View Full Version : Looking for a 12VDC power supply

02-14-2013, 09:58 AM
Basically I'm looking for a power suppply that will convert 12ish VAC to a clean 12VDC.

The back story is that it is for a motorcycle that does not have a battery. It is my understanding that the battery provides a substantial buffer that smooths out the DC coming out of the voltage regulator. My friend has a two stoke moto with no battery and no electric start so it has no DC wiring at all. He has added a Trail Tech Voyager speedometer/GPS that needs power to charge it's internal battery. The Voyager unit can accept AC or DC over a pretty wide range - sort of. The voltage regulator circuit in the Voyager has now failed twice. It has been replaced under warantee but they are now saying that they have seen this problem "on some bikes" and rec' charging the unit with a 120VAC plug in wall wort before using it.

I figure that adding a small power supply to the bike so that the Voyager unit receives a cleaned up signal should solve the problem. I know there are several ways to get there: A large capacitor across the AC signal should help. A bridge rectifier with a small cap would likely work too, but is there a durable compact, off the shelf packaged supply that includes a volatage regulator? 500mA should be more than enough, heck 100mA might do it.

02-14-2013, 11:22 AM
You'd need to step up the 12VAC a bit before you can rectify and filter it back to 12VDC. Since you're only talking 100ma, that would help.

If this bike is utterly free of electrical bits, does it even have an alternator? If so, what is the raw AC output voltage of the alternator?

02-14-2013, 01:31 PM
It has an alternator that is probably putting out 30-50 VAC which goes to a regulator that knocks in down to 12ish. It has a headlight but it is AC. the clean DC power doesn't have to be exactly 12VDC but that is the target.

02-14-2013, 02:58 PM
My Kawasaki Bayou 220 (1989) alternator puts out 45VAC, and it goes through a regulator/rectifier (nothing else) and charges the battery. I wonder if you could intercept the alternator output (mine has 3 windings) and use a motorcycle regulator. You might have to install a small motorcycle battery or gel cell (alarm system battery) to filter the output. You would need a decent wiring diagram showing the current bike setup and the setup to which you'd like to migrate. My system can theoretically put out 13a at 12VDC and is protected by a 20 amp fuse.

02-14-2013, 04:02 PM
I have my factory KTM regulator/rectifier and stator that I no longer need. You are welcome to them. I'd contact Trail Tech and see if they offer something. http://trailtech.net/stators_and_electrical_system_kits.html

02-15-2013, 09:19 AM
Does it even have a rectifier? Headlights are not specific, they don't care if the current is alternating or not. If there's no battery or DC accessories it might not have a rectifier at all.

If the gizmo can accept AC then it might be that all you need is a TVS. Alternators are very noisy and the spikes can be pretty substantial and the bike is blowing out the rectifier inside the power supply. Things like this are often cumulative, so it'll work for a while but after some period, say 50 or 100 hours, the 1ms long 200V spikes just wear out parts to failure.

I would just hit Radio Shack, a couple of diodes and a couple of caps potted into epoxy is going be cheap.

02-17-2013, 07:30 AM
I'll look into TVS - new to me. I have used MOV before but not sure if that would help here.

When you say "Radio Shack .... couple of diodes and caps," do you mean build a DC supply?

Something like the pic below without the transformer?


02-17-2013, 09:01 AM
Yes, that is a very basic rectifier. The device sounds like it has a rectifier already in it, it accepts up to 150VAC, right?

That would not solve the transient spikes from the alternator, though. Where you're gonna solve the problem is filtering. But you would not really want to try to filter AC, you want to filter out AC in this case and it's more complex than needs to be. Large value caps tend to be polarized and so you would have to be careful not to reverse the polarity, thus the blocking diode(s).

Something that occurred to me, you might try this filter. It's very basic but does work to cure alternator whine when operating HF mobile (which remember is running from already rectified power). They used a 4700uF cap to swamp the power, but it won't hurt anything to put a couple small values in, too. I'd put in a 1uF and 0.01uF as well. This is essentially just a monster low pass filter and the smaller values are for high frequency noise.