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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Oct 2020, 7:57 am   #21
Radio Wrangler
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Default Re: 12V 20A PSU Crowbar Cct

It's a fair warning. It's one thing to design a power supply that produces 13.8v into a steady load, but an SSB transceiver can swing between taking an amp or so to >20 amps and back in under a millisecond, following the audio waveform from the microphone.

There will be decoupling capacitors inside to filter off the RF components of current demand, but audio and syllabic speed fluctuations aren't decoupled, that would need a lot of energy storage, especially as the linear-mode RF power amp was designed around having a supply which doesn't droop. These current demand fluctuations are exported straight back to the power supply. The power supply has a whacking great reservoir capacitor which stores plenty of energy, but big peaks knock its voltage down somewhat.

So the voltage change of the reservoir capacitor is handled by the regulator section of the power supply. The reservoir capacitor is run sufficiently high that even after a run of big peaks, it has comfortably more voltage than the 13.8v needed and expected by the transceiver. The regulator uses series pass transistors to drop the excess voltage.

The series pass transistors are controlled by an amplifier and a voltage reference which monitor the output voltage and vary the drive to their bases to keep the output reasonably constant.

This is a feedback loop and it will have to be able to track and compensate accurately for the current demands the transceiver makes. So someone has to have planned its dynamic response. This is not a trivial task, avoiding overshoot and undershoot. Undershoot can cause distortion of the transmitted audio as well as widening of the transmitted spectrum. Overshoot can damage the transmitter. The voltage the power supply reservoir runs at will be far above the wanted 13.8v - plenty enough to damage things. Hence the use of a crowbar on the PSU output. The PSU is worth a lot less than the transceiver, and it's a hell of a lot easier to fix.

It was quite worrying when you saw someone spend a couple of thousand quid on a new transceiver back in the day, and then shop around for the cheapest PSU they could find. They hadn't sussed that one fault in the transceiver would be just that; one fault to fix, but one fault in the PSU can overvoltage the entire transceiver. You wind up with many immediate faults, and a likelihood that things which seemed to survive will provide faults in the future.

So a crowbar is a jolly good thing. It's good if the PSU survives a crowbar trip without smoke and drama. But even if the PSU expires dramatically, it's still better than what would happen if it didn't trip when needed.

Good power supplies have a crowbar.

Great power supplies don't take any further damage should the crowbar trip. You still have to fix whatever caused the trip, but nuisance trips aren't the end of the world.

The PW Marchwood is probably the gold standard in amateur radio PSUs. It does however, still have the error of sensing output current on only one of the series pass transistor emitter resistors, but that doesn't threaten destruction of the transceiver. The crowbar is a pussycat because it dumps the charge in the reservoir.

THe current demand of an SSB transceiver is especially complex because positive going parts of the microphone waveform bring up the transmitter power, and hence current demand as seems obvious. But negative-going parts of the microphone waveform also bring up the transmitter power and current demand. The difference is that the phase of the RF is reversed. This is how the balanced modulator differs from a simple amplitude modulator.

So, to the PSU, the current demand looks like the microphone audio got full-wave rectified. This makes for fast drops in current demand as the microphone waveform passes through zero.

PSU design is really a task for wily, seasoned designers. But they normally get the flashier stuff to design, the PSU gets given to the student on the project! The student larns a lot, but PSUs tend to contain the mistakes they learned from.

If you look at a linear power supply from Trio/Kenwood or Yaesu or Icom the first thing that hits you is how pretty they look and how well their styling matches the radio. The second thing that hits you is the price. These things were damned expensive. This is why so many amateurs went looking elsewhere for their 13.8v.

There's worse...

Now look inside that Icom/Kenwood/Yaesu power supply, look at the circuitry and it's as crude as you can imagine. It works, but there is no crowbar, and things were sized by skilled and highly paid accountants. Put in a pretty box so a station photo looks nice on a QSL card and the punters will buy them.

So the wise guys also went looking for an alternative make of PSU. There were crappy ones, sort of expanded CB thingies, but there were good ones too BNOS and DRAE were at the better end. Not all their products were as well protected as you'd like, but their top end ones made a laughing stock out of what was inside Icom/Yaesu/Kenwood ones.

What about SMPS type 13.8v supplies?

Well the first thing everyone thinks about is RFI. So they need good filtering at both mains input and 13.8v output. They need a good RF-tight case. The second thing everybody thinks about is.... well they don't normally get to a second thing, the fear of RFI stops most people thinking further.

In an offline inverter SMPS, failure of a chopper transistor brings the output to zero. The expensive transceiver is safe. The risk is that a fault in the control circuitry could let the SMPS run aout of control and make very high voltages. This is where the expensive repair/write-off risk lies. This sounds like a suitable job for a crowbar. Few have them.

And the matter of getting the SMPS power supply dynamics to handle an SSB transceiver (and CW pulses!) is harder than it was with the linear supply.

So there you have it, a litany of the difficulties of 13.8v power supplies, and why the Marchwood is outstanding. The article is good and explains a lot. Even if you build/buy something else, it ought to be essential reading.

So if your crowbar does nuisance trips, don't slow the crowbar. Think first about what caused the trip and fix that first. You want a fast-acting crowbar. Semiconductors are famously fast at how quickly they blow up if given too many volts, and this applies to everything in the transceiver.

David
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Old 8th Oct 2020, 8:20 am   #22
Bazz4CQJ
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Default Re: 12V 20A PSU Crowbar Cct

Well, being Mr Meany, the PSU I have for the Icom is not Icom itself, it is a linear MYDEL 304 https://www.eham.net/reviews/view-product?id=11181.

So far as I know, the circuit diagram has not yet found its way on to the web, but I'd assume that it's built down to a price and make well lack some 'nice to have features'. I'll certainly look out the Marchwood and place it file for when I next wonder in to the world on 12V rigs.

B
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Old 8th Oct 2020, 8:25 am   #23
raditechman
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Default Re: 12V 20A PSU Crowbar Cct

For simplicity get a 1.5KE15A and a couple of 25a fuses. The 1.5KE15A will blow the fuse if voltage goes above 15v It is not my idea but there is an article here
http://www.ad5x.com/images/Articles/Vprotect.pdf

John
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Old 8th Oct 2020, 8:45 am   #24
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Default Re: 12V 20A PSU Crowbar Cct

Quote:
Originally Posted by raditechman View Post
For simplicity get a 1.5KE15A and a couple of 25a fuses. The 1.5KE15A will blow the fuse if voltage goes above 15v It is not my idea but there is an article here
http://www.ad5x.com/images/Articles/Vprotect.pdf

John
Mmmm...that's something a bit like what's in Post#2 then
B
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Old 8th Oct 2020, 12:53 pm   #25
Alan_G3XAQ
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Default Re: 12V 20A PSU Crowbar Cct

Is there a need to ensure there is a 6" or longer lead in the crowbar SCR/reservoir discharge path so as not to exceed the dI/dt limit, perhaps especially with "soft" gate triggering? This is certainly significant with valve HT crowbars.

Alan
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