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I’m having a strange problem here:

When I use two AAA batteries to supply a small slave flash, it charges in 10 seconds, with 1.4A instant draw for the first second and smoothly reducing current draw until 10mA, when fully charged, at the tenth second.

I have these three small dc-dc buck voltage regulators, to lower from a bigger voltage Li battery at 8.4V to 3V, to supply my flash at 3V.

The weird thing is that, when I connect the output of any of these regulators to the flash supply contacts, it works, but it draws much less current, at 200-300mA, alhough these dc-dc regulators are rated at 3A max, 1.5A continuous.

So my flash is taking forever to charge, maybe 30 seconds.

For testing reasons, i’m using protoboard wires, don’t know if it makes any difference just to test it.

Is there any reason these different units would perform The same (be able to supply less current than they should)?

These are the ebay links of them: I don’t have a rank to post pictures yet...q

The first one is the smallest and it’s called Mini 360, and it’s based on the MP2307. Input 4.75-23V, 3A peak, 1.8A sustained

The second one is slightly bigger, nas an enable pin, and it’s based on the RT8727. Input 4.75-24V, 2A max

The third one is similar to the previous, with no enable pin, and it’s based on the MP1584EN. Input 4.5-28V 3A peak, 1.5A

ebay link for the first one

ebay link for the second one

ebay link for the third one

Edit: I may have had some bad connections. I rewired all of then I managed to measure the outputs voltages of all three regulators, when charging the flash unit.

All of them performed similar, charging to full in close to 15 seconds. The first and second regulators had some 0.1V, 0.2V voltage drop, from 2.98v to 2.85v more or less but in the end of the charging they got back to they’re normal voltages.

The third one as the charging begins it raises the voltage from 2.99V to 3.05, as to compensate for the load, and at the end of the charging it gets back to nominal voltage. It seems faster to charge, maybe a sec faster.

If I lower the output voltages of all three of them to 2.6V than it takes a much longer time to charge the flash to full..

But still, why when using two rechargeable NI-MH AAA batteries, at 2.6V (1.3V each) the flash still charges faster than all of these regulators at 2.99V?

Edit 2: I have another flash unit that uses 2 AA batteries. In this unit, when using any of the three voltage regulators I have, the voltage drops from 3V to 1V, leaving no time for the flash to charge and the voltage get back to normal. It turns itself off. If I increase the voltage to 3.2v it drops to 2V and the flash manages to keep on until it charges and the regulator gets back to it’s normal voltage. So, in this particular case, how could I prevent this voltage drop in these small units? Adding an electrolytic capacitor in parallel to the output?

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  • \$\begingroup\$ Did you measure the voltage output of each of these devices (including the batteries) during the charging process? I suspect your converters are not supplying the proper charging voltage despite their ratings. \$\endgroup\$
    – Barry
    Oct 14, 2018 at 20:57
  • \$\begingroup\$ I think you will need an oscilloscope to get to the bottom of this. I expect the flash draws power in fast pulses. The power supply can't deliver, and the voltage drops. It happens too fast for your meter to register. All you see is an averaged small drop in voltage. \$\endgroup\$
    – JRE
    Oct 14, 2018 at 22:38
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    \$\begingroup\$ If you don't have a scope, put a large electrolytic capacitor across the output of the powersupply. Mind the polarity and the voltage rating. Use something like 470uF or 1000uF. If that helps, then the cause is the high current charge pulses. \$\endgroup\$
    – JRE
    Oct 14, 2018 at 22:40
  • \$\begingroup\$ @JRE. I do have a small two channel portable scope, although I don’t know a lot on how to use it. But I could try. If this electrolytic capacitor test works, could I leave it soldered for good if I find space inside to put it? The voltage should be the same as needed for the original battery supply? 3V? \$\endgroup\$
    – Rodrigo
    Oct 14, 2018 at 23:15
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    \$\begingroup\$ Batteries can deliver bursts of high current. Regulators have a maximum current, and above that the voltage drops. Looks like you'll either have to find a regulator capable of more current, or continue using batteries. \$\endgroup\$
    – JRE
    Oct 15, 2018 at 14:28

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