Redesign, was low voltage drop transistors for power supply

[Tam/WB2TT]

Or the separate lower-current diode/capacitor rectifier from the same
winding of the main transformer. Will give the same 17V but this is
loaded only be regulator and predrivers, so droop is avoided on the
regulator control and predriver supply, allowing the same low
saturation from pass elements without added magnetics.......

Is this what you mean: connect a cap from one side of existing transformer
to cathode of diode. Anode of diode to +17V. Second diode with its anode to
cathode of first diode and cap. Cathode to filter cap with ground end of
that cap to +17V? Gives ~34V.

Tam

 

[legg]

Is this what you mean: connect a cap from one side of existing transformer
to cathode of diode. Anode of diode to +17V. Second diode with its anode to
cathode of first diode and cap. Cathode to filter cap with ground end of
that cap to +17V? Gives ~34V.

Tam

Two new diodes positive rectify the transformer winding ends (one from
each end) to positive end of new cap. New cap neg is connected to same
general return rail of regulator ~ neg end of old main cap.

This is not a multiplier. It is simply a duplicate of the main
rectifier, in parallel, at a lower power level. The main high current
rectifiers in the original bridge serve both circuits, connected to
both capacitor's negative terminals on the general 0V return rail.

Voltage stress on the regulator and drivers is not increased - it is
simply allowed to use the original headroom without the detrimental
effect of main storage capacitor droop, under load.

RL

 

[legg]

Diagram is on ABSE.

RL

 

[Tam/WB2TT]

Two new diodes positive rectify the transformer winding ends (one from
each end) to positive end of new cap. New cap neg is connected to same
general return rail of regulator ~ neg end of old main cap.

This is not a multiplier. It is simply a duplicate of the main
rectifier, in parallel, at a lower power level. The main high current
rectifiers in the original bridge serve both circuits, connected to
both capacitor's negative terminals on the general 0V return rail.

Got it! I thought you were trying to produce a boosted B+. BTW, I ran SWCAD
on my circuit and it does work. Of course, there is a half amp of AC flowing
through the series capacitor. Also, we don't know what his transformer
voltage output is, but it looks like he needs a BIG filter cap in the main
rectifier.

I have done your thing also. Works great if there is a lot of ripple on the
main supply. Thing that was most straight forward was adding about 6 turns
to an open frame transformer. (#10 stranded Teflon).

Tam

 

[N. Thornton]

I'm unclear what the problem is at this point, but if its a slight
lack of voltage, some extra turns can sometimes be added on the
transformer - just be sure to put them over the sec and not the pri!
Also For us europeans we often used to use the old tweak of adusting
the transformer mains tap from 240v to 220. But it can caue problems,
even fires, so if you do run a transformer like that you need to at
least ensure youve got proper overheat safety protection on it.

If you use a separate small TF to add a volt or two, connect it direct
to the main one, dont waste your V with 2 sets of rectifiers.

Finally, there is one for-play-only scheme for minimising V drops, and
thats a buzzing rectifier in parallel with your diode bridge.
Basically it gives you a metal connection at the V peaks. Not for
commercial use.


Regards, NT

 

[Tim Wescott]

No, the original problem per the poster was a burned out
supply. The poster's description included the impossibility
of a PNP in parallel with an NPN as the pass transistors,
so someone was in there with a "golden screwdriver" before
he got it, or his post was incorrect.

Regarding headroom/low drop out:
The LM317 needs a bit less than 2 volts headroom at
1 amp, per the datasheet. With the "Watson boost circuit"
(if needed), he'll get about 1.9 volts extra or about
16.9 total, assuming his 15 volt measurement was accurate.
That's 3.1 volts headroom. The 7812 datasheet shows a
slightly lower headroom requirement.

Burned out supply and low output from the rectifiers with a suspicion
that the original supply was for 12V, which is probably why the original
post was "low voltage drop transistors...".

 

[El Meda]

Sounds like you don't have to change a thing- the 3055 is operated in
"inverted" mode and this will give you the lowest series voltage possible:
Please view in a fixed-width font such as Courier.

Inpn
----> Iload
3055 ----->
V+ --+------------ e c ------+-----+-> Vregulated
| \ / | |
| ---- Ipnp | |
| b ----> | |
| 2955 | Rbn | |
+----e c ----+----/\/\--+ |
| \ / |
/ ---- |
Rbp b error amp |
/ | /
\ | /| \
| | /+|------------> /
+------+-------< | \
\-|--Vref |
\| |
|
|

On the 2N3055, The "e" and "c" terminals should not be inverted?