Help needed Lowering DC Voltage - Page 1
Forum section: electronics / beginners

author: ropes712
author_rank: Newbie
timestamp: February 03, 2021, 07:55:53 pm
content: Hello, I recently finished making improvements to my trigger circuit that will be used to light a match with nichrome wire (for a uni project). The capacitor discharges into the nichrome wire causing it to heat up and ignite the match. Circuit attached below.I have an issue with the power source though. I have 3 'D' batteries in series (totalling 5.5v) which I need to lower to 2.7v and 4.8 A in order to charge the capacitor. This capacitor then discharges into the nichrome wire heating it up. I understand the danger of using capacitors and safety precautions will be taken. Is it possible to use a voltage divider to achieve this? I have created one on falstad but it does not produce the required amperage, and its just glitching out and causing a lot of pain.. is there a better configuration for this? Please feel free to edit the falstad model and improve it.Best regardsRopeshere is the linkhttps://www.falstad.com/circuit/circuitjs.html?ctz=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-L4ct5kHh9mQEEXsIWxOsyAATbwVx6+IpEXzeduJtiwMALPUarCNsjGoxu15F2ZqaeTmQWd6xbCWNQoBBuLduRKCABCjwAXmfvLJcB0mJZlCW98owLQ119lHwd9hnGAGu4kiAAAiqSAdkOTJIUpQVNsbrgIIG7FgSZzGs2ajRFgqHqOWlbVrqHqYf6mGgsaS5MJmEpanBqCwScTrcOqnzqicoJaAkm5vOQ4wyPIrHEJ8vGfEYUJ-O6U4lkQgmEvKTDifxHESZuICdt2vb9oOw5CKOap1B6DEEvIKGtCRgZMHAlRUBEVqWoK1T-kgADC-gAA7+CUxR9LIuTtgUOTlP4sgdFe2l2FgE66bmZG+suVoviCyAVlWNYYZU6AQNgYZWAB6X6sWIAAHIFOUsh9AAtt4uSFLILzrrMppWLB0mJGQnQ9EU-SDDkp5FEUAzmRQ8HLjsqCHrlADS3iBLkAAyfQlAA1rkEEjGUi3pBkrzfB6dqxSaCFbZtMwiZMMmxDtwjyHiuK7NIdBcv8zoqlOi45hdUoYtdNr8psKC4MhnxbSWHr0AluEGbRdRpjCX1vLYwXWko8MmtIuk6XQaoTB6O1AyIWNGCWBgmjtBP6FaLEcVu2qClxoLqhTlE3XD6pMzm3AUODRyYumIg0OQjA8wz3BEJzIqYpQaOC6LfO8GLMhAA

author: rks96
author_rank: Contributor
timestamp: February 03, 2021, 08:31:02 pm
content: Why do you need to charge capacitor with a such high current? You could use buck converter with current limit feature and charge capacitor with 1A limit. In this case you could even leave buck converter connected to capacitor when discharging it in nichrome wire.Also check online how much current D cell battery can provide! You might have to go with an even lower current limit on buck converter.PS: I am myself a noob so you probably should take advice from smarter people.

author: tunk
author_rank: Super Contributor
timestamp: February 03, 2021, 10:37:24 pm
content: Here's two tests of alkaline D cells:https://lygte-info.dk/review/batteries2012/Varta%20High%20Energy%20D%204920%20UK.htmlhttps://lygte-info.dk/review/batteries2012/Panasonic%20Pro%20Power%20D%20UK.htmlNotice that the voltage falls very rapidly when loaded with 3-5A,so it may not be possible to regulate to 2.7V. I also think a newalkaline cell is around 1.6V, not 5.5V/3. Or maybe you're not usingalkaline cells?

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 04, 2021, 01:35:39 am
content: No, a voltage divider is not a solution.If you have a fixed length of nichrome wire that needs voltage that is less than a multiple of battery cell voltages, a switching buck regulator is a proper solution. Anything less just shortens battery life. Significantly.A battery is an excellent energy storage device. A capacitor is an energy storage device with significantly less practical capacity than a battery. Using a capacitor to change the effective voltage of the battery is simply the wrong approach. Use a very high efficiency voltage regulator.Google “buck converter”.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 04, 2021, 04:24:26 pm
content: Unibomber- are you still out there??

author: mvs
author_rank: Frequent Contributor
timestamp: February 04, 2021, 05:18:09 pm
content: Quote from: WattsThat on February 04, 2021, 01:35:39 amI think the reason why ropes712 tries to use a capacitor in his circuit, is that he wants to limit amount of energy of one pulse.He would need something like monostable timer to use battery directly.

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2021, 05:36:38 pm
content: Quote from: mvs on February 04, 2021, 05:18:09 pmNo, he wants a pulse much, much greater than the batteries can deliver, so he his charging a supercap over a longer time, then releasing the energy as a pulse.

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2021, 05:44:01 pm
content: Quote from: ropes712 on February 03, 2021, 07:55:53 pmNo, a voltage divider is the worst possible way to do this. To be anything other than ludicrously inefficient, you need a voltage regulator or a buck converter, or else a circuit that allows the battery to charge the cap at directly and then disconnects when it reaches 2.7V. This last would be the most effective but most complex--you would need a reference, a comparator (op-amp) and a relay or MOSFET switch. The easiest would be an LDO adjustable linear regulator like the LT1086CT.

author: mvs
author_rank: Frequent Contributor
timestamp: February 04, 2021, 05:51:15 pm
content: Quote from: bdunham7 on February 04, 2021, 05:36:38 pmSupercaps have high ESR, so it is not very clever. They were made as coin cell replacement, so high ESR was not an issue.I have looked in datasheets for 2.7V 3.3F supercaps, and they have ESR of around 0.2-0.3 Ohm, while internal resistance AA NiMH rechargable battery is just 0.03 Ohm.

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2021, 06:18:19 pm
content: Quote from: mvs on February 04, 2021, 05:51:15 pmI misread his diagram for some reason and I thought he was using a bigger (3kF) supercap. You're right--a 3.2F supercap probably has a DC ESR about the same or even more than 3 new alkaline D cells.