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Adjustable PWM Voltage Reducer.

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Axle Roads

New Member
I need a circuit for an adjustable voltage reducer. My analog switching instrument voltage reducer (limiter) is broken and I would like to build a better one.

Input voltage 12V - 15V
Output voltage 5V - 7V
Max Output current 3Amp (estimated)
Battery -> IVR -> Load (gages)

I'm just beginning to learn about switched-mode power supply and pulse width modification; LM317, 555. Variation in input voltage will be minimal, so I imagine even a scr might work.

This is the closest circuit I have found so far. I found it on this site. I've looked at data-sheets and prev. posts. I'm having trouble finding something with a high enough amp tolerance. Not sure if I need a transducer. If I could just make a PWM unit to 6 or 7 volts I could maybe tune my gages with a variable resistor or Z-diode. What do you think?
 

dknguyen

Well-Known Member
Most Helpful Member
It is called a buck converter. The circuit you attached is similar to a buck converter, but is not one.
 
Last edited:

ronsimpson

Well-Known Member
Most Helpful Member
"My analog switching instrument voltage reducer (limiter) is broken "
Is this in a car?
 

Axle Roads

New Member
This circuit is for a 40 year old car. I recently upgraded the alternator and shorty after the IVR fried.
The stock replacement part is $35. I need to build something tomorrow to power the fuel gauge and temperature gauge. I'm hoping to do it using common ic for around 6-7 dollars. I need help with the details so I don't wast my money buying needless parts. I'm not sure how many amps these gauges pull, It might be just enough overload a LM317 (1.5A).

I like the idea of using am SMPS and PWM. It'll be similar to how the original worked and use a smaller heatsink than the linear reducers I've found on the web so far. Example: 1, 2.

Any suggestions?
 

ronsimpson

Well-Known Member
Most Helpful Member
I have an old truck with a dead instrument regulator like yours. I tried making a regulator using a 7806 and a heat sink. I also tried the LM317. They almost worked. Some of the instruments worked well and some did not. They were off a little in the readings. The original regulator, at 12 volts is on 50% of the time. Some of the instruments are OK with 6 volts and others needed the 12 volts at 50%.

I ended replacing all the old instruments with new ones and new senders. The new instruments do not need a regulator. They work from 12 volts. They look just like the old ones (if you black out the name).
 

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MikeMl

Well-Known Member
Most Helpful Member
Axel, do you know how fast the old bimetal reducer switched on/off? As Ron says, to make the instuments read right, some are expecting their input power to pulse on/off with something like a 50% duty cycle, but the rate is important. I vaguely remember an old Ford I had you could almost see the needles twitch :)
 

Axle Roads

New Member
Thanks guys. I 'm guessing the duty cycle is slightly less than 50 percent, since I've read that other people have had success using an LM317 set at 5.3 V. I ordered an adjustable step down switch-mode power unit.. It's adjustable from 3 V to 13 V output using screw.
I ran out of time in my search for the right circuit diagram. Hopefully this will be a cheaper than stock, more or less plug and play solution. Thanks again for your help.
 

moe7404

New Member
moe in wichita ks go to allpar i think some where they have an art on puting a 7505.. the circuit that reg runs uses 5-6 volts to run some of your gauges. its a common failur
 
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