. Will that be ok?

Schematic attached. I'm connecting the supply with two short leads from the power supply to the breadboard. An in-line on/off switch is on the +ve lead.

But the capacitor is across the output of the bridge. How can the voltages be different? Don't change the capacitor voltage rating. You always want to use a cap whose voltage rating is higher than the operating voltage by at least 25%. Where are you connecting your multimeter probes? They need to go directly across the capacitor. And what kind of multimeter do you have?
BTW, post your schematics as .GIF , .PNG, or .JPG. They will be much smaller files.

 

Thanks for reply.

The DC voltages 23.4V and 29.5V are different because I opened the power supply disconnected the capacitor to check the output voltage from the bridge. That is I measured the voltage with the capacitor and without the capacitor thereby knowing what DC content is coming out of the bridge and the actual AC voltage going in the bridge.

Yes, I connect the multimeter probes across the output terminals of the power supply which is across the capacitor.

It's not a high precision multimeter but since I'm still more or less new to electronics it's not wise to buy the expensive type. It's a circuitmate DM10. I'll be looking for a better one in the future.

Thanks for advice, I tried but it's only yesterday that I downloaded circuitmaker v6.2c and could not find how to do it otherwise. Probably other schematics programs are better and easier to use.

So what do I do now? Do I check again with the 29.5V across V1?

 

PS sorry did you post an attachment with your last reply? If you did please post again because I don't see it.

 

Thanks for reply.

The DC voltages 23.4V and 29.5V are different because I opened the power supply disconnected the capacitor to check the output voltage from the bridge. That is I measured the voltage with the capacitor and without the capacitor thereby knowing what DC content is coming out of the bridge and the actual AC voltage going in the bridge.

Yes, I connect the multimeter probes across the output terminals of the power supply which is across the capacitor.

It's not a high precision multimeter but since I'm still more or less new to electronics it's not wise to buy the expensive type. It's a circuitmate DM10. I'll be looking for a better one in the future.

Thanks for advice, I tried but it's only yesterday that I downloaded circuitmaker v6.2c and could not find how to do it otherwise. Probably other schematics programs are better and easier to use.

So what do I do now? Do I check again with the 29.5V across V1?

OK, now I understand. There is usually no point (from a troubleshooting standpoint) of measuring the bridge with the cap disconnected. The bridge and the cap form a peak rectifier, so the DC output equals peak of the AC input voltage, minus a couple of diode drops (1.4V), unless the ripple is significant, which it should not be with the light loading of your "high impedance voltmeter" circuit.
The circuit should work with V1=29.5V. Using your multimeter, probe from circuit ground (the output of the second op amp section) to the positive terminal of the cap, then from ground to the negative terminal of the cap. You should get, respectively, about +24.6V and -4.9V.

 

OK thanks, will run some tests and inform you with the result.

 

OK all is fine now the readings are stable and accurate and the voltages including V1 are also stable. The voltages from the capacitor to ground are what you indicated. Sorry for mixing things up.

Referring to my post #13 and your post #14 and since this is solved would it be possible to measure higher voltages with this circuit? Can it be adapted to measure current?

Again many thanks for your help.

 

OK all is fine now the readings are stable and accurate and the voltages including V1 are also stable. The voltages from the capacitor to ground are what you indicated. Sorry for mixing things up.

Referring to my post #13 and your post #14 and since this is solved would it be possible to measure higher voltages with this circuit? Can it be adapted to measure current?

Again many thanks for your help.

You wrote:

3. Usually by changing the value of R1 one can change the scale of the meter. Can this also be achieved with this circuit? Or is it limited to 15V maybe because if the range is increased to say 30Vor more it would be too close to the +20V or exceed the +20V, thereby not allowing a correct reading?

If you are using a 1mA full scale movement, R1=V(fullscale)/1mA. You can only read up to about 20 volts without an input attenuator (high impedance voltage divider). If you add an attenuator, you should be using a FET input op amp.

 

Yes for the voltmeter I built I've used R1=V(fullscale)/1mA to change the fullscale range of the meter without the op amp circuit but it has the loading effect. I included a set of multipliers which are switched in by a rotary switch.

For the ammeter I used a set of shunts again being switched in by a rotary switch, but again having the loading effect.

I would like to remove this loading effect for another set of voltmeter and ammmeter. If I need an FET input op amp I think it requires another schematic. Can you please help with the schematics for both the voltmeter and ammeter?

 

This circuit doesn't lend itself to use as an ammeter, unless you just add a DPDT switch to remove the meter from the voltmeter circuit and connect it instead to a shunt and your ammeter probes. As a voltmeter, you don't need a FET input op amp unless you intend to add an input attenuator, and/or you are measuring very high resistance nodes.

 

Thanks for reply.

Sorry, but I thought you said that,

"You can only read up to about 20 volts without an input attenuator (high impedance voltage divider). If you add an attenuator, you should be using a FET input op amp."

I don't understand, if I change the value of R1 can I use this op amp schematic for voltages higher then 15V or as you said higher then 20V? What would be the highest voltage possible for this schematic?

Sorry but now I'm mixed up, do I or don't I need an input attenuator and an FET input op amp to be able to measure voltages higher then 15V?

I did include an DPDT switch for the voltmeter/ammeter I built with multipliers and shunts to switch from voltmeter to ammeter.

Since this circuit makes a moving coil meter high impedance in this case voltmeter, is it possible to make a high impedance ammeter?

 

Thanks for reply.

Sorry, but I thought you said that,

"You can only read up to about 20 volts without an input attenuator (high impedance voltage divider). If you add an attenuator, you should be using a FET input op amp."

I don't understand, if I change the value of R1 can I use this op amp schematic for voltages higher then 15V or as you said higher then 20V? What would be the highest voltage possible for this schematic?

Sorry but now I'm mixed up, do I or don't I need an input attenuator and an FET input op amp to be able to measure voltages higher then 15V?

I did include an DPDT switch for the voltmeter/ammeter I built with multipliers and shunts to switch from voltmeter to ammeter.

Since this circuit makes a moving coil meter high impedance in this case voltmeter, is it possible to make a high impedance ammeter?

You can measure up to 20V with this circuit, if you change R1 to 20k. If you want to measure higher than that, you will need an attenuator and a FET input op amp.
"High impedance ammeter" is basically an oxymoron. A current meter is inserted in series with an existing circuit, and ideally does not change the current that was flowing before it was inserted. It therefore ideally has zero impedance.

 

OK now I understand, so this circuit can measure upto 20V, for higher voltages an attenuator and an FET input op amp is required.

I would like to have the higher voltage option too, that is a selection of voltage ranges from low to higher voltages combined into one circuit, if this is possible. Would you be able to help with this circuit schematic consisting of the FET input op amp?

Sorry, I mixed things up. Yes an ammeter must have low impedance, but I’ve been thinking so much about high impedance that I made a mess of things.

Would it be possible to have a circuit which will lower the impedance of an ammeter?

 

OK now I understand, so this circuit can measure upto 20V, for higher voltages an attenuator and an FET input op amp is required.

I would like to have the higher voltage option too, that is a selection of voltage ranges from low to higher voltages combined into one circuit, if this is possible. Would you be able to help with this circuit schematic consisting of the FET input op amp?

I might be willing to tackle that. So what voltage ranges would you want?

Sorry, I mixed things up. Yes an ammeter must have low impedance, but I’ve been thinking so much about high impedance that I made a mess of things.

Would it be possible to have a circuit which will lower the impedance of an ammeter?

I'm assuming you want a lower impedance ammeter which would be made from your moving coil meter, with the addition of some active circuitry. It can be done, but I'm not willing to get into ammeter design, at least not now.