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regulator 12v, 25amps fpr power supply.

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rc technics

New Member
I hope some one can help me. I have a non-regulated 25-30Amp 14v DC power supply, for use on a lipo charger, which requires a regulated at 12 volts at 25amps. Currently my voltage is going up and down 13.6v to 16.3v

Please can some one show me a diagram and the parts I need to build my own regulator?

Volts: 12
Amps: 25
 

Richard S

New Member
How smooth does it need to be?

Dones it need to be small or light? If not, then just use a big transformer (300W) to change mains into 15v or so, then regulate it down using a 7812 and a bypass transistor.

Richard
 
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mvs sarma

Well-Known Member
I hope some one can help me. I have a non-regulated 25-30Amp 14v DC power supply, for use on a lipo charger, which requires a regulated at 12 volts at 25amps. Currently my voltage is going up and down 13.6v to 16.3v

Please can some one show me a diagram and the parts I need to build my own regulator?

Volts: 12
Amps: 25
If it is used for charging application, perhaps that much strictness for voltage is not called for. OK as you are asking for lithium potasium battery charging, dropping by 1.6V may be easy by using froward drop of high current diodes. you get dual diodes in TO220 or larger packages.
you may adopt two of them.
 

Richard S

New Member
Or just a 25A+ rectifier - very cheap and simple.

What's the charger like? If it is expensive, with an LCD screen and PIC inside, it will need a much more stable voltage then if it is a purely analogue device.

Richard
 
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bountyhunter

Well-Known Member
I hope some one can help me. I have a non-regulated 25-30Amp 14v DC power supply, for use on a lipo charger, which requires a regulated at 12 volts at 25amps. Currently my voltage is going up and down 13.6v to 16.3v

Please can some one show me a diagram and the parts I need to build my own regulator?

Volts: 12
Amps: 25
If the unregulated voltage drops to 13.6V under load, you don't have enough headroom to provide a regulated 12V output at 25A using a traditional linear regulator design. You could maybe do it using what is called a "low dropout" configuration which requires the use of either a PNP or P-FET pass transistor. To handle 25A, you would have to parallel some power transistors and have a really good heatsink..
 

mneary

New Member
Why has no one mentioned the proper charge mode does NOT use a voltage regulator?
My reading of the OP is that the LiPo is a red herring. OP wants 12V regulated as the input to a box called a charger. He has a DC supply that supplies 13.6 to 16.3V.

So he wants to create a device that accepts 13.6 to 16.3VDC in and provides 12.0V out. The circuit isn't complicated, but construction isn't going to be easy.

I would use an LM2940CT-12 with a current booster similar to this:

An example of current boosting of a regulator is given in Figure 9 of the attached data sheet. Note that MC7800 is NOT an LDO, it is just provided for figure 9.

The transistors will need to dissipate up to 107.5 watts, so an enormous heat sink with forced air cooling is a must. Also, you'll want four or five of the 2N2955's in parallel, with a resistance in series with each emitter. This resistor can be about 0.01 ohms if all transistors are on the same heat sink. 6 inches of 22AWG wire on each emitter will do the job.
 

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bountyhunter

Well-Known Member
you can use pwm to do the trick
Actually, a PWM switcher bucking 13.6V down to 12V @ 25A is probably going to be LESS efficient than a linear because the duty cycle is so high the switch is ON nearly all the time and you have high switching losses. I'd go with a linear.
 

bountyhunter

Well-Known Member
My reading of the OP is that the LiPo is a red herring. OP wants 12V regulated as the input to a box called a charger. He has a DC supply that supplies 13.6 to 16.3V.

So he wants to create a device that accepts 13.6 to 16.3VDC in and provides 12.0V out. The circuit isn't complicated, but construction isn't going to be easy.

I would use an LM2940CT-12 with a current booster
Find a different part to build a current boosted LDO around. The LM2940 is marginally stable even as designed, with a very severe restriction on the output capacitor's ESR to achieve stability (check the data sheet). If you add a second transistor around it to increase current, that jacks the gain up. I doubt it would be stable. That part was a royal pain in my backside the whole time I worked at NSC, stay away from it.
 
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tcmtech

Banned
Most Helpful Member
My brother has some high powered LiPo chargers and not one of them is all that fussy about the input voltage. I believe his are something like 11 - 17 volts input rated.
And being typical LiPo chargers are uC controlled they automatically adjust for input voltage variances on the go in order to get the correct charging output profiles.
 

reader1

New Member
Actually, a PWM switcher bucking 13.6V down to 12V @ 25A is probably going to be LESS efficient than a linear because the duty cycle is so high the switch is ON nearly all the time and you have high switching losses. I'd go with a linear.
You can use linear regulator all right, but it has higher power dissipation then pwm. for example, if adjustor has 2V voltage drop (1.5-4.5v) and if you output 25A, you will lose 50W. it 's a higher lost at 4v. to 100W!
PWM is not only use at higher voltage condition, but lower voltage ,say 10V and can convey to 12V, we calli it booster.

pwm has a higher efffictive than linear.
 

mneary

New Member
Thanks for the warning about the LM2940-12.

Does anyone know a fairly stable adjustable (or 12V) 1A LDO we could substitute?
 
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bountyhunter

Well-Known Member
Thanks for the warning about the LM2940-12.

Does anyone know a fairly stable adjustable (or 12V) 1A LDO we could substitute?
I am not convinced a 1A regulator will work. An LDO (low dropout reg) uses PNP pass transistor(s) and the regulator must provide all the base drive to the pass devices who are delivering 25A total. IMHO, those guys will not be able to hold a current gain of 25 with such a low voltage drop across them from collector to emitter. Maybe, but you would need some seriously honking PNP devices which can hold a beta of 25 with 4 or 5 amps current and a VCE of a shade over a volt.

Remember: since you have to parallel at least 5 PNPs to do the 25A, they must have emitter degeneration resistors to force them to share current. Those resistors will eat up maybe 0.2 - 0.3V of the total VCE drop available.
 
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bountyhunter

Well-Known Member
pwm has a higher efffictive than linear.
NO, not always. When VIN is close to VOUT, a linear may be as efficient or more efficient because the PWM switch transistor will be ON most of the time and it will also have switching losses added on to the saturation conduction losses. In high current apps like 25A load, switching losses are VERY HIGH. With 25A load, the losses in the catch diode are also very high. The belief that PWM is always more efficient than linear is simply not true: when VIN gets close to VOUT, switchers lose their advantage.
 

mvs sarma

Well-Known Member
NO, not always. When VIN is close to VOUT, a linear may be as efficient or more efficient because the PWM switch transistor will be ON most of the time and it will also have switching losses added on to the saturation conduction losses. In high current apps like 25A load, switching losses are VERY HIGH. With 25A load, the losses in the catch diode are also very high. The belief that PWM is always more efficient than linear is simply not true: when VIN gets close to VOUT, switchers lose their advantage.
while so, perhaps Pchannel mosfets can also be used as bypass elements for increasing the load current, as this would help maintaining low Vds drop.
 

bountyhunter

Well-Known Member
while so, perhaps Pchannel mosfets can also be used as bypass elements for increasing the load current, as this would help maintaining low Vds drop.
You can definitely build a high current LDO linear reg using P-FETs. This application is a good candidate for that because you have 12V of gate drive available.
 
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