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Power Supply

Overvoltage Protection

Polarity Inversion protection@, Isolation@, Short Circuit@, Sensing Power or Current

Dennis Plunkett says:

[Rather than rely on the low speed and low power protection of a Zenar diode, use a] TRISIL. Basicaly [this] is a bidirectional crowbar device (Like 2 SCRS) that clamps both rails together (To within 0.6V) [when] the breakover point is reached. These things come in 0.5kW and 1.5kW (Yep that's correct) versions (surface mount 0805, and SOIC8!). I love these things. If you use a TRANZORB then it will clamp once over voltage. And release once under.

Mark Willis says:

TVS's act just like a fast, cheap, very high power Zener, they're a little sloppy on triggering voltage (not quite as precise as a Zener, or at least weren't at one time), these work well for Auto circuits. A TVS won't clamp down to 0.6V (maybe a Trisil is a different beastie?) TVS's are rated by power & Vbr (breakdown / avalanche voltage); 1 pSec avalanche time for unidirectional units, the bidirectional's are slower, and at $0.72 in onesies for the 1.5kW units, you can do worse. General Semiconductor makes the ones I've looked at, other co's make these; Other places may be cheaper.

The wattage rating IS, btw, a "Peak power pulse" rating - I wouldn't try to use one at a steady state power dissipation of 1.5kW, "it wouldn't be pretty!" - I haven't blown one up yet, I'm sure someone will here Mark

Harold Hallikainen says

[For polarity inversion protection use] a series PTC then a shunt diode. The diode will conduct if the batteries are put in backwards, opening the PTC.

Nick Taylor says:

[this] can be improved by using an appropriate valued zener for the diode ... also gives overvoltage protection.

Roman Black [fastvid at EZY.NET.AU] says

I'm not going to pretend to be some type of automotive electronics expert, as that is not my main field. But, I will say we have never had a product fail in service, and since many units we sell are used in racing motorcycles which often have no battery(!) and butchered charging systems I must be at least doing something right.

I am a big believer in simplicity and overkill. Simplicity lets you use a big resistor as opposed to a smaller more expensive semiconductor or MOV spike suppressor etc. For the same parts cost you often get greater durability. Overkill to excess, I will use a 2w resistor in a circuit that dissipates 0.1w or even less. Resistors are cheap compared to the cost to you of a product that fails.

My theory behind this comes from 20+ years of servicing TVs, etc, and it has been my experience that the most unreliable parts are any semiconductors, any varistors, any components full of corrosive (electro caps) and reliability is directly proportional to how cool any device runs. With the new tvs, I can touch the heatsinks for the the power semis and give you a pretty good estimate whether that semi will fail in 1 month, 1 year, 3 years, or never. Any service guy who fixes 50+ units a week will know what I am talking about. :o)

For power filtering in auto applications I prefer simplicity of two large series resistors, with a cap to ground at the point between them. This is ideal for PIC apps which draw a small and regular current. I only use metal film resistors of course. Carbon resistors are a monstrosity! I use 630v polyester cap between the two series resistors (to gnd), these caps have better reliability than just about anything in that size and price range. With a 5v chip running from 12v auto, I drop about 4v across the resistors at normal running current and the other couple of volts with a low voltage drop out 3 pin regulator. I also use a 5w 24v zener to absorb transients that get through the two resistor ladder network. These big zeners last forever when spike buffered by a couple hundred ohms, mainly since it has zero temp as it doesn't conduct during normal operation. Yes I could use a 1w zener, but the 5w has a much larger silicon die and larger legs, and hence stronger mechanical coupling during heat expansion etc etc. Always us a BIG part if you want it to last forever, what the heck, costs 20c more per product!

With a very low power app I would just use the two series resistors to a 5.1v zener, with a 630v poly cap and 24v zener at the spot between the two resistors, and obviously a large 22uf or more tantalum (did I mention I hate electros?) and this should suffice. When I was a kid I used to buy the ex-military and ex-commercial circuit boards from the local scrap dealer, and strip the parts for my hobby use. Surprise I found many of the electros were shot but NEVER found a dead tantalum. Also noticed the military stuff always used tantalums. How much price difference? How much do you want a little can of corrosive sandwiched between your heasinks? Yes I will sometimes use electros but only if no other choice, and I will use 105 degreeC types which have better rubber plugs and different corrosive type. Then I just prey that I mounted them far enough from any possible heat source. :o)

With input/output buffering, again use the largest ohms value series resistor you can use, always use a buffer transistor, something like a little BC337 (800mA 50v) is tiny and cheap, but has a HUGE silicon die when compared to the in/out pins on a pic chip! If you buffer this with a decent series resistor and cap, it will be pretty bulletproof. If you need to run any decent output current from the PIC use a transistor with about 10x the current/pwr needed, again to use the maximum size silicon die and metal backing plate (never use a transistor you haven't pulled apart) etc. I like BD139/BD140, these have a huge backing plate and chunky die and are dirt cheap if you can fit the physical size. :o)

Remember that MOV varistors and the like sound nice, but many types use an unstable metal "dust" compressed into a solid, and I have replaced so many of these things over the last few years I reject that as a solution. Nothing kills spikes as reliably as a RC network, or a RZ network and with good choice of R and C (or Z) it will do so forever. Many of the MOV type solutions are a trade off of "MOV kills spikes, which at the same time slowly kill MOV" that suits many TV/telecom manufacturers, but not suitable for a product I'm putting my name on!

Also look at: tranzorb a brand of SILICON AVALANCHE DIODE + 

The following circuit uses two 1N4148 diodes to protect parallel port against higher than +5V signals and also against wrong polarity signals (power on the circuit is accidentally at wrong polarity.

          1N4148  4.7K   
parallel  >-|>|-+--\/\/\/--
port data       |          
pin             +-|<|-+    
            1N4148    |    
parallel  >-----------+
port ground           |

Adding even more safety idea: Replace the 1N4148 diode connected to ground with 5.1V zener diode. That diode will then protect against over-voltage spikes and negative voltage at the same time.

Another way to protect against over-voltage is to put a regular diode with the cathode on the signal line and the anode on the power supply line. Keep in mind that this will cause the power supply to recieve the over-voltage and may result in damage to other components on the board.


See also:

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