I've made a boost converter circuit, with a MOSFET as a switch, and a (7)555 timer operating as an astable oscillator, and it's output goes directly to the MOSFETs gate pin. The problem is that the input voltage for the boost converter is unstable (solar panels). When the input from the solar panels is high the output from the boost converter is also higher and when input's low, the output is also lower. That is because the 555 is operating at a constant frequency and duty cycle.

What can you suggest for controlling the 555, so its output would switch the MOSFET at such frequency/duty cycle, so that the output voltage of the boost converter would be stable? I need it to be around 14.7V, minimum 14.4V and maximum 15V for my CSB GP 12120 battery.

I've played around with 555's control voltage (5) pin, tried to use op amp to compare the batteries voltage to the solar panels voltage and apply the result to the 555 in a couple of ways (the fifth pin and the supply voltage). There were cases, where the output was more stable, but still, that was not even close to what I need. Maybe I just didn't do it right, or maybe 555 is just not what I need.

So in case you think I should use something else than a 555 timer, please suggest it. And please give me as many details as possible.

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    \$\begingroup\$ Is there a particular reason that led you to avoid one of the boost circuits available on the market? You add an inductor and a diode, and the output voltage is stable where you set it regardless of the input. \$\endgroup\$ Commented Jun 22, 2014 at 12:17
  • \$\begingroup\$ Now you know why most converters use a feedback path. Which is what you're asking for. \$\endgroup\$ Commented Jun 22, 2014 at 13:31
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    \$\begingroup\$ @VladimirCravero Yes, there are several reasons: availability, cost,desire to understand how devices work and being able to upgrade the device. \$\endgroup\$ Commented Jun 22, 2014 at 13:44
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    \$\begingroup\$ You can get the bare DC-DC chip for free as a sample, or in the same price range of the 555. You are using it for solar panels, you even use a low power version of the 555... But you will never achieve high reliability and efficiency with a 555, and that of course is not because you are stupid but because that's impossible. I understand your reasons though but keep in mind that the solution to your problem (that is not an answer to your question) is to use a dirty cheap readily available DC-DC integrated solution. \$\endgroup\$ Commented Jun 22, 2014 at 13:47
  • \$\begingroup\$ @VitaliusKuchalskis: If you're running the 555 as a monostable, where are the trigger pulses coming from? \$\endgroup\$
    – EM Fields
    Commented Jun 22, 2014 at 15:50

2 Answers 2


Use TL494 that has a reference and feedback loop to stabilize the voltage. http://www.ti.com/lit/an/slva001e/slva001e.pdf

If you can use microcontroller, use its PWM channel and ADC to achieve this. Any MCU can be used. For example, AtTiny series from Atmel. You need to program the MCU with what you want. MCU can also help you achieve temperature compensation of the charging voltage and many other things e.g. over-voltage and over-current protection.

Contrary to what has been said above, 555 is not unreliable and not inefficient. It can be used as a regulated booster than a simple booster (as it is in your case) but you need to modify your design. In your current design add a fixed reference voltage source and an op-amp. Let op-amp compare the voltage at the output to the reference voltage. If reference voltage is 1V, you need to divide the output voltage by 14.4 (using voltage divider network) and then let op-amp compare the voltages. Output of the op-amp should manipulate (increase or decrease) the duty factor so that the output voltage is fixed at 14.4V regardless of load or input voltage variations.

  • \$\begingroup\$ This is a good answer. I want to add, since it wasn't stated in the question, that you should add some hefty output capacitance. Preferably some .01uf/1uf/10uf ceramics for high-frequency noise as well as some larger bulk capacitors to handle larger transients, on the order of 330uf or higher. This will help "stabilize" your output. \$\endgroup\$
    – Funkyguy
    Commented Jul 8, 2014 at 15:47
  • \$\begingroup\$ I agree. Since it is PWM. there is a need for filtering. I would also add that in the most simple way, only a relay (or solid state relay; MOSFET) with proper current rating is needed that will stop the charging process as soon as the battery (or batteries) is fully charged. Knowing the charged state comprises of disconnecting the battery (or batteries) from the charging source, reading the terminal voltage at no load, loading with a resistance (with proper current rating) for a specific time and noting the drop in voltage at the end. More drop in the voltage, more empty is the battery. \$\endgroup\$
    – nvd
    Commented Jul 9, 2014 at 11:53
  • \$\begingroup\$ This is a non-constant current and non-constant voltage charging. Panel power will go unused at fully charged point. Actually same thing happens in PWM; low period of PWM pulse causes panel power to be not used. Generally, buck-boost converters are used to maximize the energy transfer from a panel to the battery by matching the impedances at input and output. They charge batteries at a faster rate due to optimum energy transfer and no "power off" state. PWM is used there as well but inductors and capacitors keep the energy flow continuous. Thus they load the panels all the time. \$\endgroup\$
    – nvd
    Commented Jul 9, 2014 at 11:54

Spehro or Andy pointed me to PWM chips awhile ago as alternatives to 555s, ring oscillators, or even triangle-wave comparator PWMs because everything gets packaged up into a small chip while you still have great control of the operation unlike a boost converter chip.

This or something like it would allow you obtain a stable voltage by adding an op-amp. The op-amp would compare a reference voltage to a divided down version of your output voltage. Then the op-amp output would be sent back into Anolog PWM Duty-Cycle Control input to this chip, and you should end up with a well controlled output voltage.

The reason a 555 timer may not be ideal for what you're after is because the equations for frequency and duty cycle off of a 555 timer are based on resistance and capacitance, see here. Resistance and capacitance are difficult to modify without physically affecting/adjusting the circuit. One hack to try and work-around this may be to replace R1 of a 555 circuit with a PNP or a Pmos transistor and sending your op-amp output to the base or gate of the transistor. That would give you 50%-100% duty cycle. If you want full 0-100% duty cycle (which you would), you could bypass R2 with a diode so that so that the "High" cycle is no longer dependent upon R2.

The beauty of PWM chips are that their input is voltage which is easily adjustable without any physical modification.


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