Over Voltage protection

Have built a couple of devices for automotive use - they work on both 12 and 24V systems - but was wanting something to protect a possible voltage spike to prevent the poor little 78 sereis regulator from going over 40 volts. Initially I put a 25Vrms (31Vdc) MOV on thinking much over that and it will clamp - however after reading datasheets, this particular MOV doesn't really start to draw reasonable current till around 100Volts!!! - it's totally backwards to my initial understanding of an MOV - but there it is in black and white - I even put 45volts across it and it didn't really draw any discernable amount of current. If I use a lower voltage MOV - then they are rated below the voltage they will be running at - which might be a problem for them. I did think of using a zener - but not so sure how one of them will last with a but of a surge across it either.

Any protection suggestions would be greatly appreciated

Cheers

Have a look to ST Automotive regulators instead of 78xx ....

Alain

I found the ST series auto regulators were still only good for upto 40 V - when running on a 24v system (with alternator running upto 30V) i figure that's too close. I did however find some devices termed as a TVS transient voltage suppressor. They are sortof (not quite) like a couple of grunty zener diodes that are back to back that have a distinct clamp voltage. They are called TRANSIL

after many car device done, the regular 78XXX never failed.

Now if you really want to have something over-rated have a look to TL783

Hi,

3-Terminal Adjustable Voltage Regulator
http://www.national.com/pf/LM/LM317HV.html

The maximum input voltage of a LM317HVT is 60 volt.

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REVERSED BATTERY AND OVERVOLTAGE PROTECTION
(12V system).

RBO40-40G (D2PAK)
RBO40-40T (TO220)

http://www.st.com/stonline/products/...re/ds/5484.pdf

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BZW06 (TRANSIL)

http://www.st.com/stonline/products/...re/ds/2972.htm


PROTECTION BY TRANSIL
HOW TO ENSURE ABSOLUTE SAFETY
http://www.st.com/stonline/products/...re/an/3568.pdf


TRANSIL™/TRISIL™ COMPARISON
http://www.st.com/stonline/products/...re/an/5628.pdf

* * *

Best regards,

Luciano

Quote:

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Originally Posted by George

Any protection suggestions would be greatly appreciated

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How much current do you need ?

I put a 68Ohm 1W resistor in series to a 47uH coil in the input in order to lower the shortcurrent of the following parts. Then I use a 30V SOIV in parallel to a Transil (P6KE33A) towards Gnd.

Using a 78xx is a bit difficult. Maybe it is good to place a 1N4007 across Vin and Vout in order to protect the device. The 78xx is dead, if the input is connected to Gnd (Power-Off or shortened to Gnd) while the capacitator at Vout is still charged.

I also use the 7805 for automotive and I have never had a problem. You are probably drawing too much current. There is a circuit on the datasheet that uses a PNP transistor as a current supply. I have gotten close to 750mA without heatsinks. If your car does have some problem with transients you can always put a zenter diode on the input. This will absorb the energy of the transients.

I'm only pulling 20mA max - so current limit isn't much of an issue. I've never actuall had an issue with the 78XX however I see on running on a 24V system that I could get surge voltages over 40V. I was also thinking of just using a Zener and a resistor to regulate the voltage - but that would lead to poor efficiencys since I want to run it both on 12 and 24V. Thanks all for your input - I think I'm going to be uding the transil TVS diodes

I use a 1.5KE24CA to keep those nasty transient spikes (like a relay opening) out of my automotive stuff. Anything over about 24 volts is clamped.

http://download.siliconexpert.com/pd...1.5ke440ca.pdf

Richard Furniss
is it suppose to smoke like that ?

I've worked in the US Auto industry for over 20 years and standard procedure
these days is to use a TVS. They are quite durable. Zener diodes can't handle
the current of a large spike. I use a 24 volt TVS in everything that I design for
automotive use.

...at 12V (13,8 volts) systems ?

What's about truck with 24V (28 volts) ?

Yes, 12 volt systems. For 24 volt systems, I would use a 33 or 36 volt TVS.