6 to 12 volt conversion

[arcturus]

Re posted, was posted accidentally in 144 forum

Hi Ron,as you know I recently converted my PV 6v to 12. I am finding that the wiper DR gets VERY hot and was thinking that I should spread the load. Looking at the write up I note that the blower motor DRs would be wired in parallel but the wiper should be wired in series.Can you please explain why that should be so?

Heater blower motor. DR = [0.6Ω 150W, (3 2Ω resistors in parallel). Bill reports a 90 degree F rise above ambient after 15 minutes of operation.

Wiper motor. DR = [0.6Ω 150W, (3 2Ω resistors in series). Bill reports a 40 degree F rise above ambient after 15 minutes of operation.
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Derek,Mounted directly onto bulkhead,obviously not a good conductor

[Ron Kwas]

Arcturus;

The explanation is that an error crept into my write-up (I apologize for this!...I will correct it ASAP, but I would like some actual measured voltage info from you to verify the correction!).

In studying the text, you will notice that 3 2 Ohm resistors in parallel calculate to 0.6 Ohms total, and 3 2Ohm Rs in series calculate out to 6.0 Ohms total...so I believe the error is a simple placement of the decimal point, but I will verify the posted calculations. (a decimal point error is seemingly minor, but this is actually a 10X value difference...and less of an error has caused caused satellites to fall from the sky!!)

Please confirm current for both Wiper and Blower Motors at 6V, and I will run the calculations again to verify the correction...according to my info, the Wiper Motor draws 5A at 6V, and I don't have actual measured current info for the Blower Motor, so will run the calculation for this when you supply a measurement. Again, I apologize for this pilot error...

As far as being mounted on a bulkhead, the sheetmetal will actually allow the heat to spread somewhat...and that is desirable, (a thermally conductive grease would help transfer the heat) but if it is not enough and you feel unacceptably high heating is still occurring, you could add a heat sink (with fins) on the other side of bulkhead OR separate the three DRs physically (to spread out the heat) further...OR reduce the R of the individual DRs and add more, keeping the total DR array R equal to the calculated value (this would also spread the heat out). Remember, the amount of heat energy produced does not change, but by spreading it out, the peak temperatures reached will be lower at any individual R.

Cheers

[arcturus]

Haven't been able to verify the numbers but as an experiment I connected three DRs in series as suggested and the wipers barely move. Should they still be the 50w 20ohm DRs or should the values not be changed.I was under the impression that the series connection cut the voltage. Please excuse my ignorance in this matter. As explained before I'm hopeless at electrics.
I still don't understand why the blower should be connected in parallel and the wiper in series as he suggests.

[Ron Kwas]

Arcturus;

You need to be working with 2.0 Ohm resistors! ...3 20 Ohm Rs in series will give a total of 60 Ohms, so the fact that Wiper Motor is not moving at all shouldn't surprise us...all the voltage is being dropped across the DR, whose R value is waaaay to high!

To answer your question: "I still don't understand why the blower should be connected in parallel and the wiper in series as he suggests. " ...this allows us to come to a DR total, vaule suitable for the two motors with very different power characteristics (Blower Mot is a low power motor just moving air, Wiper Mot must be a fairly high power motor capable of moving a serious mechanism plus in some instances, snowload)...putting them in series increases the total R (I expect this might be OK for Blower Mot), and putting them in parallel decreases the total R (I expect for Wiper Mot which is drawing a lot more current).

Again, the Rs SHOULD BE 2.0 Ohm, and for purposes of test, start out with them in Series for a total of 6.0 Ohm, and measure voltage they leave for Wiper Motor. If Motor V is less than 6V, try them in parallel for a DR total of 1.2 Ohms, and measure Motor V. Please report back with findings.

Hope that helps!

[arcturus]

Hi Ron,I had this from the post I accidentally put on the 140 forum. Seems an easier way. What's your opinion?


Not Ron or Bill!

I am presuming that you are using a series resistor pack to allow you to run a 6 volt motor from 12 (actually more like 14 volts)? Reading between the lines, your 3 x 2 ohm resistors in parallel has an equivalent resistance of 0.666 ohms, so your design motor current to create an approximate 6 volt drop across the resistors is 9 amps. At 9 amps your heat output is going to be roughly 54 watts so those resistors are going to get really, really hot. The heat dissipation rating of high wattage resistors is based upon the assumption that the resistor is mounted on an heat conducting substrate (metal chassis) that is held at a specific temperature. Mount those resistors in free air and their temperature will go through the roof. So, first observation is that the resistors need to be mounted on a large metal substrate (the car body) and make sure that nothing meltable / combustible gets close because they will likely still get quite hot.

My second observation is that using voltage dropping resistors for this type of application is very mid 20th century. My initial reaction would be to fabricate a simple pulse width modulation control just like the blower speed control on just about every car built in the last 20 years. This could be done using a 555 timer and a power transistor for the equivalent of about $15 Cdn. With a little fiddling this could be set up to operate as a variable speed control. If you are disinclined to mess with electronics, the other product of the late 20th century is the modular buck/ boost power supply. These are high efficiency devices that are available in a wide array of input and output voltage options and they are reasonably low priced. I can't advise whether this one is any good; but, it gives you an idea of what is available

https://www.amazon.ca/Converter-Regu...ag=googcana-20

As a further observation, if you have more than one 6 volt device that you want to retain, I would be inclined to put in a single buck converter capable of supplying the total 6 volt load and power it up with the switched 12v from the ignition switch (through a fuse). Then run a separate 6 volt supply network (with individual fuses) to the remaining 6 volt devices in the car. This avoids the initial start up hick-up that some converters can have on initial power up. It also allows you to mount the single converter solidly on the body for heat dissipation purposes. Buck converters are typically high efficiency devices (>90%); but, they still generate some heat and electronic components don't like heat.

The other reason to use a buck converter is that they are regulated (insensitive to load current). Whether your blower current is 9 amps or 5 amps you will get 6 volts out. If your measurement of current draw from your blower is incorrect, the voltage drop across your resistors will not be 6 volts which may result in unexpected operating characteristics. The buck converter will be unaffected by your measurement uncertainty as long as it is sized to supply the maximum current.

All this presumes that the PV was negative ground originally.
Last edited by 142 Guy; Yesterday at 16:53.

[Ron Kwas]

arcturus;

I also know 142Guy from the Swedespeed Forum, and I know him to be a technologically capable, well-informed, and helpful contributor to the vintage Volvo community whose opinions I do respect.

That having been said, I can't even disagree with his calling the DR solution "from the 20th century"...they are! I have previously stated in the tech article that DRs are a brute force and absolutely simple way of getting 6V from 12V, because they basically do this by turning an equal amount of energy (which the load is using), into heat...so its not the most technologically advanced or elegant solution, and I'm quite sure you wont find it on the International Space Station, but one thing DRs do have going for them, is they are bullet-proof in reliability because they don't use chinesium consumer electronics with questionable long-term reliability...I might just note, that these cars are currently 50+ years old! ...so the reliability goal in my recommendation is another 50 years!

If 142G ever owned a 444 which was changed to 12V and needed to make 6V, he might, as a technologically savvy guy, would probably want to install the more modern, high efficiency PWM controlled buck/boost converters, as the other solution option, but I can guarantee those electronics will never be as reliable as a DR solution...yes, I said never!

I stand by my advice!

Cheers

[arcturus]

So three 60 watt two ohms? Will have a look and see what i can find.

So far no luck. Should it be 20 watt x3? Don't think so.

[Ron Kwas]

Yes, get high wattage rated 2 Ohm Rs!

[142 Guy]

Ron:

The dropping resistor is a robust solution. Volvo used dropping resistors for speed control on the blower motor on my 242 GT and I believe on my 745 Turbo. If only the blower motors on those cars had been as robust as the dropping resistors!

Dropping resistors are an acceptable approach for running low current LEDs off of 12 volts. My primary dislike of the dropping resistor solution is the heat issue. I have linked the data sheet for some TE surface mount power resistors.

https://www.te.com/commerce/Document...ocLang=English

Two important things to note. On the first page in the table listed under dissipation you will note that if you don't mount the resistor on a heat sink operating at 25C the HSA50 50 watt resistor must be limited to 20 watts if in free air or mounted on a non aluminum heat sink. On the third page is a graph titled surface temperature rise. For the HSA50 50 watt resistor operating at 50 watts, the predicted surface temperature RISE of the resistor is 140C even when mounted on a heat sink (hence the comment in my previous post 'still get quite hot'). My advice is that once you have calculated the power dissipation in an individual resistor you purchase a resistor with a power dissipation rating at least 2 -3 times the calculated power even if you mount the resistors on the body. The cost increase is not huge and the higher wattage resistors will run cooler when operated below their rating. Do shop around since the prices vary significantly. In North America DigiKey, Mouser, Allied Electronics are good vendors to select from. I don't know about Euro or GB vendors.

My second dislike of the dropping resistor solution is that you absolutely need to know the supply voltage (does your car operate at 13 volts or 14.2 volts) and have a measurement of the current draw of the motor in order to accurately calculate both the resistance and the wattage rating. You can pad the numbers to account for uncertainty in these values; but, then you run the risk of an underperforming blower / wiper motor if you pad the numbers too much.

Note that PWM and buck are two separate technologies. The advantage of the buck technology is that most of them are controlled devices. They typically tolerate a wide range of input voltage while holding a constant output voltage and control that voltage regardless of what the load current is (as long as you respect the maximum load current of the device). The comment about Chinese consumer stuff has some merit, particularly when it comes to the knock-offs. But, Chinese based manufacturers (which may or may not be Chinese owned) run the gamut of quality and can produce high quality product or poorly designed junk for people who don't want to pay for good stuff.

I can't comment on the quality of the Uxcell converter I linked (at least it is not a knock off). However, on Digikey and the other electronic vendor sites you can find small modular buck converters in an acceptable price range. The problem with searching those sites is that you need to know what you are looking for to whittle down the literally tens of thousands of options.

The PWM option I floated was a home built solution. However, it does require some electronics knowledge and soldering skills. The simple solution I was thinking of would tolerate differences in load current; but, would be affected by changes in the input voltage. It is cheap and its primary advantage is much lower operating temperatures.

For those wanting to stick with the robust dropping resistor solution, my primary words of advice are to go big or go home on the resistor wattage rating. Oversize the wattage rating to reduce the resistors operating temperature. You don't want the resistor outlasting the 444 because the 444 caught fire!

[arcturus]

I have already had one engine bay fire and don't went another!