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Old 16th Dec 2018, 3:04 am   #1
Bazz4CQJ
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Default Switch-on Transients; Linear 12V PSU

I have a 30A linear psu which I use to run my IC-718 transceiver. The latter has an 'electronic' on-off switch, so that should need to be depressed after I've switch on the psu. Very occasionally, switching on the psu has tripped the switch on the 718 and that has come on immediately. I've assumed that this is due to some transient voltage coming from the psu and I've been trying to investigate this.

I've been setting my scope to trigger on a leading edge, with a trigger level of just 5mV and a timebase of 200uS per division, with the PSU set at the switch-on voltage of 2V. I've connected the output of the PSU (with no load) directly to the scope. However, I just don't get any sort of consistent results. Sometimes it does not trigger, sometimes it triggers showing a few very narrow spikes, sometimes it triggers with lots of spikes. There's no noise on the DC after switch-on. I'm suspicious that I have not set this up adequately for the task.

Searching Google, I seem to find most hits dealing with looking at switched-mode psu's under quickly changing load conditions.

Can anyone offer me some advice on how best to use the scope to look at my linear switch-on issue?

Thanks

B
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Old 16th Dec 2018, 7:05 am   #2
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Default Re: Switch-on Transients; Linear 12V PSU

Should the radio turn on on transients?

You'd think it ought to ignore them, but there is a possibility that Icom designed it to trigger turn on by dV/dt (rate of change) of input voltage. It might be a trick so that if you have a matching Icom psu that turning it on triggers the radio to come on, saving you the vast labour of having to press two buttons. Might be a design oversight, might be a fault.

To look for transients from the PSU, set the scope timebase to "normal" not "auto" trigger and set the trigger level to about half the psu's normal voltage, rising. You really need a storage scope. A digital storage scope should let you play with time/div and time offset controls to get a zoomed-in picture. Analogue scopes are best for some things, but this is where digital ones shine.

Trying to trigger on a transient isn't the best place to start. There may not be one. Trigger on the rising PSU voltage, see what there is, if anything, and take it from there.

David
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Old 16th Dec 2018, 11:07 am   #3
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Default Re: Switch-on Transients; Linear 12V PSU

Does the Icom have a memory for whether it’s switched on or not? My FT450D does. If you switch it off at the power supply it will power up automatically if you turn the supply on again ie retain the last state at the radio.
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Old 16th Dec 2018, 6:06 pm   #4
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Default Re: Switch-on Transients; Linear 12V PSU

I bought the 718 about 3 years ago, but it's been ages since I last used it. It's back out again as I've just put up a 5MHz dipole.
The Power on/off switch does not latch; you depress it briefly to switch on and the same for off. I don't think the switch has a 'memory'; I'll double check that. When I was using it 3 years ago, I was spooked that very occasionally, switching on the power supply brought the set directly on. I thought that the most likely cause was a switch-on transient. Other than these very occasional events, the psu /718 work perfectly well.

Bringing it back in to use again, I wanted to try resolve the issue; if it is a PSU transient, that might have the potential to cause damage, so I'd like to resolve it rather than just ignore it. I've been looking at the output using a "Picoscope" usb scope, so has storage. I used it some years ago to look at a similar problem at work, but I haven't used it since then, so I'm now a bit hazy about it. What is curious about the tests I've done so far is the absolute lack of consistency of results, which as per OP, makes me wonder about my test method.

Yesterday I fitted an X2 0.1uF cap in series with a 47R resistor across the psu primary, as well as a varistor, believing that if the problem is switch-bounce, they might help eliminate/reduce it. However, I still (appear to be!) seeing spikes on switch on.

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Old 17th Dec 2018, 1:00 am   #5
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Default Re: Switch-on Transients; Linear 12V PSU

Hi Baz

Looking at the Mydel front panel, and the Daiwa 304 cicuit tells me that there are some differences, so be alert that there still may be a degree secondary side "soft start" in the Mydel, and which may not be working.

The switch in my Yaesu FP1030 is a solid looking two pole rocker.
Most switches will have some bounce, but this is probably not your problem.

Things to consider please -

1
Trigger your CRO as per Wrangler's good post.

2
Check your secondary main diode bridge to see if all sections are OK.
Use a CRO on the bridge output, PSU loaded to 10A.
Stranger things have happened.

3
Remove the 47R in the X capacitor leg in the primary.
Of probably little value, unless I misunderstand its use.

4
Try a reverse polarity 1N540x across the DC out.
Almost any power diode will do for test
If successful, solder in a 10-20A type, PIV 100V or higher, inside the PSU.
If you do solder it in, add a 0.1 ufd chip cap in parallel.

5
Try two back to back 18 V 5 W zeners across the DC output.
Polarity unimportant if a pair is used like this.
Ensure your PSU adjustable voltage is limited below the zener rating.
Beware that most zeners are 5-10%.
Use 24 volt types if in doubt.
Use 1 W types for test.

6
Try a 2R2 10 W resistor in series with the transformer primary, and on the winding side of any MOV that you have installed.
Acts as a partial soft start.
If this works, consider an ICL as a permanent fix in lieu of the resistor.
See next.

7
Try an Inrush Current Limiter in series with the primary winding.
I would start with 10 ohms 250 VAC rated.
You may have to order it in.
The ambient (starting) resistance is not too critical, as long as it doesn't overheat long term.
Use in conjunction with the back to back zeners.

Good luck.
Merry Christmas.
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Old 17th Dec 2018, 3:21 am   #6
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Default Re: Switch-on Transients; Linear 12V PSU

Wrangler, I'm not sure I understand what the difference is between a normal trigger and an auto trigger. As per OP, it's set to trigger on a rising voltage, though that was set at just 5mV, though in the last tests I did I raised that to 50mV and the scope will capture data some mS before the trigger. So there is the transient, corresponding to the triggering event and then, on this supply, the DC starts to rise after about 6mS and reaches full working voltage a few mS later.

Thanks Radio, all those points are duly noted. Since my last post, I decided to look at another PSU which I have to see what that looks like in terms of switch-on. That is a (ubiquitous) 30V 2A Coutant LB200. That too produces a transient on switch on, a short time (~2mS) before the DC starts to rise.

Looking at both PSU's side by side, the main thing I note is how very "individual" and variable each and every switch-on event is. This leads me to the question of what transient poses a concern? Is this as simple as saying any switch-on transient should not exceed X% of the operating voltage, which in this case would be 13.8V? If so, what would X be?

B
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Old 17th Dec 2018, 4:12 am   #7
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Default Re: Switch-on Transients; Linear 12V PSU

Yes, "switch on" is a variable feast, with assorted results.
I will await the experts also, but I think it mainly depends on startup currents through transformer windings, leakage inductances and the like, and the state of charge in the secondary bridge filter caps, worst case being cold start.

Every tenth event or so for my Yaesu PSU has some transformer "thump", depending on my switch cycle timing.

Reminds me of growl and thump associated with switching of 110KV mains transformers on load.
The winding copper losses and core losses are factored into the transformer design, so as not to put the network at risk during switching.
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Old 17th Dec 2018, 4:40 am   #8
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Default Re: Switch-on Transients; Linear 12V PSU

Yes indeed, never twice the same. I've never had to concern myself with transients previously, it's just that I'd like to be sure I'm not anywhere close to "zapping" the 718.

I think I have resolved the auto and normal trigger modes. The Picoscope people use the terms auto, repeat and single to describe the basic trigger functions and I've been using the single mode, which I think translates to Wrangler's normal. It is such a long time since I used this usb scope that it's almost like a new piece of kit.

B
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Old 17th Dec 2018, 11:15 am   #9
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Default Re: Switch-on Transients; Linear 12V PSU

Auto- the 'scope trace is triggered continually then locks to a repetitive signal if present. Avoids blank screen and useful for general poking about / use as a voltage detector.

Normal- the trace is only triggered by a suitable signal but can be repeatedly thus triggered. Corresponds to Picoscope's Repeat.

Single (shot) the trace is triggered once by the first suitable signal. Subsequent triggers, if any, are ignored unless triggering is reset.

So normal will work to catch a one off transient but any subsequent triggers will change the display. Useful to try repeated attempts to see if triggering occurs. Single will catch just the first event- in this case, the one you're after.
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Old 17th Dec 2018, 11:47 am   #10
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Default Re: Switch-on Transients; Linear 12V PSU

A guy called Howard Vollum invented the triggered timebase for oscilloscopes and founded Tektronix on the strength of it. Before his invention, scope timebases were free-running oscillators with a bit of signal injected to them in the hope of synchronising them. You usually got lots od differently timed traces and had a bit of a job twiddling to get something stable.

With a triggered timebase this all changed. You set up a set of trigger conditions, and when the signal met tem, a timebase scan was set off. Just the one. If you set repetitive, the next meeting of the trigger conditions started a new scan. If you set single, it just waited for you.

This meant if you hadn't got the trigger conditions suitable you got no trace and therefore no clues what to do. So you mucked about with the trig controls til you got something.

'Auto' was added to fix this problem. A little timer runs and if it says 'It's been a while since the timebase last ran' then it triggers a run without any trigger from the signal. The trace it makes won't be timed to anything in the signal, but it makes things look alive and gives clues as to what's going on. If your signal makes frequent triggers, auto is fine and handy. If your signal is infrequent, auto-triggered traces are a nuisance, so you select normal to stop them.

On posh scopes there will be a holdoff control, this forces a delay before re-arming the timebase. With it you can make the scope ignore all but the first if triggers come repeated in bunches

David
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Old 17th Dec 2018, 3:59 pm   #11
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Default Re: Switch-on Transients; Linear 12V PSU

Mr Bungle was right about the transceiver power-on switch; if the rig is operating and then switched off at the psu, and the psu is switched back on again, then the transceiver comes straight back on! I tend to think that is a poor design, considering that it's a non-latching switch and you cannot tell what state it is in by looking at it.

When the 718 is switched OFF but still connected to the PSU it takes 5mA. If it is disconnected from the PSU for any length of time and then re-connected, there is a pulse of ~60mA while something charges then it drifts back down to 5mA.

No proposals yet for what X should be, 60%?

B
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Old 17th Dec 2018, 4:12 pm   #12
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Default Re: Switch-on Transients; Linear 12V PSU

Quote:
Originally Posted by Bazz4CQJ View Post
Mr Bungle was right about the transceiver power-on switch; if the rig is operating and then switched off at the psu, and the psu is switched back on again, then the transceiver comes straight back on! I tend to think that is a poor design, considering that it's a non-latching switch and you cannot tell what state it is in by looking at it.
That's also found on some VHF mobile gear: the idea being that you power it from an ignition-controlled source [probably with a slave-relay] and the radio then turns on/off with the car, minimising the risk of forgetting to turn the radio off and coming back to your car a few days later to find a dead battery.
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Old 19th Dec 2018, 1:56 am   #13
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Default Re: Switch-on Transients; Linear 12V PSU

I'm not sure I understand the rationale being used; seems to me that the advantages are with the rig switching itself off if the power goes off.

Doing some more experiments, both power supplies I am looking at actually produce more scary looking transients on switch-off than switch-on.

Thinking back to the days of Cambridges and Vanguards, Pye always fitted filter inductors in the 12V feed which looked like ~10 turns of 16SWG on a ferrite rod about 6mm diameter and 30mm long. Were they there to stop transients getting in or RF getting out?

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Old 19th Dec 2018, 2:39 am   #14
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Default Re: Switch-on Transients; Linear 12V PSU

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Originally Posted by Bazz4CQJ View Post
Thinking back to the days of Cambridges and Vanguards, Pye always fitted filter inductors in the 12V feed which looked like ~10 turns of 16SWG on a ferrite rod about 6mm diameter and 30mm long. Were they there to stop transients getting in or RF getting out?B
It'll do both.

It stops the power wiring injecting car-sourced noise into the transceiver and upsetting its receiver, and it also stops TX RF getting into the car's systems. OK cars with points type ignition had nothing to upset, though I did once see a Smiths electronic taco that jumped around. But once you get to later vehicles with engine management computers, ABS, electronically controlled power steering, blocking RF both ways seems like a good precaution.

David
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Old 19th Dec 2018, 4:49 am   #15
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Default Re: Switch-on Transients; Linear 12V PSU

Thanks for that David. I'm pretty sure I still have a few of those chokes in my junk box, kept for ~50 years for just a time like this. I'm now fairly certain that the original concern with the 718 "switching on" probably had nothing to do with transients, and that it was just left switched on at the set. However, having taken a look inside a 'can of worms', it's proving strangely interesting!

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Old 20th Dec 2018, 4:18 am   #16
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Default Re: Switch-on Transients; Linear 12V PSU

Behold, the fruits of my junk box. Looking at the psu which powers my Icom, it looks as if I could squeeze that filter in there, most easily, there is space right up against the output terminals. It might go in between the bridge rectifier and the regulator but not so easily. However, would I be right in thinking that putting it against the output terminals would be a good place (the best?) from an electronics point of view?

I haven't forgotten other suggestions which have been made; some current inrush limiters are in the post; not sure if they'll be here before Xmas.

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Old 20th Dec 2018, 7:38 am   #17
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Default Re: Switch-on Transients; Linear 12V PSU

have you tried it on a car battery and see what it does?,m3vuv.
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Old 20th Dec 2018, 5:45 pm   #18
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Default Re: Switch-on Transients; Linear 12V PSU

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have you tried it on a car battery and see what it does?,m3vuv.
No, I haven't done that, but may be worth a look. This is the first time I've had reason to take an interest in psu transients, so I'm on a learning curve. But in the situation where the psu is feeding the ICOM, with the ICOM is switched off it takes only 5mA current and that goes to some thing like 500mA when it's switch ed on and on receive, so that's a fairly high impedance load. Looking at the transients with either no load on the psu, or a resistive load taking ~100mA suggests that such a load has no effect on the transients produced.

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