# Input Voltage drops when use 7806

• posted

Hi, I have created simple oscilloscope circuit taken from magazine, when I use 6V battery (4 pcs AA type), the input voltage is 6V, but when I am using regulator circuit 7806, the input voltage will drop. Why this happen?, my electrics basic is not so good, sorry. TIA Yanto

• posted

Assuming you are powering the 7806 from the batteries, the output will be Vbatt - V dropout (typically 2V for the 78xx series). So if you power it with 6V, you'll get about 4V out, and not regulated properly either.

Datasheet available at:

Cheers

PeteS

• posted

In case you are confused about dropout, it is the minimum voltage difference between input and output of a regulator for it to function.

In this case, if you want to provide a regulated 6V from the 7806, power it from a 9V battery and you'll be fine, but keep in mind the power dissipation of the regulator:

P = (Vi - Vo) * Io where Vi = Vin, Vo = Vout and Io = output current.

Cheers

PeteS

• posted

Thanks PeteS So better I use higher regulator ICs like 7809 as solution? Any suggestion? TIA Yanto

PeteS wrote:

• posted

No. Use a voltage source with higher voltage than 8V, then your 7806 will provide a regulated 6V-output.

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Rikard.```
• posted

The regulator requires a "head room" of 4 volts to stay in regulation

- the input voltage needs to be 4 volts higher than the output or 10 volts if you use the 7806 and want 6 volts out. It will be in the datasheet on the part you have and you can look it up on-line easily.

Search: 7806 datasheet

Low dropout regulators require less differential to stay in regulation (different part number) LDO's work well but are more prone to oscillate if the output or input isn't bypassed with a cap - read the application notes to find out how to use the parts - any parts.

And don't forget the ripple if you are using a transformer for mains power - A rule of thumb is 1,000 microfarads per 1/2 amp of current, for about 10% ripple at 60 HZ. If the "valley of the ripple" drops lower than the differential the regulator requires - ripple shows up on the output.

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