Need design for 120VDC 60mA constant current supply

current

big

I like the simplicity of the previous circuit charging the cap via R1.

FIG. 1 ====== R1 4.7K / 5W

This is a hysteretic buck. Q1 is the switch, current is sensed across R9, and Q3 is the comparator. R6-7 sets the current limit as a fraction of the DATA+ signal input, while R8 provides hysteresis.

When the current in R9 is below setpoint, Q3 is biased off by R6-8, both 'HC132 inputs are high, driving the output low, and switch Q1 is on. When i(L1) reaches the setpoint, Q3's base rises until it conducts, 'HC132 output goes high, Q1 cuts off, and L1 freewheels efficiently through D2 and Q4.

When DATA+ goes low, Q4 turns off and the inductor flies back, returning its energy to the +120v supply.

The currents are small, so D1-2 are signal diodes and snubbing is probably unnecessary.

The DATA+ signal could also serve as the +5V supply powering the logic gate.

The whole should draw roughly 3 or 4 mA from the +120v.

Cheers, James Arthur

If the 120V was only on a charged capacitor, how large would the capacitor have to be to accomplish a brisk switching?

...Jim Thompson

that's a place to start

-Lasse

The selector coil is floating, nicht wahr? A sense resistor on the (-) terminal to ground is all it really needs, your low voltage supply will power op amps and a grounded-base transistor (base to +12V in this case) is the level translation to the HV pass transistor.

Roll your own?

There are gate drive chips out there that are good up to 600V, there are FETs that are good up there, too.

You could either wimp out and build a flyback circuit that would go up from +12V (or whatever) to +120 (actually you could probably just go up to +60 if you could deliver an honest 60mA from there), or you could be manly and go off line with a circuit that bucks straight from the power line.

Running off of some low voltage will present you with lots fewer regulatory (and liability) hurdles to jump over if you want to make this a product (but don't take my word for it).

I think if I were doing what your web site shows I wouldn't try to generate 60mA and switch it; I'd build a 60mA source that I could enable & disable fast enough to run the printer's magnet.

Gee, no bites. Doesn't anyone like talking circuits?

Here's another hysteretic bucker, from scratch:

I can think of some far simpler configurations. I'm just waiting on an answer to this previously posted question...

"If the 120V was only on a charged capacitor, how large would the capacitor have to be to accomplish a brisk switching?"

Also, since I haven't seen a flexowriter since I was a kid, what's the duty cycle and rep-rate?

...Jim Thompson

Your question was quasi-answered up thread.

The energy needed to charge to inductor is 60mA into 4H. 1/2 * L * i^2 = 0.5*4H*(0.06)^2 = 7.2mJ

The problem is charging that cap efficiently. I figured to just skip that step and charge the inductor directly.

As to duty cycle and rep rate, IHNI--I was depending on the OP. It seems they run at about 50 baud:

Cheers, James Arthur

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

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

It would definitely be convenient to run the whole thing off

John Nagle

...

Tried that in SPICE. During ON periods, the current through the selector magnet is usually 60mA, but there are 1uS spikes to 200mA every

John Nagle

That's the 1nF parallel capacitance specified for the inductor.

The 120V voltage pulses have a rise time of 1uS, with a peak slew rate of ~140V/uS, giving 140mA through the parallel capacitance, on top of the

Yep, that's right. The real selector magnet will have considerable capacitance. I didn't know the value, so I took a wild guess.

As far as getting rid of the 120v supply entirely, boost topology off say +12v on the "hot" end of the coil, FET to GND on the cold end, open the FET to dump the coil quickly?

That is, you might initially load up the boost coil (L1) to the desired total energy, then let it fly back (single impulse) through a rectifier, charging an appropriate capacitor to a desired voltage, e.g. 1uF to 120v. The capacitor then decays into and charges the selector magnet to the desired current.

In steady-state, maintain desired current with a low voltage supply through a diode and resistor, as before, or switchmode, via SW1.

Open FET to dump the coil (L2).

Like this:

L1 .-.-.-. D1 +12v>----| | | |---o--->|-----o------. | | | SW1 \\ C1 --- _) L2 | --- _) selector === | _) magnet GND === | coil GND | ||--' ||

On Tue, 17 Mar 2009 05:08:16 -0700, dagmargoodboat wrote:

LTSpice version:

Version 4 SHEET 1 940 680 WIRE 16 -144 -96 -144 WIRE 192 -144 96 -144 WIRE 288 -144 192 -144 WIRE 432 -144 352 -144 WIRE 592 -144 432 -144 WIRE 752 -144 592 -144 WIRE 592 -80 592 -144 WIRE 592 -80 544 -80 WIRE 640 -80 592 -80 WIRE 752 -64 752 -144 WIRE 544 -48 544 -80 WIRE 640 -48 640 -80 WIRE -96 64 -96 -144 WIRE 192 64 192 -144 WIRE 544 64 544 32 WIRE 592 64 544 64 WIRE 640 64 640 16 WIRE 640 64 592 64 WIRE 320 80 240 80 WIRE 432 96 432 -144 WIRE 592 128 592 64 WIRE 640 128 592 128 WIRE 752 128 752 16 WIRE 752 128 704 128 WIRE 320 144 320 80 WIRE 192 208 192 144 WIRE 240 208 240 128 WIRE 240 208 192 208 WIRE 752 208 752 128 WIRE 880 224 800 224 WIRE 880 272 880 224 WIRE 752 352 752 288 WIRE 800 352 800 272 WIRE 800 352 752 352 WIRE -96 384 -96 144 WIRE 192 384 192 208 WIRE 192 384 -96 384 WIRE 320 384 320 224 WIRE 320 384 192 384 WIRE 432 384 432 160 WIRE 432 384 320 384 WIRE 752 384 752 352 WIRE 752 384 432 384 WIRE 880 384 880 352 WIRE 880 384 752 384 WIRE -96 400 -96 384 FLAG -96 400 0 SYMBOL voltage -96 48 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5V SYMBOL ind 0 -128 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 630mH SYMBOL sw 192 160 R180 SYMATTR InstName S1 SYMBOL voltage 320 128 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(-1V 1V 20ms 1us 1us 20ms 100ms 5) SYMBOL cap 416 96 R0 SYMATTR InstName C1 SYMATTR Value 0.47µF SYMBOL ind 736 -80 R0 SYMATTR InstName L2 SYMATTR Value 4H SYMATTR SpiceLine Rser=55R SYMBOL res 528 -64 R0 SYMATTR InstName R1 SYMATTR Value 10K SYMBOL cap 624 -48 R0 SYMATTR InstName C2 SYMATTR Value 1µF SYMBOL sw 752 304 R180 SYMATTR InstName S2 SYMBOL voltage 880 256 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value PULSE(-1V 1V 39ms 1us 1us 20ms 100ms 5) SYMBOL schottky 288 -128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMBOL schottky 704 112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 TEXT -40 432 Left 0 !.model SW SW() TEXT -130 506 Left 0 !.tran 500ms

Nice. I like your dump-snubber. Here's my reply (below). I envision S1 being triggered on the rising edge of data, with its ON-time controlled either by time (a 290uS one-shot) or by a current sense limiter.

S2 would be controlled directly by the data level (high = ON).

Cheers, James Arthur

Version 4 SHEET 1 940 680 WIRE 16 -144 -96 -144 WIRE 192 -144 96 -144 WIRE 288 -144 192 -144 WIRE 432 -144 352 -144 WIRE 464 -144 432 -144 WIRE 592 -144 544 -144 WIRE 752 -144 592 -144 WIRE 592 -80 592 -144 WIRE 592 -80 544 -80 WIRE 640 -80 592 -80 WIRE 752 -64 752 -144 WIRE 544 -48 544 -80 WIRE 640 -48 640 -80 WIRE -96 64 -96 -144 WIRE 192 64 192 -144 WIRE 544 64 544 32 WIRE 592 64 544 64 WIRE 640 64 640 16 WIRE 640 64 592 64 WIRE 320 80 240 80 WIRE 432 96 432 -144 WIRE 592 128 592 64 WIRE 640 128 592 128 WIRE 752 128 752 16 WIRE 752 128 704 128 WIRE 320 144 320 80 WIRE 192 208 192 144 WIRE 240 208 240 128 WIRE 240 208 192 208 WIRE 752 208 752 128 WIRE 880 224 800 224 WIRE 880 272 880 224 WIRE 752 352 752 288 WIRE 800 352 800 272 WIRE 800 352 752 352 WIRE -96 384 -96 144 WIRE 192 384 192 208 WIRE 192 384 -96 384 WIRE 320 384 320 224 WIRE 320 384 192 384 WIRE 432 384 432 160 WIRE 432 384 320 384 WIRE 752 384 752 352 WIRE 752 384 432 384 WIRE 880 384 880 352 WIRE 880 384 752 384 WIRE -96 400 -96 384 FLAG -96 400 0 SYMBOL voltage -96 48 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12v SYMBOL ind 0 -128 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 470µH SYMATTR SpiceLine Rpar=22k SYMBOL sw 192 160 R180 SYMATTR InstName S1 SYMBOL voltage 320 128 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-1V 1V 20ms 1us 1us 290uS 100ms 5) SYMATTR InstName V2 SYMBOL cap 416 96 R0 SYMATTR InstName C1 SYMATTR Value 1µF SYMBOL ind 736 -80 R0 SYMATTR InstName L2 SYMATTR Value 4H SYMATTR SpiceLine Rser=55R SYMBOL res 528 -64 R0 SYMATTR InstName R1 SYMATTR Value 10K SYMBOL cap 624 -48 R0 SYMATTR InstName C2 SYMATTR Value 1µF SYMBOL sw 752 304 R180 SYMATTR InstName S2 SYMBOL voltage 880 256 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-1V 1V 20ms 1us 1us 20ms 100ms 5) SYMATTR InstName V3 SYMBOL diode 288 -128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value MURS120 SYMATTR Description Schottky diode SYMATTR Type schottky SYMBOL schottky 704 112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 SYMBOL res 560 -160 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 130 TEXT -40 432 Left 0 !.model SW SW() TEXT -128 504 Left 0 !.tran 500ms

And 5.5A in the 470uH choke ?:-(

...Jim Thompson

Cheers, James Arthur

Sure--it needs to be meaty, but that's standard switchmode stuff these days.

It's low duty, low frequency. This toroid might be pushed to 5.7A (we'd have to review the datasheet sat curve, but

Cheers, James Arthur

For DC in chokes I prefer gapped cores... pot cores or E-I.

...Jim Thompson

Me too. Gapped is the way to go. Here I just searched for whatever Digikey had handy--saved me the calcs for a home-rolled.

Mouser's search is impossible for this -- slow, and won't permit even a small subset of the range of Idc to search 3-7A chokes ... grrr.

Cheers, James Arthur