Voltage doubler - Page 1
Forum section: electronics / beginners

author: Ammar
author_rank: Regular Contributor
timestamp: December 16, 2016, 10:09:04 am
content: Hi all,I know Dave has done a video on this topic, but I am looking for extra advice. I have implemented a voltage doubler to receiver an RF signal and light an LED. The signal is at around 250MHz and the signal is received capacitively through biological tissue. That is, each electrode in the picture has a plate near it and the signal passes through the muscle tissue to light the LED.It works and meets my design requirements, but I was wondering, what is the best way to choose capacitor values in this setup? I just played around with LT spice until I got something that worked. Can anyone recommend an application note or the like where the choice of capacitor value is explored? I could try to figure out myself from first principles but I figure if someone has already done it, it would save me some time and effort.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2016, 11:09:05 pm
content: No replies?You could try using the LED as one of the diodes in the voltage multiplier. It might not be fast enough for RF though.

author: Ammar
author_rank: Regular Contributor
timestamp: December 18, 2016, 01:16:56 am
content: Thanks for the reply Hero999. I am particularly interested in the choice of capacitor value in a voltage doubler. Do you know of any application notes that cover the theory for this?

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2016, 09:07:05 am
content: The first capacitor will be in series with the "biological material capacitor". If you estimate that capacitance you can calculate the extra effect of the physical capacitor.How brightly do you expect the LED to glow? What's the transmitted RF power? If you assume a perfect rectifier, how much could possibly be delivered to the LED?

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2016, 11:07:52 am
content: Why are you interested in the capacitor value?The capacitor just has to have negligible impedance, compared to the antenna. At 250MHz, ta 100pF capacitor will have an impedance of just under 6.4Ohms, which is much lower than that of what a typical antenna would be. You need to be careful. If the RF power is too high, then it will destroy the LED., in which case you may use a smaller capacitor or an electrically small antenna to limit the power delivered to the LED.I've built the circuit I posted previously. I used a 100nF capacitor and high efficiency green LED. When the 0V was connected to earth (a radiator) and I held the RF input, the LED would dimly glow, due to the mains hum.

author: Ammar
author_rank: Regular Contributor
timestamp: December 18, 2016, 12:48:32 pm
content: tggzzz: I am using 50mW of transmit power. The LED glows bright enough at a voltage of 1.6V and power of around 1mW. I do have an approximation of the capacitance of the biological material capacitor, it is typically in the single digit pF range.Hero999:Thank you for your detailed reply, I think that makes sense to me now. Let me know if I have this correct. I must have been confusing the step response of an RC circuit with the AC response. I think I imagined that the time it would take the first capacitor to reach peak voltage would change with its size. That is, the bigger the capacitor value, the longer it would take to charge. So I thought perhaps the capacitor value could be "tuned" to optimise the power transfer efficiency to a known load at a known frequency.However, in this case, the capacitor will hold the peak voltage, since it is a sine wave input, not a step response. So the ideal capacitor would have a large capacitance in this scenario (ignoring things like ESR) to maximise efficiency. Have I got that right or am I off the mark?

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2016, 03:12:20 pm
content: Quote from: Ammar on December 18, 2016, 12:48:32 pmThe voltage is an intrinsic property of the semiconductor bandgap, so that's irrelevant. The question is whether there is enough power in the RF signal to give a sufficient current in the LED; you say that it is; fine.QuoteSo you can now apply the usual "two capactors in series" formula to estimate the combined capacitance of 100pF and the biological sample.QuoteYou need to visualise the voltage and current waveforms.If there is insufficient RF power coupled into the circuit, the capacitance would be irrelevant. If there is sufficient, then the droop between peaks will be determined by the capacitance, the current in the LED and the interval before the next peak.

author: Ammar
author_rank: Regular Contributor
timestamp: December 18, 2016, 10:07:53 pm
content: tggzzz, I should really rephrase my question since i think you might know the answer. What effect do the capacitors in the voltage doubler have on the voltage doublers efficiency? Given a known load (assume the load is a resistor for the sake of simplification if necessary), known operating frequency, known diode characteristics, known input voltage and current do you know of an existing method or application note that shows you how to choose the capacitance values for maximum efficiency?

author: tggzzz
author_rank: Super Contributor
timestamp: December 19, 2016, 12:13:44 am
content: Question. What is the current through a capacitor C in series with a resistor R, when driven with voltage V T frequency f?Question. What is the equivalent capacitance of 1pF in series with 100pF ? And 1pF in series with 200pF?The answers to those basic questions will give you a feel for what is happening.A full answer would take too long and have too many assumptions.

author: Ammar
author_rank: Regular Contributor
timestamp: December 19, 2016, 02:15:52 am
content: tggzzz, thank you for the suggestions. From your answer I will assume that you are unaware of an application note that covers this topic. When I put one together, I will post a link here.Just a note on the forward voltage of the LED. Different LEDs have different forward voltages. Hence, it is absolutely relevant, it is the reason the voltage doubler is required, to condition the input to reach a high enough voltage.