Charging a capacitor - Page 1
Forum section: electronics / beginners

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 06, 2024, 11:27:35 pm
content: I want to build a power supply to charge a capacitor in a flash unit. It needs to charge a 1500 microfarads capacitor to 330VDC. I can simply use a 120 to 240 step up transformer and a single diode. My question is would it draws too much current and cause damage because when the capacitor is empty the current would be very high.

author: TimFox
author_rank: Super Contributor
timestamp: October 06, 2024, 11:32:09 pm
content: Since there is no load on the capacitor while you are charging it, you could add a series resistor to limit the initial current surge.Asymptotically, the capacitor will charge to the same voltage as without the resistor (although it takes a while to get close).

author: Infraviolet
author_rank: Super Contributor
timestamp: October 06, 2024, 11:55:53 pm
content: The output of the step up transformer has a resistance, this, as well as the amount of power going in to the transfromer will limit your output current, however low the apparent "resistance" of the capacitor when initialy uncharged may be. The types of capacitor used in camera flash circuits, usually electrolytic, obviously rated for high voltages, are, I strongly suspect, rated to handle this inrushing current just fine*, I did some projects with camera flash circuit boards many years ago (before I knew very much in terms of electronics) and never noticed anything that looked like a deliberate resistance added between the transformer and the capacitor pins. These electrolytic caps will also have an equivalent series resistance of their own which will further limit how much current they'll draw when initially charging.*obviously the camera manufactuers assumed relativly infrequent flashing, if you're charging it up and dishacrging again once every few seconds you might have to start worrying about the heat generated in the cap by those regularly repeated periods of higher current during charging.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 06, 2024, 11:58:10 pm
content: So the question is what size of transformer would be good? How many VA?

author: Gregg
author_rank: Super Contributor
timestamp: October 07, 2024, 12:02:41 am
content: Three incandescent 2 to 3 watt indicator lamp bulbs in series should do the job of limiting the current cheaply and nicely

author: IanB
author_rank: Super Contributor
timestamp: October 07, 2024, 12:05:17 am
content: Quote from: BeBuLamar on October 06, 2024, 11:27:35 pmAny disposable film camera with a built-in flash contains a simple circuit that will charge the photoflash capacitor to 300 V using a 1.5 V battery. You don't need to build a supply when such modules are so readily available.Be aware that the capacitor in such a camera is only about 200 µF, and when one shorted out on my test bench due to a fault, the bang was loud enough to make me jump out of my seat. Charging a 1500 µF capacitor to 300 V is like creating an instant death device. Please be aware that if you kill yourself, I will not be coming to your funeral.

author: themadhippy
author_rank: Super Contributor
timestamp: October 07, 2024, 12:13:54 am
content: Quotehot caps aint normally a problem,its the flash tube getting to hot ya gota worry about

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 07, 2024, 12:57:09 am
content: Quote from: IanB on October 07, 2024, 12:05:17 amThank you,Before I retired I worked everyday with drive and inverter that charge 10,000 microfarad banks of capacitor to 600V and then feeding it to an inverter and transformer to boost it up to 20,000V. So I know the danger. I do have a small circuit board with 1 transistor, few caps, a few resistor and a diode that can do the job with 6VDC input. I want a circuit that can charge the cap faster and runing directly from AC rather going thru an inverter. On all the equipment I worked on they do have current limiting resistors that in the circuit for a second and then a contactor would by pass them. However the thing I have for the flash doesn't have suck a resistor. My question if I have a small enough transformer would that act as a current limiting device?

author: IanB
author_rank: Super Contributor
timestamp: October 07, 2024, 01:16:26 am
content: If you want to limit the initial current when charging a capacitor, you can just place a current limiting resistor in series with the capacitor. Assume the capacitor is a short circuit, decide on the maximum current permitted, and choose the resistor value accordingly.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 07, 2024, 12:47:46 pm
content: Quote from: BeBuLamar on October 07, 2024, 12:57:09 amThe smaller the transformer core the more coils/volt it needs, and also there is less room for wire so you end with very thin wire. And copper is not a superconductor so the current is certainly limited but if to acceptable value (for capacitor and transformer) I don't know. Just take small transformer and measure its winding resistance. I have (but not here) 4W 220V->11V transformer. I will measure its resistances ad write here later.

author: CaptDon
author_rank: Super Contributor
timestamp: October 07, 2024, 12:59:55 pm
content: Something on the order of 10 ohms total would be good. If you have 3 ohms of winding resistance (small 25va transformer) on the secondary then just add a 6.8 or 7.5 ohm resistor of perhaps one or two watt size in series. Use a diode that can handle the predicted surge current. A 1N4007 or better. Remember the P.I.V. will be double the D.C. capacitor voltage for a single diode half wave rectifier PLUS any turn-on or turn-off transient line spikes. 1500uf at 300vdc is a powerful flash.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 07, 2024, 02:02:52 pm
content: OK. I have measured my 4W trafo. 220V winding has 1200Ω and 11V winding has 3.8Ω.For made at this core trafo 120V ->240V I would expect (R proportional to voltage square): 120V winding having 350Ω and 240V winding having 1400Ω.So we have circuit AC 120V at input. Than through 350Ω we go on ideal 1:2 trafo and its output loaded with 1400Ω.So at ideal trafo primary we have 60V and at secondary 120V.Input current is (120-60)/350 = 172mA. Output current is 120/1400 = 86mA (current is transformed 2:1).I don't expect 1500uF capacitor will have anything against 86mA loading current.

author: Seekonk
author_rank: Super Contributor
timestamp: October 07, 2024, 02:08:39 pm
content: Instead of resistor, I would use a capacitor to limit current as in a voltage doubler or tripler. I built a MOV tester this way to safely limit current. Production would sometimes pot stuff and then assemblies would get mixed up before labeled.