On Friday, October 9, 2020 at 4:51:25 PM UTC-4, T i m wrote:
Hi all,

I was given a 'dead' cheapo 20V Lithium drill battery, charger and
wall wart to look at by a mate tonight.

Plug PSU (wart) into wall socket, clip 'remote' charger interface onto
battery, switch socket on ... nothing.

I took the covers of the charger clip module and connected it onto the
battery, measured 19 or so volts on the battery pins but nothing
coming from the PSU.

However, after a few minutes I felt the PSU was slightly warm so
assumed it was doing something?

I disconnected the PSU output leads from the PSU at the charger module
and connected my bench PSU at 20 and with the current limited to the
same 400mA as the PSU and the green 'Charged' LED came on constantly.

I rigged up a load (a couple of 12V 60W headlights in series) onto the
battery and let it run for a while and then hooked it back up to the
charger, charging LED now flashing, indicating it was charging.

DMM on the unloaded o/p of the PSU, nothing.

After leaving it disconnected for a while ... I cracked the PSU open
with my toffee hammer and gave the board a look over, nothing
obviously burned and no dry joints etc. I checked the bridge rec and a
couple diode over and they seemed ok, but an axial series diode (SJ220
from memory) on the output that was driving a cap that went to the
remote charger module seemed to be short?

I found the spec somewhere that suggested it was a 2A 1000V (?) device
and I have a couple of questions please?

I know Schottky diodes have fast switching times (and a low forward
voltage drop but not sure if that would be relevant here?) so is a
Schottky required in what I think might be part of a 'charge pump'
circuit because of the frequency of the output through the SMPSU and
assuming I decoded the markings correctly, why would it need a 1000V
device there?

Cheers, T i m


Use the Schottky diode. Sometimes a fast/soft recovery diode will work, but I've run into problems where the drive IC or transistor will run hot or fail or the secondary voltage will be lower if the Schottky diode is subbed from original spec. Also, sometimes those diodes check very leaky even when they're not - depending on your meter and the polarity of the leads, even when not using the diode scale. You can put in a fast recovery to see if the circuit works, but I wouldn't run it more than a few seconds.