Zap a volt regulator w/ no cap??
Zap a volt regulator w/ no cap??
(OP)
Hi guys:
First of all, forgive me for being a dumb ME meddling in electrical work.
I have an application in a machine that has a 24-volt power supply. I have a tilt sensor (analog output) that needs 12 Vdc as its input supply. Until now this part of the machine used an embedded circuit that the sensor mounted directly to and that provided its own 12 V.
I converted this part of the machine's control to be read from a PLC analog input module, so I had to come up with my own 12 V source. What I did was add an LM2940CT-12 voltage regulator (TO-220 package) to the circuit right before the analog sensor. This provided my 12V and worked great for about a day and a half, then my analog sensor fried. When I measured the voltage from the regulator I found that it was now the full 24V input...so apparently the regulator failed first and then cooked the sensor with 24 V.
When I took a look at the spec. sheet on the LM2940, I see that they recommend putting capacitors from the input and the output to ground...I didn't do this. Other than this mistake, I can't see anything else wrong with the circuit. Could the absence of these caps have caused the regulator to fry? If not, what else could have caused it? I'd like to know exactly what my problem is before I toast another sensor (they're $175 a pop).
Thanks for any advice (or chastisement) that you can provide.
Don
Kansas City
First of all, forgive me for being a dumb ME meddling in electrical work.
I have an application in a machine that has a 24-volt power supply. I have a tilt sensor (analog output) that needs 12 Vdc as its input supply. Until now this part of the machine used an embedded circuit that the sensor mounted directly to and that provided its own 12 V.
I converted this part of the machine's control to be read from a PLC analog input module, so I had to come up with my own 12 V source. What I did was add an LM2940CT-12 voltage regulator (TO-220 package) to the circuit right before the analog sensor. This provided my 12V and worked great for about a day and a half, then my analog sensor fried. When I measured the voltage from the regulator I found that it was now the full 24V input...so apparently the regulator failed first and then cooked the sensor with 24 V.
When I took a look at the spec. sheet on the LM2940, I see that they recommend putting capacitors from the input and the output to ground...I didn't do this. Other than this mistake, I can't see anything else wrong with the circuit. Could the absence of these caps have caused the regulator to fry? If not, what else could have caused it? I'd like to know exactly what my problem is before I toast another sensor (they're $175 a pop).
Thanks for any advice (or chastisement) that you can provide.
Don
Kansas City





RE: Zap a volt regulator w/ no cap??
Also, I don't know how much current your circuit draws, but make sure that you calculate the power dissipated by the regulator and make sure that you have adequate heatsinking if needed.
RE: Zap a volt regulator w/ no cap??
RE: Zap a volt regulator w/ no cap??
Look at so called wall warts with a regulated 12 V output. They won't cost you more than 20 - 30 dollars. And they are quite good and very reliable.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Zap a volt regulator w/ no cap??
They absolutely require the caps - no if or maybe.
It did oscillate.
It will fry.
Non-'low head' regulators are less touchy and are less expensive.
I do not see how you add a voltage regulator piece-meal to a PLC in any kind of a clean and robust manner.
I would look for small DIN RAIL power supplies with all the Certs and hence some pedigree.
Something like these: (Note: there are a huge number of different makers and styles)
http:
They will all cost far far less than $175!!!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Zap a volt regulator w/ no cap??
----------------------------------
Sometimes I only open my mouth to swap feet...
RE: Zap a volt regulator w/ no cap??
Don
Kansas City
RE: Zap a volt regulator w/ no cap??
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Zap a volt regulator w/ no cap??
Linear regulators dissipate heat according to P = V.I where I is the load current and V is the drop across the regulator, i.e. the differential between input voltage and output voltage. With a high input-output differential you'll see from P = V.I that you can quickly get to a fairly high dissipation even with light load currents far below rated current of the regulator. A TO-220 package can't dissipate much power without a heatsink, probably less than 1W before it reaches a damaging temperature.
----------------------------------
Sometimes I only open my mouth to swap feet...