Boost output voltage on a 7805 regulator

[stj]

Re: Boost output voltage on a 7805 regulator

the pcb has pads for fixed resistors, you could find the correct value and then dump the pot.

i suspect these are also sold with fixed output.

[stj]

Re: Boost output voltage on a 7805 regulator

hmm.
http://www.kerrywong.com/2015/07/24/...odule-testing/

[fzabkar]

Re: Boost output voltage on a 7805 regulator

I like the MP1584 chip because I have a need for an 0.95V regulator.

MP1584, Monolithic Power Systems, Step-Down Converter, 3A, 1.5MHz, 4.5V - 28V in, 0.8V - 25V out:


The chip's reference voltage is 0.8V and its Vout range is specified from 0.8V to 25V. However, I can't seem to find a readymade PCB with this same range. Some suppliers do state 0.8V in their specs, but I'm not certain that they are referring to their PCB.

Here is one example with a multiturn trimmer:

http://klinikrobot.com/product/dc-dc...e_frame|41-996
http://klinikrobot.com/components/co...6d6ab22e7c.jpg

[mariushm]

Re: Boost output voltage on a 7805 regulator

The output voltage is configured using two resistors, the ratio between them tells the chip how much the output voltage should be.

See page 15 in the datasheet:

You have a R2 resistor = 40.2 kOhm going from the feedback pin (4) down to ground and another resistor R1 which varies depending on the output voltage desired. In the circuit board that you show pictures of, the resistor is replaced by that multiturn potentiometer.

The application notes in the datasheet say (on page 10) that R1 should be fixed and in general should be under 40kOhm (but they chose a 40.2 kOhm probably because it was an often used value). It's not clear from the pictures what resistor they chose instead of 40.2 kOhm but you could easily figure out the 4th pin and look where the trace goes from there, the circuit should be very similar to the example circuits in the datasheet on page 15.
Once you know the value of the resistor (or you could just desolder the resistor and replace it with a 39kOhm resistor or a 39.2kOhm or a 40.2 kOhm, all three standard values easy to find) then you can configure the output voltage to any value you want, the formulas are right there on page 10:

Vout = Vfb * [ (R1+R2) / R2 ]

R2 x Vout = Vfb * (R1+ R2) ==> R2 x Vout = Vfb * R1 + Vfb * R2 ==> R2 x Vout - Vfb * R2 = Vfb * R1 ==>
R1 = R2 * ( Vout - Vfb) / Vfb = (R2/Vfb) * (Vout-Vfb)

If your R2 is 40.2 kOhm and as we know Vref / Vfeedback / Vfb is 0.8v (the reference voltage) then

R1 = (40200 Ohm / 0.8v) * ( Vout - 0.8v)

So if you want 0.95v out then you have R1 = (40200 / 0.8) * (0.95 - 0.8) = 50250 * 0.15 = 7537 ohm

[fzabkar]

Re: Boost output voltage on a 7805 regulator

Thanks for the explanation. Hopefully it will help others. :-)

Actually I am well aware of the requirements and calculations for the feedback resistors in the potential divider. The reason for my question is that I require a readymade PCB that includes 0.8V in its Vout range. Some PCBs on eBay specify a range that starts at 1.8V. I don't know if this is a typo or whether they actually mean 0.8V. The PCB is not for myself -- if it were, then I could simply make the necessary changes to the divider.

Here is a thread where a user of a Seagate HDD needs to repair a damaged 0.95V Vcore regulator:

http://forum.hddguru.com/viewtopic.php?f=1&t=33643

... and another one:

http://forum.hddguru.com/viewtopic.p...29204&start=20

The second person is a technician, so he managed to repair his drive. However, the first person is an end user with no soldering or electronics experience. Nevertheless, I believe that an inexperienced person would not have too much difficulty soldering the SMPS unit to suitable Vin/Vout/Ground points on the HDD PCB, after preadjusting its Vout level, of course.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

I would connect an overvoltage protection diode across the output terminals so that the instrument cluster will not be damaged in the event of a regulator failure.

1.5KE6.8A, 1.5KE6.8AG, 1N6267A , 1500 Watt Unidirectional Mosorb Zener Transient Voltage Suppressor, Standoff voltage = 5.8V, Breakdown voltage = 6.45V (min) - 7.14V (max):

https://cdn.badcaps-static.com/pdfs/...5a8e4abcff.pdf


I would also place a suitably rated PolySwitch or PolyFuse in series with Vin.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

hmm.
http://www.kerrywong.com/2015/07/24/...odule-testing/

My dad and I went shopping at skycraft, it's a local surplus store, fill with random unwanted junk. Anyways while I was gathering stuff to build lm317 design, my dad found a dc-dc converters for like $8. It's similar to the one above but has two 470mfd caps and two inductor coils, I think it's the same chip. I told my dad "eh we'll try it," turns out works pretty fucking well.

We also found more information on the limiter, it's apparently a 12 to 6v limiter, that makes sense to me. So we set the thing to 6v now the gauges work properly especially temp, it always read low and didn't seem right.

SO now I got a brown bag with 6 resistors, diodes, lm317 acouple of shitty caps and wondering what I can build with it since the dc-dc converter has already fixed the problem.

[Th3_uN1Qu3]

Re: Boost output voltage on a 7805 regulator

Build a class A audio amplifier maybe? Yes, the LM317 can do that. http://www.electroschematics.com/668...dio-amplifier/

[budm]

Re: Boost output voltage on a 7805 regulator

My dad and I went shopping at skycraft, it's a local surplus store, fill with random unwanted junk. Anyways while I was gathering stuff to build lm317 design, my dad found a dc-dc converters for like $8. It's similar to the one above but has two 470mfd caps and two inductor coils, I think it's the same chip. I told my dad "eh we'll try it," turns out works pretty fucking well.

We also found more information on the limiter, it's apparently a 12 to 6v limiter, that makes sense to me. So we set the thing to 6v now the gauges work properly especially temp, it always read low and didn't seem right.

SO now I got a brown bag with 6 resistors, diodes, lm317 acouple of shitty caps and wondering what I can build with it since the dc-dc converter has already fixed the problem.

Thanks for the follow-up, so it is 6V limiter:
http://www.rockauto.com/en/moreinfo....jsn=__GIX__3__

[stj]

Re: Boost output voltage on a 7805 regulator

30$ for a bimetal switch? bandits!!

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

Thanks for the follow-up, so it is 6V limiter:
http://www.rockauto.com/en/moreinfo....jsn=__GIX__3__

Yeah, which was confusing because the all the hack designs I saw, called for 5v
And Chrysler/Dodge I didn't find any specification up until this past sunday night.

30$ for a bimetal switch? bandits!!

$38.99

(waits for him to respond.)


Right now, my dad and I are trying to fix the AMMETER gauge, it's doesn't get a reading, sits dead in the middle. I'm not sure how it's works. I tried to hook up my DMM and set it on ammeter, but it didn't get a reading as well.
Then it started to rain, so I called it quits for tonight.

[stj]

Re: Boost output voltage on a 7805 regulator

if it's the old inline ones that go between the battery & car, it just has a coil in it that creates a field in the direction of current flow.
show the back of it.

dont put your meter inplace of it, some of the old ones can shunt over 100A!!

btw, those bi-metal things arent voltage droppers, they will simply generate 12v pulses that then get averaged out by a capacitor.
as the cap dries out the thing will just be pulsing into the dash.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

if it's the old inline ones that go between the battery & car, it just has a coil in it that creates a field in the direction of current flow.
show the back of it.

dont put your meter inplace of it, some of the old ones can shunt over 100A!!

btw, those bi-metal things arent voltage droppers, they will simply generate 12v pulses that then get averaged out by a capacitor.
as the cap dries out the thing will just be pulsing into the dash.

Thanks for the heads up, but nothing happened. I have several fused inputs on my meter.

Anyways I'm not sure what's wrong, I've found the wiring diagram and I get 12v from one lead and 12v from another lead, it's a very simple design.

Can a bad cell in a battery that otherwise will function cause this problem?

[stj]

Re: Boost output voltage on a 7805 regulator

no.

it's current flow.
either it's inllne with the battery like a fuse, or it's inline with a parallel shunt resistor to save it from most of the current - more likely.

check the resistance between the terminals on the meter - with it disconnected.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

no.

it's current flow.
either it's inllne with the battery like a fuse, or it's inline with a parallel shunt resistor to save it from most of the current - more likely.

check the resistance between the terminals on the meter - with it disconnected.

It's reading 0.0 ohms.
Shorted?

[stj]

Re: Boost output voltage on a 7805 regulator

could be - show a foto of it.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

could be - show a foto of it.

Well here's the photos.

[fzabkar]

Re: Boost output voltage on a 7805 regulator

I believe the ammeter should connect between the alternator and the battery.

[budm]

Re: Boost output voltage on a 7805 regulator

DCR of 0.4 Ohms is normal for that electromagnet coil.
The needle assembly is not binded up?
http://www.madelectrical.com/electri...p-gauges.shtml

Amp Meter wiring:

[stj]

Re: Boost output voltage on a 7805 regulator

what i was thinking.

it's pretty shit btw, the ones i used to see actually had the scale marked in ampsx10 (10,20,30 etc)