Capacitor Voltage Divider

[fatman57]

I am trying to make a capacitor voltage divider. I used an online tool to check the values before purchasing any parts. Upon connection it does not behave as I expected. Across positive (+) and negative (-) of 22µF capacitor a very low voltage was measured and I was hoping for 8v.

My second attempt pictured below, I added a load, in this case a Peltier and still got the same results. I have never built one of these before, might anyone be able to advise as to where I am going wrong?

Thanking you in advance.

 

[(*steve*)]

Is this for DC? If so, capacitors won't work.

What are you trying to do?

 

[fatman57]

Is this for DC? If so, capacitors won't work.

What are you trying to do?

Oh, I have seen circuits for DC, like this:
http://www.learningaboutelectronics.com/Articles/Capacitive-voltage-divider.php

 

[Alec_t]

That circuit works only if you draw no current from it. Here's what happens as soon as a load draws current :-

 

[fatman57]

It would have been good if it was stated in the article! Thanks Alec_t - much appreciated.

So is the only 'simple' way with DC to use resistors, I didn't want to do that. Is there another?

 

[fatman57]

Is this for DC? If so, capacitors won't work.

What are you trying to do?

Just to confirm, I am trying to get a 8v output to power Peltier (a max draw of about 6 amps).

 

[Alec_t]

Resistors would waste 42W of power if you need to drop from 15V to 8V at 6A. What you need is a switched-mode power supply (buck converter). Cheaper to buy one than build one.

 

[Chemelec]

Or build a PWM Circuit to drive the Peltier.

 

[(*steve*)]

Or a switch mode regulator.

 

[Kabelsalat]

In this realm, we can't fly without complex machinery around us - nor can't capacitors be used for DC like transformers are used for AC. If it was possible however, any power supply would be ridicilous cheap to buy.

 

[fatman57]

Or build a PWM Circuit to drive the Peltier.

Thanks, in its most simple form wouldn't this be a transistor and capacitor - reading the voltage level in cap and keeping it up to required voltage by pulsing a higher voltage in?

Do you have any links to some basic circuits?

 

[fatman57]

Or a switch mode regulator.

Thanks, do you have any links to simple circuits?

Thing is in my current project I need to power 6 Peltiers, I was going to power each 3 on their own equipment. I fear this will get pricey and very time consuming if I have to make everything myself, but don't mind...

 

[Alec_t]

What is the maximum rated voltage of the Peltiers?

 

[Chemelec]

Here is some Info for you:
http://chemelec.com/Projects/PWM/PWM.htm

Have you ever used Peltier devices Before?
Where do you Live?

 

[fatman57]

What is the maximum rated voltage of the Peltiers?

About 18V I think...

 

[fatman57]

Here is some Info for you:
http://chemelec.com/Projects/PWM/PWM.htm

Have you ever used Peltier devices Before?
Where do you Live?

Hey, I live in London UK. I have used them before to play about with - my current project uses them and this will be my first proper application. I am an amateur in electronics but have a strong interest for many years, I am a programmer by trade.

Great link, thanks! Will have a look at that next week.

Got some step down buck converters today, it seems the current demanded by Peltiers is not directly proportional to the voltage, so even though 12V chomps through 5A, 5V takes much less amps then it should. I'm hoping 8V (in middle of 12v & 5V) will be lower than half, so a 3 amp buck converter can be easily purchased. Also looking into changing the pot to a digital one to be controlled by Arduino. Should be able to do some testing next week to see how the buck converter plays out for my requirements...

There is a pattern there if you hadn't seen it: 12v, 8v, 5v ...my three planned power settings. Got a computer PSU and that does 3v, 5v and 12v so something in the middle of the two largest values would be great.

 

[Alec_t]

A PWM circuit running from 15V with a 8/15 mark/space ratio (duty-cycle) would give you an average of 8V. The long thermal time constant of a Peltier does the averaging. It's a doddle to control the duty-cycle (and hence average voltage) with the Arduino.

 

[fatman57]

A PWM circuit running from 15V with a 8/15 mark/space ratio (duty-cycle) would give you an average of 8V. The long thermal time constant of a Peltier does the averaging. It's a doddle to control the duty-cycle (and hence average voltage) with the Arduino.

Thanks, 12v is such an easier thing to find these days so thats what I usually try to aim for. Would be great to build one some day (always wanted to, power control always a barrier I seem to encounter) but just to get this thing finished I will see what parts I can buy.

Thinking out loud (I know mains is not something to mess with) but a pair of capacitors and diode rectifier to AC source sounds a simple (and apparently efficient) way to get a fixed DC voltage. Maybe that could be an easy way to get 15V...? If it works of course!

 

[Chemelec]

QUOTE: Thinking out loud (I know mains is not something to mess with) but a pair of capacitors and diode rectifier to AC source sounds a simple (and apparently efficient) way to get a fixed DC voltage. Maybe that could be an easy way to get 15V...? If it works of course!

NOT VERY PRACTICAL at That Current.

Peltier devices work most efficiently when operated at rated current.
I would recommend running them at the 12 Volts from a Battery or a good 12 volt supply.
Ideally with a Control Circuit to turn it On and Off, based on the Cold or Hot side Temperatures.

 

[fatman57]

NOT VERY PRACTICAL at That Current.

I thought it could supply as much current as required, capacitors will be able to cope with these high currents in this application, no?

Peltier devices work most efficiently when operated at rated current.

Does this mean they are also best at their rated voltage? Or is 5amps @ 5V as efficient as 5amps @ 12V?