Agreed. The question is, given that the capacitor feeds the coil, and the HV feeds the capacitor, how to prevent the HV feeding the coil? The resistor can't be any higher if the capacitor is to recharge fully.
You could switch the HV off with e.g. a FET during the on state, but that requires a transformer or a high-side driver. If you're going to that level, it's getting to the point where you may as well go the whole hog and use an H-bridge, which lets you recover the energy stored in the coil on switch-off to use for the next switch-on.
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.
If the 120V was only on a charged capacitor, how large would the capacitor have to be to accomplish a brisk switching?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
[ use two power supplies, and a class G type amplifier that takes current from both]
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.
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.
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
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.
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.
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).
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
+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.
For DC in chokes I prefer gapped cores... pot cores or E-I.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
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