[cctalk] Re: Rainbow H7842 PSU Fault
That’s a good idea, I will try that From: Mattis Lind Sent: Tuesday, May 2, 2023 7:55 AM To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts Cc: Rob Jarratt Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault Not quite sure what you mean here. I had advice from a friend to bench test the control module by providing 14V to the input of the 7812. On the good PSU I can see the PWM operate, on the bad one the PWM is shutdown. And what happens if you feed in exactly 12 V on the output of the 7812? Would the PWM work then? By using a lab supply you could check if the circuit is sensitive to variation in the 12V supply. /Mattis Regards Rob
[cctalk] Current Loop Schematics
Hi everyone! I'm working on a project where I need to build an RS-232 to Current Loop adapter. I've seen some schematics on-line and the parts list seems pretty small, but does anyone have a specific set of instructions and schematics that they trust? Or does anyone in the UK have a spare adapter they'd like to send me? :D Thanks! -- -Jon +44 7792 149029
[cctalk] Re: Current Loop Schematics
It seems surprisingly hard to find that, though this https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.47841/ is close. Conceptually it's really easy. The main tricky part, certainly if you're driving a mechanical terminal like a Teletype, is the current source. You need something that can drive 20 mA into an inductive load. The classic answer was to use a fairly high voltage with a big series resistor so the inductive impedance is much less than the resistance. A current regulator would be a modern replacement. Also, for the case of the inductive load, you'd need a shunt diode across the switch to absorb the voltage spike from the inductor when the current is switched off, otherwise the switching transistor will be quickly destroyed. paul > On May 2, 2023, at 7:42 AM, Jonathan Katz via cctalk > wrote: > > Hi everyone! > > I'm working on a project where I need to build an RS-232 to Current > Loop adapter. I've seen some schematics on-line and the parts list > seems pretty small, but does anyone have a specific set of > instructions and schematics that they trust? Or does anyone in the UK > have a spare adapter they'd like to send me? :D > > Thanks! > > -- > -Jon > +44 7792 149029
[cctalk] Re: Current Loop Schematics
On Tue, May 2, 2023 at 1:34 PM Paul Koning via cctalk wrote: > > It seems surprisingly hard to find that, though this > https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.47841/ > is close. > > Conceptually it's really easy. The main tricky part, certainly if you're > driving a mechanical terminal like a Teletype, is the current source. You > need something that can drive 20 mA into an inductive load. The classic > answer was to use a fairly high voltage with a big series resistor so the > inductive impedance is much less than the resistance. A current regulator > would be a modern replacement. Also, for the case of the inductive load, > you'd need a shunt diode across the switch to absorb the voltage spike from > the inductor when the current is switched off, otherwise the switching > transistor will be quickly destroyed. The common version of the Model 33 Teletype with a current loop interface [1] has a bit of electronics inside. A little driver PCB with a couple of transistors on it, a power transformer for it and so on. The result is that although the interface is 20mA or 60mA current loop you are not driving the receive magnet directly and there are no high voltage spikes at the interface terminals. [1] This is by far the most common version of the Model 33 in the UK. If you do have to drive the solenoid directly then (as you're in the UK) look out for the RSGB [2] Teleprinter Handbook. Although, not surprisingly, this is biased towards amateur radio RTTY operation, it has a lot of information on mechanical teleprinters, how to drive them (with circuit diagrams) and so on. [2] Radio Society of Great Britain. Basically our equivalent of the ARRL. -tony
[cctalk] Re: Current Loop Schematics
Led Driver ICs may merit consideration, e.g. https://uk.rs-online.com/web/p/led-driver-ics/7377975 for 20 mA. However, they usu have a not insignificant voltage drop (a few Volts). I have used lots of 10 mA versions for sensing switch / relay closures, typically with an ACPL-217 as sense element. And, these days, the catalogs are replete with LED drivers. The classic TLR solution, e.g. 1.25 V drop with an LM317, is written up by Horrowitz and Hill as Current Sources - Three terminal regulator as current source [3rd Edn $9.3.14 A, pp620] and many data sheets / ANs. The LED drivers seem to work just as well, 10+ years ago the LM317 was my standard design. Martin -Original Message- From: Tony Duell via cctalk [mailto:cctalk@classiccmp.org] Sent: 02 May 2023 14:06 To: General Discussion: On-Topic and Off-Topic Posts Cc: Tony Duell Subject: [cctalk] Re: Current Loop Schematics On Tue, May 2, 2023 at 1:34 PM Paul Koning via cctalk wrote: > > It seems surprisingly hard to find that, though this > https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.47841/ > is close. > > Conceptually it's really easy. The main tricky part, certainly if you're > driving a mechanical terminal like a Teletype, is the current source. You > need something that can drive 20 mA into an inductive load. The classic > answer was to use a fairly high voltage with a big series resistor so the > inductive impedance is much less than the resistance. A current regulator > would be a modern replacement. Also, for the case of the inductive load, > you'd need a shunt diode across the switch to absorb the voltage spike from > the inductor when the current is switched off, otherwise the switching > transistor will be quickly destroyed. The common version of the Model 33 Teletype with a current loop interface [1] has a bit of electronics inside. A little driver PCB with a couple of transistors on it, a power transformer for it and so on. The result is that although the interface is 20mA or 60mA current loop you are not driving the receive magnet directly and there are no high voltage spikes at the interface terminals. [1] This is by far the most common version of the Model 33 in the UK. If you do have to drive the solenoid directly then (as you're in the UK) look out for the RSGB [2] Teleprinter Handbook. Although, not surprisingly, this is biased towards amateur radio RTTY operation, it has a lot of information on mechanical teleprinters, how to drive them (with circuit diagrams) and so on. [2] Radio Society of Great Britain. Basically our equivalent of the ARRL. -tony
[cctalk] Re: Current Loop Schematics
If it’s specifically to interface to a model 33 Teletype, here’s a design that works well and is pretty simple. https://altairclone.com/teletype.html Mike
