Subject: Re: [Shop-talk] Which diodes for a battery charger?

From: "Karl Vacek" <[email-masked]>
Date: Thu, 7 Mar 2013 10:12:46 -0600

I bought the stud-mount diodes Randall suggested from Surplus Electronics.
They arrived yesterday and I put them in.  But the outputs were pretty much
the same as before I changed the diodes.  Max DC out in "Charge" was only
around 11 volts, with 4 to 6 volts AC, which I took as indicating a bad
diode as before.

After stewing all night and checking this and that, I finally had a
fortuitous stroke of near-average intelligence, attached it to a battery,
and checked again.  D'oh - it's fine.

Are open-circuit voltages in battery chargers always misleading?  I didn't
expect the transformer output to go up with a load.  Doorbell and toy train
transformers don't seem to do that.

Anyway, it's working again - thanks to all!

Karl

-----Original Message-----
From: Randall
<snip>
here's some that should be good enough.  60 amps continuous, over 1000 amps
peak. $3 each.
<snip>
_______________________________________________

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Archive: http://www.team.net/archive

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From: "Karl Vacek" <[email-masked]>
Date: Thu, 7 Mar 2013 12:22:44 -0600

So if I measured min/max rather than just the default voltage (rms on my
Fluke), I should see something like the 15 volts it's currently showing,
while charging a very dead battery ?

Also - I'm still seeing a little under half a volt AC at the battery.
Normal?

Karl

-----Original Message-----
From: Randall
Subject: RE: [Shop-talk] Which diodes for a battery charger?

Think of the output of the transformer as a sine wave.  It goes smoothly
from peak to peak, but spends very little time at each peak.  Diodes only
conduct when the anode is more positive than the cathode.  So in a battery
charger, the diodes only conduct (to produce output) when the voltage is
near the peak, higher than the battery voltage.  The rest of the time, you
are reading battery voltage.  Without the battery, the average voltage is
much lower.

-- Randall 

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From: "Jack Brooks" <[email-masked]>
Date: Thu, 7 Mar 2013 15:59:57 -0800

>Are open-circuit voltages in battery chargers 

>always misleading?  I didn't expect the transformer 

>output to go up with a load.  Doorbell and toy train 

>transformers don't seem to do that.

Karl,

I did the same thing with a furnace motor I used to build a rotary tumbler.
It would only spin at 600 RPM without a load.  Once I put the fan back on,
it went right up to 1,170, the right speed for the winding I selected

Jack.

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