I have an old Craftsman battery charger that the two Selenium Rectifiers
are shot. They are available from Craftsman but they are $50 each. Can
I just use a couple of big-assed diodes? If so, any recommendations?
You can use diodes, but you need to make sure that they are cooled
properly. Voltage drop on a diode bridge is about 1.4 volts, so, for
example, at 50 amps it will need to dissipate 70 watts. I hope that I
got my math right.
Also selenium rectifiers are not that expensive on ebay. I sold one a
while ago.
i
Ignoramus17926 fired this volley in
news:qK6dnRzW4fprqZbOnZ2dnUVZ snipped-for-privacy@giganews.com:
Ig, there's no good reason to re-install seleniums unless it's physically
impossible to arrange for proper heat-sinking of silicon diodes.
In addition to having a higher forward resistance than silicon diodes,
selenium rectifiers are somewhat 'leaky' in the reverse-biased direction,
yielding a much less efficient rectification. There are lots of stud-
mount silicons out there that usually suit the purpose perfectly.
However, on modern consumer battery chargers, selenium rectifiers usually
have their own finned sinks, and are 'air mounted'on flying leads, or
simply mechanically mounted on a cathode-common pallet (not a heat sink,
itself), while silicon diodes are often bare-chip affairs that have been
sweat-brazed directly to a cathode-side heat sink. So in either case,
some mechanical work will be necessary to replace them.
LLoyd
This does work, but you may need a power resistor in series, depending
on the overall design, which may have depended on the high resistance
seleniums to limit surge currents.
Joe Gwinn
Hmm I was going to say you could measure the voltage drop with and without current flowing and then figure out the resistance. (But that assumes you have working Selenium rectifiers.) What's the maximum current?
George H.
I didn't do any math :-) Just added a 220 VAC fan to blow on the
diodes... and O Yes, made a nice shiny aluminum plate to mount the
bridge on :-)
>Also selenium rectifiers are not that expensive on ebay. I sold one a >while ago.
>
>i
Big silicon rectifiers -- mounted to a heat sink about the area
of the original selenium plates. If you have single plate selenium
rectifiers then 20V PIV is quite adequate -- and I doubt that you will
find any smaller than 50 PIV so you should be fine there.
Current rating probably twice the maximum shown on the current
meter just to be safe.
Are the two rectifiers on a single stack, with just three
terminals? If so -- and depending on the polarity -- it might be easier
to select reverse polarity diodes so both can be mounted on the same
plate without insulation. Normal silicon rectifiers put the cathode on
the mounting stud, so with two of them mounted to a plate, the plate
will be the positive output. If the Selenium rectifier has the
negative in common instead (and the center tap of the transformer
connected to positive output -- perhaps through the meter), then you want
diodes with the anode to the stud instead. The proper selection here
allows you to mount them both on the same plate (heat sink) without
needing to get the insulating kit to keep them from shorting together.
Hmm ... going to the Digikey web page, there are rectifiers with
a maximum PIV as low as 5V, so let's select ... a range of voltages and
currents:
So -- from the search (among others) we have these two:
S25BRGN-ND $3.16350 -- 100V 25A reverse polarity
S25BGN-ND $3.60750 -- 100V 25A standard polarity
Either of these should do well for the charger (unless the
current meter goes higher than 25A).
No idea why the prices are to six figures after the decimal
point. :-)
Or -- there are modular half-bridges in various formats, and one
of these may well do for it all.
B483B-2-ND
B483B-2T-ND
Way more voltage than you need (400 V PIV) and plenty of current (35A).
Mounts to the panel with two screws, three spade terminals, common
cathode '+'. No common anode '-' version shown. Price not shown. (They
say "call".) So -- the individual diodes above are a safer bet.
Enjoy,
DoN.
I agree.
And -- when they die -- they *stink*. :-)
"selenium"? You mean "silicon", don't you? I can't imagine a
selenium rectifier being used, in a modern battery charger -- consumer
or industrial.
Really cheap construction.
Find or buy -- or *make*.
Enjoy,
DoN.
Do you have one half of the selenium rectifiers still alive?
Perhaps put the maximum current through the rectifier (based on the
meter on the charger) and measure the voltage across it (assuming that
you have a bench power supply capable of that).
Or -- take the open-circuit voltage from the charger (easy to
measure with the rectifiers replaced), and the maximum current (from the
meter on the charger) -- and divide the voltage by the current to get
the resistance. Then multiply the voltage by the current to get the
wattage needed in the resistor. (You can also measure the resistance of
the secondary windings in the transformer, and subtract that from the
total resistance calculated.) This resistor might keep from cooking the
transformer.
Good Luck,
DoN.
My first recommendation is to tell us more about the battery charger. Maybe the model number would let people look on the internet and see the specs. Or maybe just post the specs. Does the transformer have a center tapped secondary?
My second recommendation is to replace the selenium diodes with silicon diodes.
My third recommendation is to look on Ebay. Most all common diodes are made overseas, so it is just a question of who imports them to the states.
On Ebay a 50 amp bridge is $.99 with free shipping. And unless you are talking about a charger for a forklift, that is probably as big as you need.
Dan
It would be a pretty low value. A 0.25 ohm resistor would drop about 12
volts at 50 amps. For limiting surge currents you would not need more than
about 5 or 10 watt resistor. Steady state like shorting the leads would b
e 600 watts provided the transformer could supply 50 amps continuous.
Dan
[ ... ]
[ ... ]
Only one wire going to each Selenium rectifier? Then the other
side is bolted direct to the chassis, perhaps? Then determine which
direction the rectifiers were pointed (positive to chassis or negative
to chassis) and pick the proper silicon rectifier from those I found
(oops -- and already snipped from this article) -- or bigger ones
depending on what current you need to handle. I would not expect a
Craftsman one to be that high a current. :-)
O.K. Recovered the part numbers (from Digi-Key):
======================================================================
======================================================================
Good Luck,
DoN.
If it's old enough to use selenium rectifiers the Net may not help. Is
this a lead-acid charger or for NiCads which require close control of
charge termination?
If lead-acid, how does it reduce the current as the voltage rises?
This is what a change to Silicon diodes may affect.
I recently fixed some old golf cart chargers that had ferroresonant
regulation, a cheap and simple circuit that depends on clever design
and component choice to work properly. Fortunately I studied magnetic
amplifiers in the Army.
If it's ferroresonant there will be a large AC capacitor connected to
a separate winding on the power transformer. They regulate the AC
voltage from the transformer, not the DC from the rectifier.
jsw
Only if it passes through the diodes.
In my stash, acquired at a flea market, not for sale:
formatting link
I couldn't resist rectifiers with leads that look like winch cable.
A megger like this can check large surplus rectifiers for reverse
leakage and breakdown.
formatting link
I paid $39.13 for one last June. It's a ripoff of an old Biddle
megger.
jsw
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