VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt Voltage Reducer/Converter

We are working to install the original Harrison heater (6 volt motor) in a 1949 3100. The truck has been converted to 12 volts.

Should we be looking for a resistor to drop the voltage? Or should we use a converter? The motor does not run at full speed all the time.

In either case, are there any tips or tricks to selecting the proper size?

Interesting side note. While drafting this post I learned that there you can argue both sides of the "converter" or "convertor" spelling and not win or lose! "Converter" seems to have the edge.

I used this when I converted my 53 to 12 volts. The fan seems to work OK with it but only seems to have one speed instead of two. I have no clue as to whether that's because of this converter or a blower switch issue.

https://www.amazon.com/gp/product/B01CUA4KK8/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

Thanks Tiny!

I thought I remembered that you had used one on your ‘53.

I’m not surprised that you have only one speed. I am still trying to understand the nuances and the electrical implications of voltage reduction in a DC circuit.

Did you install the converter before or after the control switch? That might have an impact on how the converter works.

Time for some more research. We have a switch that appears to be a variable speed rheostat. Plus I want to try and confirm the actual amperage draw of the motor at full speed. It needs to be mounted in the housing with the fan installed to do that correctly.

It's connected between the battery and the switch.

Thanks!

Just a note. There are 12v heater motors available now that offer the same connecting screw spread and the same shaft diameters as the 6v versions. You'll have to shop around a bit to get the right dimensions and then disassemble the heater to get it all together correctly but it should be nothing you guys can't handle.

Getting all your gauges to work correctly... now that's a different issue. Maybe that is where a converter is better suited.

Overall the conversion was very straightforward. I think that every website that deals with pre-1955 vehicles has a reference article about this conversion.

We added a resistor to reduce the voltage to the fuel gauge. That is the only gauge that needed any adaptation.

We also added a resistor to reduce the voltage going to the points.

Back to my original question about voltage converters for the heater motor.

I think I am going to order a 20 amp unit. Based on the resistance I measure the motor could pull 14 amps. I know that is it an accurate way to determine current draw with an electric motor. There are induced currents due to the armature rotating in the field that are part of the equation.

I notice that the converters have 2 separate ground wires. There is a ground wire for the higher voltage input circuit and a separate ground wire for the 6 volt output circuit.

Do those grounds need to be isolated? That will be tricky because the blower motor is grounded to the chassis through the heater box mounting.

Are the two ground wires actually separate as in the resistance between the two shows an open (infinite resistance) or do they measure a short (zero resistance) between the two wires and therefore both are the same ground?

Either way, I would say a ground is a ground. If you know for sure they are both intended to be grounds then they are both to be grounded. It might have been easier to supply two external ground wires than to have connected them internally and then brought out a single wire. Although that does seem a little peculiar.

I guess you could have a situation where the 6v output (and ground) is connected to a separate chassis than to the 12v source chassis.

I have not ordered the converter yet. I was getting ready to when I saw the 2 separate ground wires.

My first thought was that a ground is a ground. Then I had second thoughts that maybe this was for 2 isolated circuits. We are dealing with solid state devices.

Not knowing the item you are talking about if you are using the unit on the same vehicle I would be connecting the 2 ground wires to the 1 point and most probably a mounting point.
Tony

It is a 20 amp version of the 10 amp unit in Tiny’s link.

FWIW we treated it as a ground is a ground. We made no special provisions for isolating the circuits. The only "issue" is the one speed fan that is either on high or off.

That one speed thing was in the back of my mind. Did the common ground create that scenario?

At least I know that I will not burn up the converter with a common ground.

Have you measured the voltage going in and out of you switch?

No, we haven't. I'm good with it so there's no motivation to tinker with it further.

I went out and looked at the converter that I'm assuming you're referring to. Red wire, yellow wire, two black wires. Since the two black wires don't identify to either the red or yellow, it would seem that there is no set requirement for which goes where.

Also looking at the converter, I'm going to say that its chassis is NOT part of the circuit. As in, it probably doesn't even connect to ground. This means that in and out grounds have to be connected into the circuit and not just rely on the outside chassis for ground.

All that being said, it does appear that both grounds are just grounds but connecting just one and not the other will probably NOT complete the circuit. Both will need to be grounded. I would say connect each ground wire to a converter mounting screw that will be in turn grounded to the vehicle chassis. 12v to the red wire, 6v to the yellow and walla. "It's Alive".

And besides, if I'm wrong, what's a little smoke amongst friends? 😄

I have learned that it is hard to put the smoke back.

The converter was delivered yesterday. I checked a few things with an ohmmeter.

- There is no continuity between any of the wires and the aluminum case/chassis.

- The 2 black wires (labeled In- and Out-) have continuity between them so are obviously connected internally.

- There is very high resistance among any of the pairs of wires except the black ones. I connected the red and black wires. The initial reading was around 32 kilo ohms. Even more interesting is that the reading immediately started going up at a fairly steady pace. I disconnected the meter when the reading had reached 70 kilo ohms. The yellow to black and red to yellow had the same behavior. I assume that the small current from the ohmmeter was creating heat internally.

I will take some voltage readings after we install the heater and controls next week.

I'm going to say that what you have is a standard transformer. A winding for the input and a winding for the output. Picture a wire wrapped around a pencil. One end of the wire is connected to 12v and the other end to ground. This is called the primary side of the transformer. And let's say this wire's length in total is 12 inches long. Then take a second wire (the secondary side) which is only 6 inches long and wrap it around an identical second pencil. Connect one end of that wire to a 6v load and the other end of that wire to the same ground as the first.

The effect that you have is the primary wire will attempt to transfer that 12v to the secondary wire but because it's shorter, it will only transfer 6v. The transformer on your power pole (out by the road) is the same thing. It receives let's say 440v into the transformer but because of the secondary wires that are only a 1/2 in length, the output is only 220v.

And the resistance that you saw change was a capacitor that will basically charge up via the Voltage that your meter generates. In this circuit it is used to keep from smoking the winding between the 12v (and the 6v) and the ground.

I believe if you would have left your meter on the red and black it would have charged up to read infinite resistance. And if you swapped the meter on the same two wires, you would see it go from low to high resistance... again.

When you checked between the red and the yellow, you were actually checking the resistance of the whole transformer. That would probably tell you if the transformer was blown but not much else.

I agree that it is nothing technically earth-shattering. Just 2 coils and an iron core. Plus the capacitor.

The charging rate was relatively slow because an ohmmeter does not produce much current (by design).

It is a decent looking unit with the die cast aluminum finned case.

I am trying to decide if it needs to be mounted as flat as possible on sheet metal to help dissipate the heat. It will be hard to find a flat surface behind/under the dash.

Rusty I have another theory, mount the unit with an air space for cooling. To use the sheet metal as a heatsink clean metal and heatsink compound would be required.
Tony

FWIW on my 53 we just attached it loosely to a handy bracket left it hanging in open air.

Thanks for those good inputs.

Tony’s comment about heat sink compound reminded me about the first generation GM electronic ignition module. There was a small packet of a silicone type substance in the box with the replacement unit. The instructions stated that you needed to wipe the mounting area clean and apply a layer of the substance before installing the module. This compound would improve heat transfer from the module to the mounting plate in the distributor.

While these modules were fairly reliable I know that myself and others kept one in the glovebox. If it failed you were obviously stranded.