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Old 27th Mar 2019, 5:16 pm   #1
60 oldjohn
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Default Low voltage supply spike protection?

In a few months time I will have a Radiomobile 4220 radio circa 1950's to repair. The owner would like the FM to AM converter board (discussed elsewhere on this forum) fitting. I will have to build a 12V positive earth to 3V 20mA supply, I expect it will need spike suppression, what would be needed keeping component count and physical size down? Hopefully fitting parts on the converter board.

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Old 27th Mar 2019, 6:26 pm   #2
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Default Re: Low voltage supply spike protection?

You might get away with a simple dropper resistor/zener combination. With a couple of additional capacitors across the zener you could probably do it with about four components. The zener will naturally clamp any spikes on the 3v output. With it being a car radio you won't be bothered about keeping power consumption down, so a 1 or 2 watt zener could be used, as you are dropping quite a bit of power. I haven't done the maths but that's the sort of value I would expect to use.
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Old 27th Mar 2019, 8:56 pm   #3
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Default Re: Low voltage supply spike protection?

You could use a small transient voltage supressor on the dc input, there can be some horrible transients on automotive wiring harnesses,
Just look at the EMC test limits for automotive transient emissions in ISO 7637-2
a couple of hundered volts isnt uncommon.
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Old 28th Mar 2019, 12:07 am   #4
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Default Re: Low voltage supply spike protection?

Thank you for the replies, I intend using a SMD LDO voltage regulator, maybe a HT7330-2 its 3.0v output with max input 30v. Another problem, this set has AC voltages in it. Maybe have it to screen as well. I will look out for some small Transient voltage suppressors.







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Old 28th Mar 2019, 11:04 pm   #5
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Default Re: Low voltage supply spike protection?

For transient suppression on the 12V (14.4V?) side, there are plenty of dedicated TVS and other parts about specifically aimed at automotive use.



As noted there can be plenty of nasty high voltage spikes about and things can get quite nasty for a long (in electronic timescale) period if the battery ever loses connection when the alternator's charging it. I wouldn't use any regulator that isn't "automotive" rated- generally that means at least 42V transient input rating.
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Old 29th Mar 2019, 2:54 am   #6
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Default Re: Low voltage supply spike protection?

I have seen a car blow both head light bulbs on main beam at once after going over a bump on a corner.
It was caused by a brief short because the battery to starter cable had been rubbing on the engine mounting.
The alternator put out a pulse of about 40 volts and killed both bulbs.
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Old 29th Mar 2019, 12:13 pm   #7
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Default Re: Low voltage supply spike protection?

You could use a voltage regulator intended for the automotive environment, these are generally specified to withstand +/- 60V input voltage during fault or transient conditions, and for extra safety you could add something like a small resistor in series with, and a low-impedance electro across the input, to take out any more viscious transients.

I have designed industrial kit for mounting on fork-lift trucks using the above techniques and never had one come back suffering from over-voltaged input, despite the fact that the motors on these trucks generate some horrendous transient voltages when switched.

Adrian
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Old 29th Mar 2019, 2:36 pm   #8
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Default Re: Low voltage supply spike protection?

I have never had an "automotive" regulator fail even on motorcycles where electrics are much of a second thought. Still the resistor/zener route sounds the most bullet proof.
 
Old 29th Mar 2019, 3:09 pm   #9
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Default Re: Low voltage supply spike protection?

Quote:
Originally Posted by Refugee View Post
I have seen a car blow both head light bulbs on main beam at once after going over a bump on a corner.
It was caused by a brief short because the battery to starter cable had been rubbing on the engine mounting.
The alternator put out a pulse of about 40 volts and killed both bulbs.
It can put out a lot more than 40 volts.....modern alternators are usually clamped to 30 or 40 volts.

For 12 volt systems the test level for pulse 5 load dump (loss of battery connection ) can be up to 87 volts and up to 400mS
The regulator can respond fast , but the magnetic flux in the rotor has different ideas, always keep battery terminals tight!!
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Old 29th Mar 2019, 3:16 pm   #10
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Default Re: Low voltage supply spike protection?

It was not a modern alternator.
The car was made in the mid 1970s and the regulator was s very simple Lucus slab insede the alternator.
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Old 29th Mar 2019, 3:39 pm   #11
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Default Re: Low voltage supply spike protection?

I presume the Rover P4 will have a Dynamo, but I still expect Transients.
I like Alan's idea in #2, is anyone able to draw me a circuit along those lines? I am not very good at circuit design. I would like to keep to the minimum components, and using SMD components to keep size down.


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Old 29th Mar 2019, 5:29 pm   #12
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Default Re: Low voltage supply spike protection?

I recall that the old valve Radiomobile radios include what the circuit refers to as a 'Spark Plate' on the 12V input together with the usual suppressor chokes and capacitors. I've never really understood how the Spark Plate operates, but it's presumably a temporary breakdown surge suppressor like a varistor.

If you feed your voltage converter from the 12V supply within the radio ahead of the vibrator pack, perhaps via well-shielded coax, then it will be benefiting from that internal surge suppression.

Martin
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Old 29th Mar 2019, 9:20 pm   #13
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Default Re: Low voltage supply spike protection?

Martin, good idea, I may do that as I am hoping to make room inside the case by replacing some of the capacitors, I'm expecting the replacement caps will be smaller.


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Old 29th Mar 2019, 9:52 pm   #14
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Default Re: Low voltage supply spike protection?

One post deleted.
Tempting, but please keep strictly on topic.
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Old 29th Mar 2019, 9:59 pm   #15
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Default Re: Low voltage supply spike protection?

My understanding of the "spark plates" used in old car-radios/two-way gear was that they were a version of what we later came to call a feedthrough capacitor: the idea being to provide a truly low-impedance path-to-ground for ignition-interference etc.

I've got a couple of references to them from 1940s US maintenance-manuals if anyone's interested.
Thankfully, these sorts of things have been obsoleted by cheap semiconductors from the likes of Littelfuse and ST.

https://www.eetimes.com/document.asp?doc_id=1272768 is another example of what can be done.
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Old 29th Mar 2019, 10:04 pm   #16
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Default Re: Low voltage supply spike protection?

One point that may make this project a non-starter, or at least difficult to implement, is the fact that the electrical system of the vehicle would appear to be positive earth. The FM/AM converter board you are intending to fit will be almost guaranteed to be negative earth (unless it has been specifically designed for positive earth working). The zener regulator design would lend itself to either positive or negative earth working, but the module will be difficult to reference to earth without a lot of careful thought and insulating etc.
Alan.
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Old 29th Mar 2019, 10:20 pm   #17
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Default Re: Low voltage supply spike protection?

When I was in the electronics industry, automotive kit had to withstand some really nasty transients. Big, fat 28V MOVs, about 20mm. diameter by 4 thick (the production line workers called them lollipops!), 3W Zeners across transistors switching relay coils, capacitors across the supply lines, resistors and capacitors on all external inputs -- basically the lot. We found dissipative voltage regulators preferrable to switched-mode types, as the former allowed much less supply-line noise downstream; it's possible that things could have changed since then, though. Low-current drain modules might use just a simple Zener diode shunt regulator.

It's also good practice generally to keep all connections as short as possible, since there is no shortage of RF noise around a petrol engine with spark ignition. And nowadays there will likely be Bluetooth and possibly even wi-fi, as well.
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Old 29th Mar 2019, 10:51 pm   #18
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Default Re: Low voltage supply spike protection?

Merlin Maxwell, 12 months or so ago I built a tester for single and 3 phase regulators for M-cycles. Out of about 10, I had two failed completely, and 3 well out of spec. Actually the BIG shunt Mosfet types all seem to regulate at higher voltages than older series regulators.
I had one fail on my Cagiva Alazzurra about 20 years ago. Lost power, no spark, no battery (dead!) and s?c regulator. No idea why, I was riding home from work, about 13 miles done and one mile left, fortunately nearly all downbank (from the top of Barragaroo for TT enthusiasts).
One more a year later, but my fault. I was giving a helpful jump start with a car battery, and connected pos to neg. Instant S/C regulator.
Les.
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Old 30th Mar 2019, 12:12 pm   #19
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Default Re: Low voltage supply spike protection?

Quote:
Originally Posted by julie_m View Post
it's possible that things could have changed since then, though. Low-current drain modules might use just a simple Zener diode shunt regulator.
All kinds are used. This week I've seen a SMPSU, a linear and a zener in different modules. What is becoming most common is a SBC which is a linear regulator, bus interface (CAN, LIN) and voltage supervisor.

There are all sorts of things to trap the unwary. The load dump is quite nasty (this is the surge when a battery terminal is loose) as there's a lot of energy. Sometimes the electronics have there own protection but sometimes one of the rectifier diodes in the alternator is replaced with a zener and as mentioned this limits the voltage of the pulse, but it's still relatively long lived. TVS diodes are common, but have to be rated so that they don't conduct and try to clamp at 28v in case someone jump starts the car from a truck (which is more common than you might think).
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Old 30th Mar 2019, 3:39 pm   #20
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Default Re: Low voltage supply spike protection?

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Originally Posted by duncanlowe View Post
What is becoming most common is a SBC which is a linear regulator, bus interface (CAN, LIN) and voltage supervisor.
I can see why they would want to integrate all that into one device. Any device is going to need power and bus drivers, after all .....
Quote:
Originally Posted by duncanlowe View Post
There are all sorts of things to trap the unwary. The load dump is quite nasty (this is the surge when a battery terminal is loose) as there's a lot of energy. Sometimes the electronics have there own protection but sometimes one of the rectifier diodes in the alternator is replaced with a zener and as mentioned this limits the voltage of the pulse, but it's still relatively long lived.
Yes, I remember our in-house built load dump test rig. A 3U equipment housing, most of the space inside of which was taken up with capacitors .....
Quote:
Originally Posted by duncanlowe View Post
TVS diodes are common, but have to be rated so that they don't conduct and try to clamp at 28v in case someone jump starts the car from a truck (which is more common than you might think).
Back in those days (1995 - 2002), we weren't making anything to withstand 24V continuously; that must be a relatively new thing. I wonder if someone hadn't had one batch too many of 24V modules incorrectly fitted with 28V MOVs (whistles innocently, as though she had never, ever in her life seen such a thing happen), and needed an excuse to stop it happening again?
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