On 02/16/11 04:56 pm, Dave Platt wrote:

N5BIA offers a kit, and I was wondering whether this could be beefed up
to handle 25A by adding 12ga wire to all the charging-current traces and
substituting higher-current pass transistor(s).

... and huge heatsinks.

Of course. His kit includes only the PC board and the PCB-mounted
components anyway, so I would have to provide my own enclosure and heat
sink (well, maybe it does include a clip-on heat sink; I don't recall).

I'd have to recalculate
the resistor values to suit a flooded battery, of course. And what about
using a P-channel MOSFET device, such as the STP80PF55 that the "Micro
M+" uses?

The thing about a low-Rds-on MOSFET, or a low-Vce-sat PNP, is that it
really only gains you a benefit under one circumstance: when it's
"hard on", acting as much as possible like a short-circuit. This will
happen only during the "bulk" fast-charge stage... and only if the
charge controller "sees" that the raw (unregulated) power supply
circuit isn't capable of pushing more amps into the battery than the
design allows.

If the charge control circuit finds it necessary to reduce _either_
the charge amperage, or the voltage being delivered to the battery, in
order to charge the battery safely, then the pass transistor will be
"partially off". There will be a significant voltage across it
(roughly speaking, Vsupply - Vbattery) and lots of amperage, and so it
will be dissipating a lot of energy as heat.

At that point, the actual Rds-on of a MOSFET, or the Vce-sat of a PNP,
will matter not at all. You'll have to dissipate (Vsupply-Vbat)*Icharge
watts of heat in the transistor.

My idea of using a MOSFET was to avoid the voltage drop of a junction
transistor so that it could be fed from a regular P/S that has been
cranked up only a little -- as with the "Super PwrGate," which has a
voltage drop of no more than 0.5V; any idea what West Mountain Radio
uses to accomplish that? So Vsupply - Vbattery would be low, and also
the power dissipation.

Now, if you happen to have been careful (or lucky) enough in the
design of your "raw" power supply, things will look good. By "careful
or lucky", I mean that you've put together a raw supply which just
happens to run out of "oomph" at exactly the right moment... the
effort of delivering 25A into the battery just happens to cause the
supply to sag down to the right voltage (equal to a voltage in the
range you want to be charging at). Under those conditions, the
whole system will be running "flat out", the pass transistor will be
turned on as hard as it can be, and heat dissipation in the transistor
will be minimized by using a low-voltage-drop transistor of some sort.

However, this approach has pitfalls... it will be finicky to get right
(component selection will be difficult) and it will probably be very
sensitive to variations in the AC power-line voltage. In real life,
you'd find that much of the time, either you aren't getting the full
25A of charge current (line voltage too low), or the voltage and/or
current are potentially too high and the charger is having to back off
turn down the pass transistor (at which point there's no longer an
advantage to a low-voltage-drop transistor).

If you want to deliver a high charging current, with good control and
low losses, under a fairly wide range of conditions, I think you'd
probably want to use a different approach... use a buck-mode switching
regulator rather than a linear pass-transistor system. With that
approach, the pass element would almost always be fully-on or
fully-off, and thus there'd be a real benefit to a low-resistance
MOSFET or a low-saturation power bipolar part.

I'm trying to avoid switching-type circuitry, since I already have
RFI-quiet regulated power supplies that are capable of supplying the
desired maximum voltage and current. It's just a matter of reducing the
voltage at the appropriate stages of the charging/maintenance process.

I was looking at the Super PwrGate until WMR's tech guy pointed out that
the absorption voltage is too low and the float voltage possibly too
high for flooded batteries. If I knew that it used a UC3906 and did not
use SMT components I might be willing to try modifying one, but I don't
have that information.

"Perce"

On Feb 16, 3:20*pm, "Percival P. Cassidy" wrote:
On 02/16/11 10:15 am, raypsi wrote:

snip

... I built a flooded lead acid charger. I only bought one part the
IC UC3906, I already had a solderless breadboard, the resistors and
capacitors in my junk drawer
The hardest part was figuring the resistor values and how to make
those values, and that was easy.
The heatsink fan came from an old CPU fan heatsink, and I mounted the
pass transistor to that heatsink fan.
I think that the UC3906 was the best think since sliced bread

I've seen schematics and board layouts for SLA chargers based on the
UC3906. VK3EM's uses SMT components, while N5BIA's uses discrete
components -- but both are only for charging currents of 2A or so.

For what current did you build yours? If high-current, what pass
transistor(s) did you use?

N5BIA offers a kit, and I was wondering whether this could be beefed up
to handle 25A by adding 12ga wire to all the charging-current traces and
substituting higher-current pass transistor(s). I'd have to recalculate
the resistor values to suit a flooded battery, of course. And what about
using a P-channel MOSFET device, such as the STP80PF55 that the "Micro
M+" uses?

"Perce"

There is no design on the internet for charging a flooded lead acid
battery at the levels I use. There is a reason for this it's called
lithium ion cells.



I used to make a living selling NTE parts. NTE always bought the cream
of the JAN type transistors, so they could sub tonnes of transistors
with one number. When the regulator went south on my car alternator I
used a NTE180 to drive the field winding with a zener regulator I
mounted the heat sink right to the positive side of the battery; when
that car died the day the big tsunami hit back on boxer day 2005 I
took the NTE180 out.

I use an NTE180 30 amp PNP transistor and 1 foot of 14 gauge wire to
set up the main charge current, I use a 23amp PS from the filtered
unregulated side to the input of the 14 gauge wire to collector of the
PNP. the UC3906 looks at the voltage across the 14ga. wire and
directly drives the PNP. I snake the wires for the emitter and base of
the PNP thru the heatsink fins. I used the TI data sheet for the
UC3906 to calculate the settings for the different charge cycles.
The 14 ga wire and the pass transistor are the only off the solderless
board parts
I wound the 14 ga wire in a pancake style coil so it fits right over
the heat sink

I had to use single turn trim pots and series resistors to set the
different charge cycles from the equations provided by the data sheet
on the UC3906

I did alot of research on my power supply, the others that used a
UC3906 on the internet could not be used to recalculate the resistor
values for the UC3906 for a large flooded lead acid battery. There
were to many mistakes as I found out.
Using the TI data sheet, is the best way to go, it looks like rocket
science but it's easy.\\


73 OM
de n8zu