Led voltage indicator

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
I would like to make a led voltage indicator that works on a 400v capacitor. currently I have to hook a multimeter up to it and watch it charge. I have seen a circuit that uses an IC and one that just uses resistors and leds. And I am not sure where to start.
 

mozikluv

Joined Jan 22, 2004
1,435
take your pick first, the ic way or the resistor way. then we can start from there. also pls provide where you saw the circuit.:)

moz
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
radioshack is the only store that I know of in my area and I prefer to buy things in a store rather than online.
The highest the cap will be charged is 400v but I would like to see it charge up with multiple leds
 

Ron H

Joined Apr 14, 2005
7,063
radioshack is the only store that I know of in my area and I prefer to buy things in a store rather than online.
The highest the cap will be charged is 400v but I would like to see it charge up with multiple leds
Does your store have LM339's? You can drive 4 LEDs with one chip, 8 with 2 chips, etc. We can show you how to do this.
What are you charging the cap with?
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
ya they have the LM339's
I am charging the cap from a transformer thats connected to the mains (in the US so 120v) I am using lightbulb as a current limiter and a full wave bridge rectifier to get DC
 

Ron H

Joined Apr 14, 2005
7,063
ya they have the LM339's
I am charging the cap from a transformer thats connected to the mains (in the US so 120v) I am using lightbulb as a current limiter and a full wave bridge rectifier to get DC
What is the wattage of your light bulb? How many LEDs do you want to use? You will need a DC power supply for the circuit. Do you have something in mind?
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
The bulb is 3 way and 50-200-250 I switch it to 250 because it gives me the best charging time...As for leds well do the voltage increments have to be the same? If so maybe 12.. If it can be made so that as the cap gets closer to 400v the increments get smaller 8 should be enough. Can it run off batteries, like a 9 volt.
Thank you for your help it is much appreciated
 

Ron H

Joined Apr 14, 2005
7,063
The bulb is 3 way and 50-200-250 I switch it to 250 because it gives me the best charging time...As for leds well do the voltage increments have to be the same? If so maybe 12.. If it can be made so that as the cap gets closer to 400v the increments get smaller 8 should be enough. Can it run off batteries, like a 9 volt.
Thank you for your help it is much appreciated
That must be one hog of a capacitor if the charge time is long enough to be seen on a multimeter. What is the capacitance? Is the peak voltage around 170 volts?
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
the cap is 3900uf at 400v I have 4 of them and the current bank has 3 so 11700uf and these are my smaller caps I had to store the bigger ones (around 12in tall 3in wide)
 

Ron H

Joined Apr 14, 2005
7,063
the cap is 3900uf at 400v I have 4 of them and the current bank has 3 so 11700uf and these are my smaller caps I had to store the bigger ones (around 12in tall 3in wide)
I would expect it to be fully charged in 2 or 3 seconds. Does that sound about right?
We could do a nonlinear scale, but I don't know what you are looking for. Seems like 12 LEDs in a linear scale would be easier, at least to start with.
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
to charge 1 it takes a just a few seconds but when charging a bank of three it takes a little longer... I mainly want this so I dont have to have my multimeter hooked up to the caps every time I charge them... also I may eventually charge the caps from a battery based circuit so I assume that will take longer... the 12 LEDs in a linear scale sounds great.
thanx again
 

Ron H

Joined Apr 14, 2005
7,063
to charge 1 it takes a just a few seconds but when charging a bank of three it takes a little longer... I mainly want this so I dont have to have my multimeter hooked up to the caps every time I charge them... also I may eventually charge the caps from a battery based circuit so I assume that will take longer... the 12 LEDs in a linear scale sounds great.
thanx again
I'm going to assume that the fully charged voltage is around 170 volts. If this is not correct, measure the voltage and post it here. I'll put in a pot for trimming full scale.
A nine volt battery won't last long. I would use 8 NiMH cells, or 6 alkaline cells.
 

Ron H

Joined Apr 14, 2005
7,063
I charge the cap up to 400V... and which is better 8NiMH or 6 alkaline
NiMH has a flatter discharge curve than alkaline, meaning the voltage will tend to be more nearly constant during usage. NiMH is rechargeable, meaning you will need a charger.
Regarding the circuit, I have started drawing it up, but I think you should be aware that it will have a LOT more parts than if you used an LM3914. With the LM3914, it will require one IC, a pot, 10 leds (or one 10-LED light bar) and about 4 resistors. You can use it in either bar or dot mode.
With the LM339s, it will take 3 ICs, the 10-LED light bar (or 12 discrete LEDs, if you want to use all 12 comparators), a pot, and more than 20 resistors. You will only have bar mode unless you add more ICs, which are probably not available at RS. Bar mode requires more current, because at full charge, all LEDs are on. This will reduce battery life, but this may not be an issue for you.

I think I have seen LM3914's at RS stores.
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
geez I didnt realize what a difference not using a 3914 would be I thought IC were more interchangeable I feel bad now that I took your time for something that could have been done easier... I guess I will go with the 3914. http://www.national.com/pf/LM/LM3914.html has a schem that looks pretty simple my only question is how to use it with something thats 400v.
thank you for all your help and patience's
 
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