OT: Need Informed Opinion re: IVR

I have a diagram someone sent me a long while back when I was searching for in Instrument Voltage Regulator for converting the old King-Seeley gauges in my old International from 6V to 12V, without letting the smoke out of either the gauges or the senders.

Parts required include (1) 7808 voltage regulator; (2) 1 uf 25V tantalum capacitors; (1) 2N 5881 transistor, and a corresponding insulating washer/transistor socket; (1) IN4002 diode; and (1) 470 ohm 0.25 watt resistor. Not counting the heat sink, which wasn't priced, the original diagram came with a parts list that total around $8. Unless prices have gone haywire, today it should be able to duplicate for under $10, I would think.

According to my friend the electronic expert, this should change a 10V to 18V input to a 6.8V output, and according to him, there should be no worries about voltage spikes.

So does anyone WITH ELECTRONIC DEVICE EXPERIENCE agree with my friend? Should this work as he says, as long as an adequate heat sink is used?

The drawing isn't clear where exactly the 6.8v output line is connected to the transistor. It should be connected to the emitter, as it is an emitter follower circuit. Given that, it should work just fine.

I have no doubt this circuit will work, although it's a bit of overkill for the application. I would have used a simpler circuit.

The LM7808 device is an 8 volt regulator. The 1N4002 diode provides a .6 volt drop, and the base-to-emitter junction of the 2N5881 drops the voltage another .6 volts to get you to 6.8 volts. The 2N5881 has a maximum collector current of 15 amps, so you have up to 15 amps available to drive your gauges. Note that at 15 amps, the 2N5881 will need to dissipate 120 watts, so you'll need a healthy heat sink to do that. I would ditch the two capacitors from this design (potential failure points that don't do much), but otherwise it looks fine.

Note that the TO-3 package of the 2N5881 is a bit of a pain to work with. If you can get by with only 1 amp, the LM7808 can do that on its own. You can put a pair of 1N4002 diodes in series with the voltage regulator output to get your voltage drop.

Another solution is to use an LM350K adjustable regulator, which can provide up to 3 amps, requiring only a couple of resistors in addition to the VR. The LM350K is a TO-3 package. In a TO-220 package you can get the LM317 which is good for 1.5 amps.

As I understand it, this IVR is supposed to be capable of 10 amps constant, and near 25 amps peak...enough to run a 6V radio as well as the gauges.

At 10 amps the 2N5881 needs to dissipate 60 to 70 watts. At 25 amps you're looking at over 150 watts. It's going to take a monster heat sink to keep that transistor cool.

If you really need 10 amps steady-state and 25 amps peak, a switching regulator is the way to go as it doesn't need to dissipate a lot of heat like a linear regulator. However, building a switching regulator is a non-trivial exercise.