Burning up flash units, Did a bit of investigation Options

[Jon5ja]

Hillhopper posted a few weeks ago about people burning up their flash units. I finally had a few minutes to make some measurements with one of the newer Vivitar 2000 flash units that use the DF500 board.

Skip to the bottom if you don't want to read the technical stuff

For starters, the transistor in the unit, a 2SC2500, is rated for 2 amps DC, or 5 amps "pulsed" at 30% maximum duty cycle. In normal operation with alkaline AA batteries, we're probably in pretty good shape because of the internal resistance and limited current capability of the AA alkalines. However...

With a fairly-freshly charged set of four AA NiMH cells (measuring just over 5.0 volts), I measured a peak startup current of over 6 amps, and with a duty cycle of 83%. This is also about what would be read when recharging after a full-power flash, and is significantly above the peak rating of the transistor as well as the allowable short-term average current. In fairness to the design, I fired it about 100 times in a row with 10 to 15 seconds between flashes, and although I made it smell funny, it didn't burn up. I was uncomfortable, though, with these current levels.

Adding a 0.47-ohm resistor in series with the battery, plus a 0.1-ohm shunt I was using to measure the current, the peak current dropped to a more reasonable 3 amps peak, and about 2.5 amps average. This means a series resistance is needed that will, at 3 amps, allow the voltage to the flash unit to drop to:
(5V - (3A * 0.57ohm)) = 3.29 volts

Based on that information, to limit the current to the same levels with a hot lead-acid battery at 6.5 volts, a series resistance is needed of:
(6.5V - 3.29V) / 3A = 1.07 ohms

----

So... if I were using one of these flash units (and I am :-) I would:
1.) add a 0.47 or 0.56 ohm, 3-watt MOF (metal oxide film) resistor in series with my four-cell AA NiMH battery packs
0.56 ohm, 3w MOF at Mouser
2.) add a 1.0 or 1.2 ohm, 5-watt MOF resistor in series with my 6V lead-acid battery.

These resistors will see about double their rated dissipation for a few seconds, but I've never had one fail under these circumstances given the short duration and how well the MOF resistors handle overload. Unfortunately, Mouser is in the process of replacing the old series of these resistors with "RoHS compliant" (containing no lead) so they don't have stock on the 1.2-ohm, 5W MOF. In the 6V lead-acid scenario, I'd probably just use two of the 0.56-ohm, 3-watt resistors in series as a substitute.

One other note -- charging the flash unit for excessive time from a 6V lead acid battery can overheat the flash unit also, regardless of the current-limiting resistor. When using a flash controller board that does the refresh thing, I don't think I'd let it run for more than about 10 seconds on the 6V battery, or maybe 15 seconds from a NiMH AA pack -- less if I could get away with it.

Jon

[headnurse69]

Thank you, Jon, for posting this. Very much needed info. Maybe you could share those brains and that knowledge with the rest of us by osmosis?

I just got brave enough to build my first Viv 2000 slave and Butch told me to add the resistor... I only had a smaller (1/2 watt) .47 ohm and I'm holding my breath till I can get up there with the 3 watt one that I ordered. At least it should only be the resistor that burns up and not the whole unit. I hope...

[treetop]

Jon5ja Once again you show us how much you are missed thanks for the research I have a couple of these ready to go and have been holding off so this is great news. Others had suggested not clipping the resistors so I tried that and it seemed to make the capacitor discharge very quickly. What do you recommend about the resistors?? Thanks again and hows the family should be a great Xmas for you enjoy it .

This post has been edited by treetop: Nov 29 2005, 12:57 AM

[Passthru]

To late for me though, Both units fried and pitched

[gypsy]

jon,
you used to send along a 0.47 ohm / 5 watt resistor with your flash boards way back when. So would 2 of these resistors in series with the red / pos battery pack lead work ? 0.94 ohm.

gyp

[Vic]

Great work Jon5ja :cool: :cool:

[Hill Hopper]

Jon, thanks for the investigation and sharing your results. It sure is great to see you on the board again.

[Jon5ja]

Headnurse, your hopes are confirmed :-) it should burn up the resistor before anything else.

Treetop, you really have to cut both the resistors to get any kind of battery life. That 1M resistance across, say, 300 volts is 0.09 watts... if the inverter was 100% efficient, that'd be like pulling 15 mA average off your battery to keep the flash unit ready to fire. It's really only maybe 50% efficient, so that's more like 30 mA average. That would run down a 4 AH battery in less than 6 days, maybe twice that if you turned it off in the daytime.

Gypsy, you will probably be OK with two of the 0.47-ohm resistors in series -- WAY better than without them, ha.

[Acrowe]

Jon you are the man I still use 4 D-cell batteries to power my flash will the same calculations work for these as for the 4 AA batteries?

Post some pics of the babies when you get a chance, I would love to see how they have grown.

[Jon5ja]

Anthony,

If you're using four NiMH D-cells, you can treat them the same as four NiMH AA-cells; however, if you're using alkaline D-cells, I'd be inclined to be more cautious and probably treat them the same as the 6V lead-acid.

Jon

[Coondog]

Man.. I miss having you around the board Jon... Thanks a million for the time and update.. I have been reluctant to try any more since they have been blowing up lately!!!

[Zig]

You guys say to put these resistors in series with the battery. Forgive my stupidity but is there a picture that someone can post to show this. I am not sure how the resistors should be installed. I have toasted a few boards myself and have been hit with voltage by toching the wrong place so I am a little hesitant on what to do. Thanks for the help.

[Acrowe]

Jon, mine are NiMH batteries. Even back in the day you built board for the viv 2000 I would install the resistor you included just for saftey sake.

Does anyone know what the radio shack part number is? Or if I go to a electronics supply parts house exactly what I ask for.

Zig I don't have a pic but all you do is where the positive wire leaves the battery soldeir this resistor in the middle of the wire before you tie it into the board [Battery +]-----[Resistor]----[Flash Board]

Anthony

[Zig]

Acrowe thanks... I thought that's how it was done but was not sure. So if I am using D-cell batteries I need two .56ohm, 3 watt resistors between the positive wire from the battery to the flash board and the resistor will go in the middle of the positive lead going to the flash board?

Battery(+) Battery (+)
---------------resistor-----resistor------------------flash board

[KZ3]

Thanks for the info Jon. I thought there had to be more to the story than just cutting the 10k resistor. I burned two D500s up, one was cut and the other was not cut.


:hags: