Boost output voltage on a 7805 regulator

[Mad_Professor]

Hey all I'm helping my dad fix his instrument cluster, the old regulator died and is $40 mechanical regulator. We also replaced the fuel sending unit as it had corroded away and lost it's pick up tube and no longer worked.
We are using 7805 voltage regulator with a 10uf cap, and it works except voltage is too low and is causing the fuel and temp gauges to read inaccurate.
Full is read as 3/4 tank full on gauge and temperature reads too low when the engine is at operating temperature

Measurement of 7805 Vout is 4.94v.
The fuel gauge is 16 ohms and temp gauge 14 ohms and fuel sending unit is 8.5 ohms when full and 75 ohms when empty. Did not measure temp sensor for ohm reading. When I measured the whole fuel sending circuit, it read 24.8 ohms when full and 91 ohms when empty.

When using a power brick/wall wart that outputs 5.2 voltage, the gauges read proper.

The voltage from the vehicle is 13.5v with engine running, and 11.9v with no engine just on battery, the change in voltage did not change the output voltage on the regulator.

This is what were using.
http://www.digikey.com/product-detai...BPMS-ND/804682

We're using a panny FC 10uf 50V capacitor on the Vin of the 7805.

I want to boost the output voltage of the 7805 up to 5.1 volts or just at least over 5 volts to remedy the issue.

[stj]

Re: Boost output voltage on a 7805 regulator

put a 1n4007 between the 7805 ground tab and ground to create a volt-drop.
add more till you get what you need.

or switch to using an ajustable regulator.

[eccerr0r]

Re: Boost output voltage on a 7805 regulator

Unusual application...

Wire your 4.94 volt 7805 like a LM317T where GND = ADJ, and with the "bottom" resistor 27 ohms and "top" resistor 1.5K ohms. However these need to be fairly accurate else it will be off (don't substitute, and you may need 1%, 5-10% resistors you will get significant error).

Be ware that the ground tab will now be floating it must be isolated from the chassis. Be ware that this eats power now. You're better off using a LM317T.

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

put a 1n4007 between the 7805 ground tab and ground to create a volt-drop.
add more till you get what you need.

or switch to using an ajustable regulator.

Wouldn't that diode fluctuate with temperatures?

Unusual application...

Wire your 4.94 volt 7805 like a LM317T where GND = ADJ, and with the "bottom" resistor 27 ohms and "top" resistor 1.5K ohms. However these need to be fairly accurate else it will be off (don't substitute, and you may need 1%, 5-10% resistors you will get significant error).

Be ware that the ground tab will now be floating it must be isolated from the chassis. Be ware that this eats power now. You're better off using a LM317T.

Something like this?
http://www.rason.org/Projects/regulator/regulator.htm

R1 would be top and R2 would be bottom correct?

[fzabkar]

Re: Boost output voltage on a 7805 regulator

If you decide to stick with a linear regulator, you might like to consider adding a power resistor in series with the supply. This will reduce dissipation in the regulator. Of course you would need to allow for the minimum input voltage and the maximum load current.

[budm]

Re: Boost output voltage on a 7805 regulator

What is the maximum circuit current requirement. It sounds like you need precision power source with low sensitivity to wide temperature range.
How was it done before?

[fzabkar]

Re: Boost output voltage on a 7805 regulator

What is the maximum circuit current requirement. It sounds like you need precision power source with low sensitivity to wide temperature range.
How was it done before?

In the opening post the OP refers to a $40 mechanical regulator. I can't imagine that these would be very accurate. In fact the last time I remember seeing one was in a 1967 Chrysler Valiant (aka Plymouth).

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

What is the maximum circuit current requirement. It sounds like you need precision power source with low sensitivity to wide temperature range.
How was it done before?

In the opening post the OP refers to a $40 mechanical regulator. I can't imagine that these would be very accurate. In fact the last time I remember seeing one was in a 1967 Chrysler Valiant (aka Plymouth).

It's a VRC-601 limiter.

I'm not sure, it's a pretty simple design, The gauges are wired in series with the sensors then grounded via body or engine block. The voltage limiter pulls power from ignition source and used a poly film box capacitor 0.47 uf 250v rated to input. I can find some information on it here. But Specification wise... nada.
http://www.allpar.com/history/mopar/electrical2.html


The bits of data I can gather is dodge recommends voltage to be between 4.99 - 5.02.

[budm]

Re: Boost output voltage on a 7805 regulator

You should at least put 0.1uF cap on the output.
You can also add the diode to the GND pin to raise the Voltage and see how well it maintain the output Voltage, It probably performs better than the VRC-601. But you are probably better off using LM317 and 1% resistors to set the output Voltage.
Did you use heat sink as shown in the link? You should, just do not Gnd the heat sink if you add the diode.

[eccerr0r]

Re: Boost output voltage on a 7805 regulator

Wouldn't that diode fluctuate with temperatures?

No, or at least not that much. But you'd get increments of 0.7 volts. Maybe with a schottky you could get less... that may work.

Something like this?
http://www.rason.org/Projects/regulator/regulator.htm

R1 would be top and R2 would be bottom correct?

Yes.
And the decoupling caps would be good too.

Oh... and the values I picked out (27/1k5) ... calculated to get about 5.2 volts with your 4.94 volt regulator... Might need to adjust that if that's not what you wanted... Increasing the 1k5 will decrease the voltage, though it is already pretty close to minimum load...

[Mad_Professor]

Re: Boost output voltage on a 7805 regulator

No, or at least not that much. But you'd get increments of 0.7 volts. Maybe with a schottky you could get less... that may work.

Yes.
And the decoupling caps would be good too.

Oh... and the values I picked out (27/1k5) ... calculated to get about 5.2 volts with your 4.94 volt regulator... Might need to adjust that if that's not what you wanted... Increasing the 1k5 will decrease the voltage, though it is already pretty close to minimum load...

What's the difference from using a resistor from diode in this scenario?

I'm curious how did you get those values?

You should at least put 0.1uF cap on the output.
You can also add the diode to the GND pin to raise the Voltage and see how well it maintain the output Voltage, It probably performs better than the VRC-601. But you are probably better off using LM317 and 1% resistors to set the output Voltage.
Did you use heat sink as shown in the link? You should, just do not Gnd the heat sink if you add the diode.

why is LM317 better? other than "it's designed for it."

[eccerr0r]

Re: Boost output voltage on a 7805 regulator

The diode has discrete increments in voltage. And yes they are somewhat temperature sensitive but many things are.

The formulas are on the 78xx datasheets though I did try to pick standard values and did a little rounding. I hope that I don't have a math error, but I chose R2=27 as this seems to work OK and only results in one equation with one unknown.
R1=4.94V/((5.2V-4.94V)/(R2=27)-0.0065)=1578 ohms. Actually you should go 1575 but there's no standard value there unless you do 1500+75.

The LM317 is better because it doesn't leak as much current through the ADJ tab as much as the 7805 in the GND tab, not to mention the lower minimum voltage (1.25V vs 5V). These affect the resistor calculations (see datasheet) - the LM317 leakage is low enough it almost can be neglected; the 7805 is significant.

[stj]

Re: Boost output voltage on a 7805 regulator

i would use a switching module.
i have a bag of these.
http://www.ebay.co.uk/itm/121645448049
you can get them single for about a $ and everybody sells them - Ebay, DX, BangGood etc.

[budm]

Re: Boost output voltage on a 7805 regulator

Or this:
http://www.ebay.com/itm/4-38V-to-1-2...sAAOSwfcVUHPKq

[Andrew F. Ali]

Re: Boost output voltage on a 7805 regulator

I'll go with the e-bay regulator budm suggested.

[R_J]

Re: Boost output voltage on a 7805 regulator

I would splurge and go for the one with the built in meter http://www.ebay.com/itm/5A-DC-DC-adj...ht_5674wt_1274

[eccerr0r]

Re: Boost output voltage on a 7805 regulator

Oh hell, why not get an Keysight E3634A with a Tripp Lite APS2012SW.

[stj]

Re: Boost output voltage on a 7805 regulator

Or this:
http://www.ebay.com/itm/4-38V-to-1-2...sAAOSwfcVUHPKq

but that would need a recap

[stj]

Re: Boost output voltage on a 7805 regulator

btw, what does the original regulator look like?
it would be interesting to fit the new pcb inside it - keeping the original look.

[budm]

Re: Boost output voltage on a 7805 regulator

i would use a switching module.
i have a bag of these.
http://www.ebay.co.uk/itm/121645448049
you can get them single for about a $ and everybody sells them - Ebay, DX, BangGood etc.

The trim pot needs to be changed.