Simple 0-120VAC bar voltmeter help

[qsiguy]

I'm attempting to make a simple AC bar graph voltmeter using the LM3914. I know how to make a DC voltmeter that will cover various voltage inputs. Can someone tell me what would need to be added to read an AC input from 0-120VAC? I believe I just need to convert the ac to dc and get it in the proper range, just not exacly sure what parts would be required. I assume a rectifier and maybe a step down transformer? The input would be tapped off from a variac that drives a high voltage transformer at varying input voltages. Just need a level indicator for the input of the transformer.

Thanks for any help. Will probably just use on of the simple sample circuits on the data sheets for the 3914 for the voltmeter section.

 

[Nigel Goodwin]

You just need a rectifier and an attenuator (two resistors), you can attenuate the AC, then rectify it, or rectify it, then attenuate the DC.

 

[qsiguy]

Can you elaborate a little more? About what value resistors would I need to get proper reading on a dc voltmeter designed to read up to about 15 VDC max? This value I know how to change, used 15VDC as an example. I assume one resister on each AC line, is this correct? Also, any particular value for the rectifier I should look for?

Thank you.

 

[Nigel Goodwin]

Here's a circuit which might help you?, R1 and R2 form an attenuator, also known as a potential divider. You can work out the values from simple ohms law!.

But essentially, if R1 is 9 times as large as R2, then 90% of the voltage will be dropped across R1, leaving just 10% across R2 - so 120V in would give 12V out (divide by ten). However, you need to consider how much current your display requires?, and select the values accordingly. You will also need to make it adjustable somewhere, in order to calibrate the meter.

 

[qsiguy]

Thanks that's basically what I thought but wanted confirmation.

 

[Hero999]

It's much better to put the divider after the bridge rectifier as it's associated volt-drop will make less difference. I would also add a capacitor to smooth out the pulses and give a true peak reading.

 

[qsiguy]

So to make it adjustable would I just make one of the resisters a pot? If so, which one would be best? Also, can one of you give me a approximate resistor value to start with or does it matter as long as they are 90% / 10% to get voltage down to 12 volts?

Hero999, about what value of cap would be used for your diagram? Obviously at least 15-20 volt but what uF value?

Thanks again guys. I know this is a simple one but I don't want to blow something up, haven't experimented much with ac-dc circuits mostly all just DC stuff.

 

[audioguru]

If the DC input voltage range of the LM3914 is 12V then it needs a supply voltage of about 14V which will cause it to get too hot. Use a 6V supply and set its max DC measurement to be about 3V or 4V.

 

[Hero999]

So to make it adjustable would I just make one of the resisters a pot? If so, which one would be best? Also, can one of you give me a approximate resistor value to start with or does it matter as long as they are 90% / 10% to get voltage down to 12 volts?

It depends on the maximum required input and output voltage and the input impedance you want to drive.

Google will help:
https://www.google.com/search?clien...tial+divider&sourceid=opera&ie=utf-8&oe=utf-8

Hero999, about what value of cap would be used for your diagram? Obviously at least 15-20 volt but what uF value?

That again depends on the resistor values but I would start with 100uF.

Thanks again guys. I know this is a simple one but I don't want to blow something up, haven't experimented much with ac-dc circuits mostly all just DC stuff.

Be warned, this circuit isn't isolated from the mains, it might be a much safer better idea to use a small transformer to provide isolatio as well as lower the voltage.