I need help with choosing DC/DC converter parts.

I want to build D-size battery powered lantern working with typical E14 or E27 compact fluorescent lamps.

Here is my awesome idea :)

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It will be supposed to work on D-size NiMh or Alkaline batteries.

Typical compact fluorescent lamps work normally with DC (there is rectifier bridge at the input anyway). I was experimenting with some old compact lamps and most of them start at 80V DC. All lamps I tried work normally at 100V DC.

I've been messing around with some low voltage easy to use converters (like step-down LM2574), I have oscilloscope, I'm able to tune-up simple converter efficiency for specyfic load needs. However I'm not inverter expert, I don't know integrated chips to build such thing.

Converter requirements:

  • output power at least 10W about 4W due to D-size alkaline capabilities

  • efficiency at least 80%

  • minimum input voltage 4V (3.2V would be awesome)

  • output voltage 110V (it may be 230V)

  • not complicated (i need to build this in a week - gift for sister)

The question is:

Are there any easy to use parts to build affordable DC/DC 3...7V to 110V converter?

It may be 4V, I wrote 3V because its possible to deep-discharge alkalines to 0.8V, thats 3.2V with 4 cells.

I know that LED lamps are more efficient, I'm considering using them too, but I want to stick to "typical home light sources".

I saw alkaline D discharge charts, it looks like single D size Duracell Procell can deliver 1W for 5 hours.

GP 700DH NiMh battery with 7Ah capacity can deliver much more power (14A discharge for 30 minutes or 7A for 1hour).

  • \$\begingroup\$ Not the best solution but look at this: homemadecircuitsandschematics.blogspot.ro/2012/02/… \$\endgroup\$
    – Cornelius
    Apr 23 '14 at 16:32
  • \$\begingroup\$ This has been done, without generating mains-like voltage, have a look on Hackaday. \$\endgroup\$
    – John U
    Apr 23 '14 at 16:34
  • \$\begingroup\$ A more practical power source may be lithium ion or lithium polymer batteries. D cells are not very good at providing high currents, and don't store a whole lot of energy for their size. \$\endgroup\$
    – Grant
    Apr 23 '14 at 16:53
  • \$\begingroup\$ @JohnU I want to use standard, not hacked lamp. \$\endgroup\$
    – Kamil
    Apr 23 '14 at 17:34
  • \$\begingroup\$ Lithium will be extremely not practical in this case. In this case I need batteries easy to replace. I don't want to use 2 sets of lithium batteries, LiIon charger etc. \$\endgroup\$
    – Kamil
    Apr 23 '14 at 17:37

Yes this is possible, but not practical. Since you don't need isolation (the whole thing, batteries and all is insulated), a basic boost converter could in theory do this. 100 V out will mean you have to select parts carefully.

At that voltage you're not going to find a integrated buck converter chip, but you should be able to find one that can drive a low side switch. The 100 V is then divided down and fed back into the chip, which should have a fixed internal reference voltage to compare this feedback input to. The chip therefore never sees the high voltage directly.

However, the real issue is the current this will draw from a few batteries. You want 10 W out. Let's say the boost converter will be 85% efficient, so that means it will require 10 W / 85% = 11.8 W in. At 6 V from the batteries, that will be 2 A. A few D cells aren't going to live long with that kind of drain.

  • \$\begingroup\$ Wikipedia says, that Alkaline D have 12-18Ah capacity. In worst case - I will drain no more than 0.2C current. It's this really bad? And I will probably put 7-8 Watt lamp in there. 10W was just initial idea. \$\endgroup\$
    – Kamil
    Apr 23 '14 at 17:44
  • \$\begingroup\$ I need transformer, right? \$\endgroup\$
    – Kamil
    Apr 23 '14 at 17:46
  • \$\begingroup\$ You are right. I just saw Duracell Procell D size battery datasheet. Looks like they have 3Ah at 1A discharge current. At 2A discharge their capacity drops terribly :( \$\endgroup\$
    – Kamil
    Apr 23 '14 at 17:51
  • 1
    \$\begingroup\$ In addition, at 2A draw their voltage drops to 1.2V each within a few minutes. Now you only have 4.8V from 4 cells, so you are drawing closer to 2.5A. They don't even test their batteries at 2.5A, and those are high quality batteries. Lower quality dollar store types ones will be much, much worse. \$\endgroup\$
    – Grant
    Apr 23 '14 at 19:36

Im answering my own question to say, that "my awesome idea" was bad.

  • I thought I can drain more energy from D-size alkaline (they are able to deliver just 1W for reasonable amount of time), here is Duracell Procell D-size datasheet
  • I realized converter will be more complicated as I thought (to get reasonable efficiency)
  • I realized that cost will be bigger, and for that cost I can make more effective LED lamp instead of CFL

I changed my mind and I will put some 1W leds into that lantern. I will be able to control current for specyfic types of batteries, dim light etc. and overall lantern cost will be similar to inverter with CFL.

  • \$\begingroup\$ you should make a constant current source for the LEDs. LEDs output light based on current, not voltage. The forward voltage changes based on current and temperature, and you need at least the minimum voltage to turn on, but voltage is NOT the controlling factor. \$\endgroup\$
    – KyranF
    Apr 23 '14 at 23:22

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