need help with isolation

Thread Starter

acat800

Joined Dec 28, 2011
9
Hi, I have a project I'm working on for a snowmobile. The project is to measure the exhaust gas temperature of the engine and for the most part I have all the bits working but I ran into one major problem last night that I need help with.

The power comes from the engine stator which produces 12VAC, I then rectify and regulate that down to 12VDC and 5VDC which powers a micro controller and other componenets. The thermocouple gets connected to the microcontroller via a digital converter. All works fine when I connect to my computer using the USB 5v, but last night I finally put the project on the snowmobile and it seems I smoked some components. What I now believe happened is this....

the mains from the snowmobile stator are grounded. When the thermocouple touches the exhaust, this is also grounded so I essentially have a ground loop. I'm wondering, what is the best way to isolate the AC from the DC circuit? A 1:1 isolation transformer? Would a set of diodes work? Or is there another suggestion?

Thanks in advance!
 

t06afre

Joined May 11, 2009
5,936
Does your snowmobile have a battery. Then your circuit should be referenced to this. We do not allow automative question on this forum. But since a snowmobile is not driven on public roads. I think it should be allowed.
 

Thread Starter

acat800

Joined Dec 28, 2011
9
No, there's no batttery. Sorry about the "automotive" question, I thought the new rules didn't included restrictions on automotive type questions. This isn't really about fixing my car though, it's regarding a ligit microcontoller project so hopefully it's OK to post here.
 

JDT

Joined Feb 12, 2009
658
I would not connect your equipment to the snowmobile's electrical supply at all. It will be a very poorly regulated and noisy supply. No good for sensitive electronics.

Use a separate battery.
 

shortbus

Joined Sep 30, 2009
6,984
An alternator system without a battery is not as well regulated as one with a battery. Spikes and surges are going to be present in the system. Maybe one of the more knowledgeable guy's will help with a surge suppressor circuit.

Back in my younger days a battery eliminator was a popular item used on Triumph and BSA motorcycles. Here is one of many links to one of those. But they were for generator bikes, don't know about using one with an alternator. http://www.oregonmotorcycleparts.com/BEC.html With the alternator you may need a LCR type circuit.
 

t06afre

Joined May 11, 2009
5,936
Using a battery is an good idea. At least during circuit testing. Then you are happy with the design. You may use a small one or two watt DC to DC converter. Can you post a simple schematic of your setup. Do you use a filter cap after the rectifier. Or better a PHI filter
 

Thread Starter

acat800

Joined Dec 28, 2011
9
Below is the diagram of my power supply. It doesn't show the mains grounded but I have continuity from the leads to the chassis.
 
Last edited:

t06afre

Joined May 11, 2009
5,936
Does your snowmobile have any lights or indicator lightbulbs. If this is the case. It must also have some sort of regulator. If not any bulb would have a very short lifetime
 

Thread Starter

acat800

Joined Dec 28, 2011
9
yes, the AC is regulated. Like I said, I believe the issue is a ground loop. The microcontroller operates very stable alone, when I connected the thermocouple is when I ran into troubles.
 

t06afre

Joined May 11, 2009
5,936
If you do your testing using a battery. You should be safe. From what I can see from your schematic. I would have inserted a phi filter before the lm7812 regulator. At least used bigger cap say 4700uF
 

Thread Starter

acat800

Joined Dec 28, 2011
9
I did do my testing with a laptop, I don't have the option to use a battery on the machine unless I want to carry around spare batteries. The idea is to get it all working off of the supplied power. So, my question is, would an isolation transformer make sense to isolate the grounds or is there a better way?
 
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