12v to 6v voltage regulator at about 15 amps

Hi, i have a 1963 VW bug that is very stock, but the 6v lights just dont work very well (dim at idle, then not too bright at speed) it is very simple to use a later 12v generator and then swap out all the lights, battery, coil and relays and it works great. BUT i have a 6v wiper motor and also a 6v gasoline heater that i will need to keep at 6v (the 12v parts are just not available or too expensive)

I have found an article that recommends building a cheap voltage regulator using a 7806, ( 6 volt, 1 amp regulator) and 2 1uF 35v tantalum capacitors, then a NPN transistor which the original poster says is a 2N5881 npn transistor. here is the scematic.

the pdf article is attached and the link to the original article is here http://www.studebaker-info.org/tech/6-12V/6-12-6.html 
The questions i have
1. what is a 2n5881 been replaced as? i cannot find it at jameco and the original posted doesn't give any specs for it.
2. how would i "pump up" the amps available from 10 to anything else (looking for about 15 amps) thinking that the npn transistor is the key but i am new to this and wouldn't even begin to know where to look.
3. what happens if any item fails? can i put some sort of safety into this so it quits rather than passing any addition voltage through

additional info
the heater runs at about 85 watts so 85 watts / 6volts = 14.616 amps 
the wipers run at much less than that so building 2 of this would work great. 
Any ideas?
Thanks in advance for any insight to this 


Picture of 12v to 6v voltage regulator at about 15 amps
sort by: active | newest | oldest
starvin (author) 5 years ago
I had not thought about mouser..thanks! I do know the 2n5881 is obsolete and have just found 20 of them at a reasonable price but there is a 25.00 minimum order so i will most likely buy them all.

hmmmm transorbs. not heard of them.will have a look THANKS!!
Treth starvin5 years ago
You may want to look at 2N3055 15A max, but different manufacturers have different gain figures, try to find a high gain part, I think STM were good.
Alternatively consider 2N3771 these are 30A Max and possibly better for your requirements. Even 2N3772 at 20A might be OK. These are all available from Farnell in UK, so Mouser might be better.

Do a search on transorbs as they are good for removing spikes. An example (not great but easy to understand) http://www.mglavionics.com/Transorb_power_supply_surge_protection.pdf

And here is a thread on the difference between a zener and transorb.
http://lofi.forum.physorg.com/Zener-VS-Transorb_7575.html
Treth5 years ago
This is a lot more involved than I realised, but you seem to have everything I can think of covered!

Something to consider as the heater is a resistive device why not just put a high power resistor in series with the heater? You have to disipate the heat whether you have a linear regulator or a passive resistor.

You need a resistor(s) that dissipate a similar power 85W and the resistance will be approx 0.42 ohms. You can parallel or series connect to get what you want, say two 0.22 ohms in series each at say 50W bolted on a heat sink/chassis. You could also place these near something that might benefit from a bit of heat on a cold day, like the windscreen wash bottle.

You can then use someting similar to the circuit shown to power your motor and use an easier to get transistor and keep the current rating lower.

Hope that helps with thoughts.
starvin (author)  Treth5 years ago
while MOST heaters are resistive this one actually uses a coil and spark plug to fire gasoline in a chamber over which air is drawn via a blower into the car. The coil and spark plug i can swap to 12v but the fuel meter and blower are 6v and i need to power them somehow.
I would use the above but i CANNOT seem to find the NPN TRANSISTOR marked as 2n5881. and being less than knowledgeable about how they are numbered i thought i would ask the collected mass of instructables what i could swap it with. If need be I will use a lower rated transistors but don't know which one to pick.

and yes a gasoline fired heater sounds scary but they were used for decades. this one will be checked and double checked (with a carbon monoxide alarm placed in the car as well)
Treth starvin5 years ago
Interesting and yes sounds scary!

Not sure of their terms and conditions but
http://www.futurlec.com/Transistors/2N5881pr.shtml appear to have them and have a link to the datasheet.

You could also checkout Farnell, Mouser, RS Components.

Your concern about failure is also appropriate as battery lines can have spikes and surges as items switch in and out. Look up transient suppressor or transorbs, there must be lots of data out there.