N
Norm Dresner
- Jan 1, 1970
- 0
Cleaning out my Hell Box, I came across two ancient boards which I'm fairly
sure were each dual SMPS's. Judging by the date codes on the IC's -- and
remembering when I probably bought them -- I'd guesstimate they were
manufactured around 1973. Each PS channel was implemented with an NE550 and
an NE555 as the controlling elements and there were 1 mH inductors too. But
there were a few "strange" [at least to me now in the 21st century]
features.
1. Each channel had a "Four Terminal Low Inductance" capacitor (2000
mfd/7.5v). I've heard of these things before but never actually held one.
Exactly how low was [is] low inductance? What's the likely need for such a
beast in a SMPS that includes a 1 mH inductor?
2. Each channel included what I believe was a Germanium Diode (fwd vdrop =
..430v) and each diode was mounted on the board with its leads vertical
passing through a ferrite bead. Why Ge? Why the ferrite in a circuit that
contains what appears to be ample RF bypass with mica capacitors?
3. Each channel had a 1N753A 18v Zener diode in it. Using my el cheapo DMM,
I saw a fwd vdrop of ~950 mv in each one. Is this typical? From the traces
on the PCB, it appears that Vcc for the ICs was created with this Zener, a
capacitor or two, and a 2N3904 so the NE555 was IIRC pretty much being used
at the limit of its range.
4. Each channel had a bunch of 2N3904 plastic transistors and two TO-225
power transistors, an MJE340 and a 2N4919. These are pretty high voltage
transistors and I'm wondering why they might have been selected for use in a
circuit which contains a 2200mfd 7.5v capacitor which probably served as a
smoothing element.
FWIW, the boards were manufactured by Singer Business Machines and marked
"Proto Only" which may explain some of the unusual compnent choices.
Thanks for satisfying my curiosity.
Norm
sure were each dual SMPS's. Judging by the date codes on the IC's -- and
remembering when I probably bought them -- I'd guesstimate they were
manufactured around 1973. Each PS channel was implemented with an NE550 and
an NE555 as the controlling elements and there were 1 mH inductors too. But
there were a few "strange" [at least to me now in the 21st century]
features.
1. Each channel had a "Four Terminal Low Inductance" capacitor (2000
mfd/7.5v). I've heard of these things before but never actually held one.
Exactly how low was [is] low inductance? What's the likely need for such a
beast in a SMPS that includes a 1 mH inductor?
2. Each channel included what I believe was a Germanium Diode (fwd vdrop =
..430v) and each diode was mounted on the board with its leads vertical
passing through a ferrite bead. Why Ge? Why the ferrite in a circuit that
contains what appears to be ample RF bypass with mica capacitors?
3. Each channel had a 1N753A 18v Zener diode in it. Using my el cheapo DMM,
I saw a fwd vdrop of ~950 mv in each one. Is this typical? From the traces
on the PCB, it appears that Vcc for the ICs was created with this Zener, a
capacitor or two, and a 2N3904 so the NE555 was IIRC pretty much being used
at the limit of its range.
4. Each channel had a bunch of 2N3904 plastic transistors and two TO-225
power transistors, an MJE340 and a 2N4919. These are pretty high voltage
transistors and I'm wondering why they might have been selected for use in a
circuit which contains a 2200mfd 7.5v capacitor which probably served as a
smoothing element.
FWIW, the boards were manufactured by Singer Business Machines and marked
"Proto Only" which may explain some of the unusual compnent choices.
Thanks for satisfying my curiosity.
Norm
