Need help on a 12ac to dc circuit.

Ok, I'm trying to install a HID headlight on my snowmobile. The stock headlight is powered by a 12v ac line. My plan was to install a bridge rectifier and a capacitor to power the HID and I have a timer relay hooked to a battery to ignite it. I can not use the battery as a powersource because the charging circuit only puts out ~2amps.

So I thought that I could intall a bridge rec and get 12vdc. But here is what happens. After hooking up the bridge I measrue ~12AC on the input and 12vDC on the ouptut. If I swith the meeter to ac and measure the outputs I get 20+VAC.. I ran across this problem because I installed a large cap across the output and then measured the voltage and got 24vdc. I have tried three different bridge rectifiers but get the same results. What am I missing here? I'm usng a 35A 200 PIV Bridge. Its from allelectronics.com pn FWB-352.

I'm totaly stumped on the summing of the voltage. And ideas?

I'm presuming the AC voltage is directly off the engine?, as common on small motorbikes. The voltage output on these varies with engine speed, and you can't get any more power out of them without reqinding them (if there's room).

Personally I would suggest you forget the idea, or totally redesign the electrical system, starting with a much larger battery and much more powerful alternator, with a regulator system.

The machine came with a 60/55 headlamp. So there should be plenty of power to push the 35w HID. I even had it operating once. But for some reason its now sending out way to much voltage.. I'm still trying to figure out why there is ac voltage on the output side of the bridge? Which builds up in the capacitor.

After the picture was taken I installed everything fired it up and bam blew the ballast. Didn't know why and then I started measuring things again and that's where I ran into this problem. I have check the regulator with my other snowmobile and its output is right.

BTW the machine is equipped with a regulator/rectifier.

Nigel Goodwin said:

I'm presuming the AC voltage is directly off the engine?, as common on small motorbikes. The voltage output on these varies with engine speed, and you can't get any more power out of them without reqinding them (if there's room).

Personally I would suggest you forget the idea, or totally redesign the electrical system, starting with a much larger battery and much more powerful alternator, with a regulator system.

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jwiedmayer said:

The machine came with a 60/55 headlamp. So there should be plenty of power to push the 35w HID. I even had it operating once. But for some reason its now sending out way to much voltage.. I'm still trying to figure out why there is ac voltage on the output side of the bridge? Which builds up in the capacitor.

Click to expand...

12V AC (which is RMS) gives around 18V DC when rectified and smoothed, it's supposed to be like that. I've never played with HID, but I seem to recall they are quite critical about their supply?. Your supply just off the engine will vary all over the place.

There can't be AC on the capacitor, the capacitor would short it out - what you're getting is the rectified and smoothed DC.

After the picture was taken I installed everything fired it up and bam blew the ballast. Didn't know why and then I started measuring things again and that's where I ran into this problem. I have check the regulator with my other snowmobile and its output is right.

BTW the machine is equipped with a regulator/rectifier.

Click to expand...

I would expect the ballast to blow, you're abusing it!.

Nigel, thanks for quick input. Why does my volmeter read ac on the output of the bridge rectifier? Is it just picking up the peaks? It measure 24Vac yet 11Vdc on the outputs.

The other idea I had was. To install another rec/reg on the circuit that has the headlight on it and install a battery. But I was wondering what the effect would be of have the ac regulator in series with the regulator/rectifier. I would think one might be battling the other.. Any thoughts?

What current does it draw?

Use a rectifier with a low dropout switching regulator on the output.

jwiedmayer said:

Nigel, thanks for quick input. Why does my volmeter read ac on the output of the bridge rectifier? Is it just picking up the peaks? It measure 24Vac yet 11Vdc on the outputs.

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Most meters on AC will read a DC voltage as well, but won't give any indication of the real value.

Every digital multimeter I've used reads 0V when measuring an AC voltage on the DC range. This is because they measure the average voltage. I suppose if the AC isn't a true sinewave and is more positive or negitive then it will read a small voltage but other than that it will be 0V. This is dangerous if you're working on a mains circuit and you don't know whether you're expecting an AC or DC voltage, it's always good practice to try both ranges.

I believe it draws less then 10amps at iginition and then stablizies down to 3-5amp.

Do you have any examples of a low dropout switching regulator?

Hero999 said:

What current does it draw?

Use a rectifier with a low dropout switching regulator on the output.

Click to expand...

When I measure the inputs of the rectifier on the dc scale the meter reads . When I measure it on ac it read around 11 volts at idle. The output reads around 11v dc and 24+ on the ac scale. Then is I hook up the capacitor to the outputs it will go up to around 24 volts dc and stablize. I'm still lost at how the meter will read ac and dc on the output? Lets pretend it was 12AC then wouldn't the output be ~16.8 assuming a true sine wave and the rms theory? It's really weird that the ac read is nearly twice the dc. Any other thoughts?

I explained above, your meter, on the AC range, will also read a DC voltage - but it won't give the correct value. Simple answer, DON'T have your meter set to AC when you're measuring DC.

It probably isn't true sinewave, the only way to find out would be to look at it on an oscilloscope.