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My third transformer for my Streetglow neons is blown, and SG tech support says my alternator is putting out too much voltage. (15V according to my cheap digital multimeter) I need someone to double check my equations for a 3 volt drop. The transformer says it uses 5A max.

Voltage reduction:
V=IR
3=(5amps)(Ω)
3/5= 0.6 ohm resistor

Power of resistor:
P=(I^2)R
P=(5^2)(0.6)
P= 60 Watts

So I need a 0.6 Ω, 60 watt resistor? That sounds ridiculous!

I did find a similar resistor (0.5 Ω, 50W), and it's HUGE.

That's 5A max, do they give you a rating for the actual current those consume? I highly doubt it's anywhere near 5A.

Regardless, this won't accomplish what you need. You need a voltage regulator to provide power to your light setup. Since the voltage consumed by this resistor will be entirely dependent on actual current flowing.

Without turning your car on, measure the current going into the lights, since that should be about 12V.

Thanks! I'll have to install the new transformer before i can test the current in operation though. A voltage regulator is probably gonna be more than it's worth.

PM me when you need another hand with this. I'm literally never on here.

It would be so much easier to use one or more power diodes in series. Here's one that can handle 8 amps forward current and drops 2.6 volts across it, and costs a whopping 77 cents:
http://www2.mouser.com/ProductDetail/Fa ... HiuQ%3d%3d

There are many other diodes available that can handle large currents with various voltage drop. The main advantage is that the voltage drop will be absolutely constant and will never vary due to fluctuations in the load or source voltage. -And it's way cheaper.

But I'd do some troubleshooting first. Your car supplies DC, and your neons are burning out a transformer. Transformers use AC, so there's clearly a chopper circuit in the mix. I'm skeptical that the problem is your Alternator's output. I'd want to see the details of the neons circuit.

I took a look at the spare tranny, and the label is very ambiguous as to whether it uses 5a or 0.5a. Have a look:



My input wire goes straight from the battery through a 10a fuse to a 30a Bosch relay and back to the transformer. The transformer is supposed to be grounded by its mounting screw to the car body, and i also added a ground wire to the engine subframe. The transformer outputs to the four neons connected in series with 15,000V 18awg GTO wire. All this came with the streetglow kit except for my relay and extra ground wire. I talked to SG tech support and they said the transformer is highly sensitive to anything over 12V and that's as far as they would help me. I dunno of any vehicle that runs 12V with the engine running anyway...sounds fishy.

The diode sounds like a good idea...i didn't know they could reduce voltage like that.

edit: Here are the installation instructions FWIW. http://www.streetglow.com/site/Media/Ma ... g201mi.pdf

I think it means 5 amps. If you look at the dot after 12V, there's the same thing after 9500V, and it calls for an 8 amp fuse, so that would be a reasonable choice to protect a 5 amp circuit.

From your description of the hookup, I assume you've actually measured that you're putting 15V into the Neon unit. I'm sure it's chopping the DC to run through its transformer. Yes, a diode is a great way to drop a small amount of voltage, but most common diodes are small signal diodes made of a single P-N silicon junction that typically drops 0.7 volts DC. Most of those common diodes would be instantly fused if you shot 5 amps through them, but there are lots of high-current high-power diodes used in power distribution systems, and that's what you can use in this case. The one I found for you is, I think, a really good choice. Give it a decent heat-sink and it will drop 2.6 volts for you.

If SG claims the device is sensitive to even a small amount of overvoltage, it's worth at least trying that solution, and I think a power diode like this is the perfect solution for high-current where you need to drop a small bit of voltage. Just put it inline in the direction of the circuit arrow, anode to the battery and cathode to the Neon unit.

Battery----|>|----Neon unit

By heatsink, do you mean just screw that mounting tab onto something metal? I've got some Arctic Silver...i could put that in-between.

Generally, yes. A power diode is likely to get hot if it's passing something like 5 amps. This will be normal, but the spec sheet will probably call for heatsinking it. Just follow whatever the spec sheet says. The tab for sinking may already be physically insulated from the anode and cathode, or it may *be* the cathode or anode, in which case you'd need to use a mica washer and heatsink thermal paste, and bolt it to a little finned heatsink. Either way, the spec sheet will specify any heatsinking requirements.

Thanks. I ordered a couple in case the first one burns up. I'll try mounting the first one to the steel base of the transformer for simplicity, then come up with a better idea if it burns up. I hate drilling holes in my engine bay haha.

John Deere Boy wrote:Thanks. I ordered a couple in case the first one burns up. I'll try mounting the first one to the steel base of the transformer for simplicity, then come up with a better idea if it burns up. I hate drilling holes in my engine bay haha.

I'm on board with this plan!

Do a simple continuity test with your digital multimeter from the heatsink tab to the anode and from the heatsink tab to the cathode. See if the heatsink tab is electrically connected to either pole of the diode. Hopefully, it's not connected to either. I checked the datasheet and it doesn't say.