Design help / ideas current sensor (2023 Update)

You could sense the AC line current with a split-core CT.

Slit open the AC line cord outer jacket and slip the CT over the hot or neutral wire. No need to expose any conductors. Or hack an extension cord to do the same thing.

Scope the output tx/rx and decide what to do next. That one makes 0.1 volt per amp so there will be a pretty healthy signal to work with.

You could build a box with a plug and an outlet, in which case you could slip an AC wire through a small toroidal inductor of something. Similar idea. You could get a lot of signal voltage that way, enough to drive an SSR directly with no local power supply.

An RF detector shouldn't be hard either.

<snipped>

A reasonably sensitive reed switch inside a solenoid with a few tens of turns should do it. This sort of thing...

Couple things occur to me:

If the mode is constant envelope (like RTTY or FM) then the current draw or RF output will be kinda constant during the over, if SSB or CW then the detectable signal will fluctuate with modulation.

12V 100W is 8.3 amps, assuming 100% efficiency but I reckon most rigs are 50% or so which means the RX idle current of <2A should be very easy to distinguish from 15 amps TX. Does your power supply have an ammeter display, many do and it can be instructive?

Reed contacts can be used to sense cable current (and much smaller than what you are considering) but hysteresis effect is very large and having closed on TX current may not release if the RX current is not vastly lower.

Sensing voltage drop in the power cable is possible, sketch attached shows one way that has been used. (LM301 is great at sensing right up to positive rail, uA741 less so but usable. Or reconfigure to use later parts sensing in the negative return). Use ohms law and knowledge of the power cable wiring gauge to determine how far apart the sense pickoff taps should be, I reckon 5 milli-ohms resistance may be enough?

piglet

Great ! At least I know I'm on the right track. I like starting with the reed switch idea and if I have to work up from there. The switch would have dry contacts and be passive which is pretty cool. Inductance has never been one of my strong points so what is a good place to start for a coil. Thing is, I really need something that is completely passive and works on the AC field alone.

I don't want to do anything "in circuit".

No problem. Look for reed switches with a long body - so you can wind enough turns of wire onto it - and with lower Ampere-Turns values (fewer turns of wire will be needed to activate).

No real inductance involved here, just wind from 1 to 20 turns (wild guess range) of wire around the reed.

Some gottchas to beware of: the reeds could react fast enough to buzz at

100/120Hz, the power supply input current could be so spiky even at low load that getting reliable discrimination needs experimental fine tuning, reed contacts love to weld themselves so do not shunt with a capacitor (unless via a series resistor) and use well within ratings.

piglet

If you have a SWR bridge in line, you could tap off the forward power detector.

Cheers

Phil Hobbs

When using a reed relay to react on AC current, adding 2 shorts (a small length of copper tube over the reed relay) on both sides of the coil will generate a more stable field. The same mechanism can be seen in AC relays, where part of the core is surrounded by a copper short. BR, Wim

Ok. Will try the reed first but also will get the parts for the split coil sensor I've seen. I like the idea of a snap on module and any circuit could also be powered from +5v I would have available. Don't like the cost of those sensors though.

Let me ask this -- put the reed switch inside a smaller lengthwise piece of copper tubing, maybe a touch of silicone also. copper would help keep any chance of breaking the glass down also. and then wrap that assembly on the cord with maybe some 20 g solid hookup wire. 12 -15 turns or so?

and the reed switch AT rating ? the lowest I see is 6 to 10 on digikey -- is that sensitive enough?

Try using a Hall effect device. Maybe something with an Arduino like this:

mkr5000 wrote: ==============

** For what purpose ??

Any any practical design, the actual purpose is * everything *. Why omit it from your post?

..... Phil

Asking for a "relay contact" seems to imply OP wants a binary OK/over current detection, there's a simple over-current detect for a DC bus on Jim Thompson's legacy site:

It looks straightforward enough to adapt to higher voltages and currents and also optically isolate if that's what's desired.

well I guess I could have stated it this way -- output of an amateur radio xmtr to KNOW when it's on the air. how bout that as a purpose?

or is it? ....hmm

Well if it's drawing "next to nothing" power from the DC supply when not transmitting and then a quantifiable more than next to nothing whenever it's transmitting you could try setting the trip point somewhere between next to nothing and more than next to nothing and see if that does ya I think, some hysteresis would help and that topology has some.

Is it? Am I? Should I?

Mark Kholber = mkr5000 wrote: =============================

** For what purpose ??

Any any practical design, the actual purpose is * everything *. Why omit it from your post? Over and over.

Think you are lying about something. Time to own up with the full, unabridged story.

What model ham radio? Whose ham radio ? Yours or not. Why is there NO indication right now ?

..... Phil

Or, why ask for a solution for igniting an explosion device.............

Acme alarm clock mounted to a bundle of dynamite not good enough for people these days? It was good enough for Wile E. Coyote.

On a sunny day (Tue, 4 Jan 2022 14:43:59 -0800 (PST)) it happened Phil Allison snipped-for-privacy@gmail.com wrote in snipped-for-privacy@googlegroups.com:

Most ham transmitters have a mike with a push button Hang an optocoupler or relay on that contact and be done with it It is just babble talk he does?