Voltage doubler not working - Page 1
Forum section: electronics / beginners

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 10, 2013, 04:04:32 pm
content: After Dave's video today I decided to try to breadboard a voltage doubler.For some odd reason it is not working. I am feeding the voltage off of a digital pin off my AVR.This is going into a 0.1uF capacitor then I have a 1N404 diode going to ground in parallel and I am reading between the cap and the diode. Since the pin is running off a 10ms delay I am reading as AC voltage on my meter. What I am getting is 2.76 volts coming off the pin and 2.75 volts after the doubler.Any ideas what is going on. The diode should be in the right direction because if I flip it I am getting negative voltage. The cap is non polarized.Maybe some ascii would helpIN ------- ||-------|-------------Out| DiodeGND---------------|

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 10, 2013, 04:21:21 pm
content: Quote from: blewisjr on May 10, 2013, 04:04:32 pmDave was using 'true' AC (the voltage went negative), you are (I assume) using 'pulsed DC' to feed it.Also, using such small voltages, the diode drop might be a significant factor.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 10, 2013, 04:29:16 pm
content: Yes using pulsed DC to feed the cap. When pulsed the 5V output of the pin is actually 2.7 ish volts. Being pulsed at 10ms.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 10, 2013, 04:35:13 pm
content: Simulated it in LTSpice. With 0-5V pulsed DC, the output lies EXACTLY on top of the input. If I shift the input to -2.5 to 2.5, then the output is 0 to 5(ish)V.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 10, 2013, 04:37:40 pm
content: It will do. The first cap/diode stage will give you a square wave that oscillates between 0 and 5V if you drive the circuit from +/- 2.5V. However, if you already have a signal that's oscillating between 0V and 5V, you'll get exactly the same signal as output.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 10, 2013, 04:51:48 pm
content: Yes looking at my scope I am driving at +- 2.7 V really not sure why I am not getting 5V out of the doubler. Maybe I am confused and I am not suppose to be lol. Maybe a true doubler will only work off mains or battery and not the signal generated by a uC.

author: mikes
author_rank: Regular Contributor
timestamp: May 10, 2013, 05:26:42 pm
content: How are you driving it below ground with a digital pin? Are you sure your scope isn't set to AC coupling?

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 10, 2013, 05:32:14 pm
content: yes scope is set to AC coupling. So I guess the doubler won't work driven off a digital pin.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 10, 2013, 06:04:57 pm
content: It will work, but what you built now is just the first part where the AC is shifted to a pulsed DC. Since this is what you already have not much is happening. Go ahead and build another stage and you will see the magic happening.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 10, 2013, 06:05:54 pm
content: Ah ok so I need a two stage. Cool I will have to pick up some more diodes then.

author: ConKbot
author_rank: Super Contributor
timestamp: May 10, 2013, 08:19:41 pm
content: In general for CW multipliers, you *dont* need the output to be pulled below ground, but it does need to be pulled down to ground to discharge to first cap. I.e. a totem pole output will work, but an open drain/collector/emitter/source output wont. For example, you can use a 555 (which has a surprisingly beefy output) to drive a 3 stage multiplier to get a 10-15v output for diving mofet gates.But yes, the thread starter needs another diode and smoothing capacitor on the output for his circuit to work.

author: lewis
author_rank: Frequent Contributor
timestamp: May 12, 2013, 10:16:37 am
content: Try this. It works well from all totem pole outputs (such as a microcontroller port pin)edit - to get stable DC at reasonable current, you'll need 20-50kHz switching frequency with a 0.1uF series cap. Try increasing to 1uF or 10uF if you want to use a lower frequency, but do pay attention to capacitor polarity.edit2 - that drawing is even better than my last one.

author: hlavac
author_rank: Frequent Contributor
timestamp: May 12, 2013, 05:21:17 pm
content: You can use second pin that is switched to opposite of the first pin instead of your "gnd".

author: MasterOfNone
author_rank: Regular Contributor
timestamp: May 12, 2013, 08:07:04 pm
content: Quote from: hlavac on May 12, 2013, 05:21:17 pmGreat idea hlavac, but I thinking if he places the Scope ground lead on the inverted AVR pin (which is now the ground reference), then he could short-out the AVR. But this would only happen if 0V on the AVR Power Supply is connected to earth.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 13, 2013, 11:55:37 am
content: Power for the avr is coming from a lab supply so of the ground on the lab supply is connect to the mains ground it would probably short the chip if I used a second pin as gnd.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: May 13, 2013, 12:22:12 pm
content: To stop it shorting you could try sticking a small decoupling capacitor on that inverted pin and then using the point after the decoupling cap as ground. In theory you should get a 10V P+P AC signal instead of 5V DC plus.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 13, 2013, 12:34:19 pm
content: I will give it a shot when I get home from work. Worst case is I blow up a ATtiny2313A. I am sure there is a better way to do this but I am trying to up the signal voltage so I can swap a piezo buzzer to a larger piezo transducer.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: May 13, 2013, 12:46:47 pm
content: Quote from: blewisjr on May 13, 2013, 12:34:19 pmActually I don’t think my last idea will work.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 13, 2013, 12:54:31 pm
content: I wonder if running the signal through an op amp first would work....

author: MasterOfNone
author_rank: Regular Contributor
timestamp: May 13, 2013, 01:17:54 pm
content: If you have a negative rail on your Power supply to feed the op-amp then it might.

author: lewis
author_rank: Frequent Contributor
timestamp: May 13, 2013, 01:50:04 pm
content: You can't use a CW multiplier type circuit on pulsing DC, the output needs to go below ground to charge the first capacitor. You can do it with a full bridge configuration as others have suggested if you don't mind your ground reference moving about all over the place.The circuit I posted on the previous page works very well, I use it all the time. It works like this: When the MCU's output is at ground potential, the series capacitor is charged through the top diode. When the MCU output goes high, the capacitor is now put in series with the high output thereby reverse-biasing the top diode and producing 10V at the right-hand diode's anode. This voltage is dumped into the smoothing capacitor giving you stabilised 10V DC. The trick is to configure the circuit so the series capacitor gets charged, then gets put in series with the MCU's output. The CW multiplier cannot do this unless the MCU can pull its output below ground.

author: blewisjr
author_rank: Frequent Contributor
timestamp: May 13, 2013, 02:16:53 pm
content: Ok I see what you are doing there. Wish I could have thought of doing that. I just do not think I understand enough to come up with that on my own but it makes perfect sense looking at your diagram.

author: IanB
author_rank: Super Contributor
timestamp: May 13, 2013, 02:20:05 pm
content: Quote from: blewisjr on May 10, 2013, 04:04:32 pmYou should re-watch Dave's video. You have not built a doubler. A doubler has two capacitors and two diodes.

author: lewis
author_rank: Frequent Contributor
timestamp: May 13, 2013, 03:32:08 pm
content: ^^ that's the kiddie...