Re: Need design for 120VDC 60mA constant current supply

John Nagle wrote:
langwadt@xxxxxxx wrote:
On 11 Mar., 15:47, Jim Thompson <To-Email-Use-The-Envelope-I...@My-Web-
Site.com> wrote:
On Sun, 08 Mar 2009 10:25:51 -0700, John Nagle <na...@xxxxxxxxxxx>
wrote:



I'm looking for a design for a 120VDC 60mA constant current
supply. This is what's needed to drive old 60mA Teletype machines.
The classic solution is to run a constant voltage supply through a
2K 10W resistor, but that approach dissipates 8W when idle, which
requires a big resistor for a board-mount design. I'm currently
using a big 10W thick-film resistor, and it works fine, but adds
1.5" to the board height and needs ventilation.
So I'm looking for a switch-mode constant current supply.
This design from EDN looks promising:
http://www.edn.com/article/CA217668.html
But the components are only rated to 40VDC, and even the "HV" version
only goes to 60V. Also, it's a design from 2002; there are probably
better switching regulator components available now.
The On Technologies NCP3065 part (a constant current driver
for LED strings) looks promising, but won't go to a high enough voltage.
Any suggestions?
John Nagle
If the 120V was only on a charged capacitor, how large would the
capacitor have to be to accomplish a brisk switching?


1uF @ 120V and 4H @ 60mA store the same energy, 7.2mJ

that's a place to start

Right.

It looks like we have some good designs for an efficient driver
which requires an external 120VDC supply. The next step is to get
rid of the high voltage supply, and use some kind of up-converter
to charge up a capacitor with the 7.2mJ needed to pull in the
selector magnet. At 45.45 baud, one bit time is 22ms. Even
better, there can only be one 0 to 1 (SPACE to MARK) transition
every two bit times, so the charging system has maybe 1.8 bit times
or so to charge the capacitor. 75 baud is as fast as the mechanical
Teletypes ever got, so a 13ms bit time is the worst case, giving a
charging time of 23ms. So design for a 20ms charging time and a
1-2ms discharge time.

This is something like a "hammer driver" for a daisy wheel printer.

It would definitely be convenient to run the whole thing off
+5VDC. Right now, 90% of the hardware volume is the open
frame 120VDC 200mA supply.

John Nagle

+5v is a pain--you can't charge the boost inductor quickly, and then
you need to stagger the switch timing. +12v wastes a little power,
but it makes the control circuitry simpler and faster.

Cheers,
James Arthur
.

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