Diesel Engine Lubrication Oil Pressure Sensor


Thread Starter

Pravin Shinde

I have noticed at one of my applications that the pressure sensor I used to measure the Oil pressure for a diesel IC Engine is causing (?) the failure of the the IC 7805 regulator which is being used to power the sensor with the +5 V DC supply. The sensor make is not known but the Land being Denmark. Well; the sensor has three pins a Supply (+5 V) a ground and the third being the signal output. I'm reading the signal in with an Op amp stage. But the culprit is the 7805 which, without any signs/symptoms of heating or burning gets off the bussiness. I will be thankful to anybody suggesting a check or solution on this. Are these type of sensors Piexo type sensors? What's the Pieo type Pressure sensor though? I have heard and seen those working on strain gauge principle.

- Pravin

Sounds like the regulator is failing due to excessive current draw, have you measure the current, it is also possible it is a 5 volt powered loop device, but does not seem likely and most automotive sensors I have had experience with are 12 vdc rated. The sensor it self if it is a high quality type has the bridge function built in and produces a voltage ouput proportional to the pressure for driving an analog gauge, you could easily replace it wihta 4-20 ma device, but wiht 12 vdc, the loop will not have enough voltage available for good operation.

It is possible that the sensor is defective, I would use a bench supply which allows me to observe the current and the voltage, power the thing up and measure the voltage as a function of pressure (say 0 to 50 psi). get on the web and look up oil pressure sender units for desil engines, or call around Denmark to see if there is good engine shop / mechanic who can get you a line or senders there.

Hope this helps.

Does not really matter what the operating principal of the sensor is as it will not affect your control system. Is your control system supply "crossed" with the generatos starting batteries? This often causes "bangs in the night". The other option would be to use an interface relay as these are far more tolerant than ICs. I would check for crossed voltages first and if there are still problems, many manufacturers have these types of sensors. If in doubt, replace it.
The problem may not be the sensor/7805 side but it is probably the power supply coming into the 7805.

I assume it is a 24VDC diesel power system?
They have *very* large spikes and the 7805 blows at 35VDC. I would add a series diode and inductor to the 7805 input pin, and a big capacitor (220-1000uF).

Curt Wuollet

Do you have it properly heatsinked ? At 12 V the bare TO-220 package is good for < .15 amp (give or take, depending on environment) at 24 V (many diesel electricals are 24 V) this is much more like < 50 ma. Also some 7805s will have problems with input/output differential. Many automotive sensors use power like they have it to throw away as 100 ma. is insignificant in charging system terms. Even if the 7805 doesn't expire, it will shut down if it gets too hot.



Hi Pravin,
You can damage a 7805 IC only on these conditions,
Supply Voltage in excess of 35 V Dc,
Reverse supply voltage,
---Look for rectifier fault , leak.
Feed back from output
----if the output capacitor is bigger than supply input capacitor.
Oscillation internal to the regulator.
The last reason could be the possible reason. Wire up 2 nos 0.1 mfd ceramic capacitors from supply to ground and from output to ground. These are to be as close as to the IC. Do not use filter capacitors more than 10 mfd at the OUTPUT.
Load the output to atleast 10 ma ( you can wire up a LED lamp)
These ICs will not destroy if once wired properly.
Hi. What other people have suggested could be right.

Look at application notes (at IC sites) for voltage regulators. To answer the possibility of output being momentarily greater than input, (when input is off/disconnected, but output C is still charged), there are some (older) solutions using a diode. Some circuits have diodes everywhere! TVS, Transorbs, Varistors are all more noticeable recently, so learn from application notes at their sites, about the "automotive environment".

Google for "voltage regulators for automotive environment" see words used such as "harsh", "hostile", and see voltage regulators made for that environment. Are you free to change the 7805 for one of those? Look at the data sheet for the 7805, and the internal circuit. I have not yet seen a data sheet for the 78xx,(nor 79xx,) make a claim to cope with reversed voltages (out > in, in < 0, out < 0 ), or other "harsh" conditions.

The words "harsh" "hostile" lead to you adding "EMC" to your thinking. Google for "Electronic Component EMC Requirements" one result is http://www.autoemc.net/Standards/OEMstandards.htm which leads to ford's interesting ES-XW7T-1A278-AB:Component/Subsystem EMC Requirements and Test Methods. If you ignore most of the words in the document, but absorb some of the data (magnitude, polarity, and where/how they originate, how to create trouble) it will enhance your thinking about sources of "your" troubles.

What you learn because of the 7805 today, will help later for the op-amp. Hope that helps.

Thanks Louis and all friends for your valuable suggestions. These have been of valuable use to me.