# Getting absolute values from DC generator

#### xxl

Joined Nov 3, 2020
1
Hi,

I am using a DC motor as a generator. When I spin it in one direction I get positive voltage and in the opposite direction I get negative, as expected.

But I would like to get always positive values, no matter the direction it rotates? Is it possible?

#### crutschow

Joined Mar 14, 2008
34,811
You can run the output into a bridge rectifier.

What is the minimum output voltage you need from the generator?
If it's over a couple volts, then a MOSFET bridge rectifier would give a lower voltage drop and better efficiency.

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#### jrjong11

Joined Nov 11, 2020
4
You can run the output into a bridge rectifier.

What is the minimum output voltage you need from the generator?
If it's over a couple volts, then a MOSFET bridge rectifier would give a lower voltage drop and better efficiency.
thanks for the tip crustchow.Will try this one out. Will update you shortly.

#### Tonyr1084

Joined Sep 24, 2015
7,987
@xxl and @jrjong11 are you the same person?

I agree with Crutschow on the bridge rectifier solution.

Joined Jul 18, 2013
28,820
Many previous post here if you do a search!.
Commonly known as a steering diodes or steering bridge.
Max.

#### crutschow

Joined Mar 14, 2008
34,811
Below is the LTspice simulation of a MOSFET bridge rectifier:
Note there is very little voltage drop (just do to the MOSFET on-resistance) when the voltage is above the MOSFET's threshold voltage (about 1V for the MOSFETs shown).
Below that voltage the current is carried by the intrinsic MOSFET substrate diodes.

If the voltage is above 20V peak, then additional circuitry (Zeners and resistors) must be added to keep the MOSFET's gain-source voltage below that level.

#### MisterBill2

Joined Jan 23, 2018
19,351
There are also other options aside from a bridge rectifier, they depend on what kind of generator you have, and how often the rotation reverses. The diode bridge is fine for lower currents and voltages that are not extreme, but for currents over just a few amps the losses are not minor. At some point a reversing switch becomes a better choice. Back many years ago there were mechaical switches driven by the same shaft that drove the generator that would reverse the connection to hold the polarity constant. I think that they were intended for battery charging applications. If the motor/generator has a wound field then reversing the field is also an option. Most of them will be more efficient than all of the generator output passing through 2 diodes of a bridge with the resulting 2 diode voltage drops. But the options depend on the current, the voltage, and the needed efficiency.

Joined Jul 18, 2013
28,820
but for currents over just a few amps the losses are not minor.
Quite simple to minimize with the use of high current Schottky rectifiers.
Max.

#### Sensacell

Joined Jun 19, 2012
3,500
What is not clear is if the TS is needing power output, or a signal voltage?

Are you measuring something? Or is it a power source?

If a signal is all that is needed, then an active rectifier using opamps would provide the most accurate output, without the voltage drops associated with simple bridge rectifiers.

#### Tonyr1084

Joined Sep 24, 2015
7,987
xxl has only one post and that was back on Nov. 3. Either they've lost interest or may be in jail. Maybe Covid got him/her. 24 days later and we haven't heard back, maybe they forgot their password or even their user name.

Doesn't look like we're going to get any further clarification.

#### MisterBill2

Joined Jan 23, 2018
19,351
xxl has only one post and that was back on Nov. 3. Either they've lost interest or may be in jail. Maybe Covid got him/her. 24 days later and we haven't heard back, maybe they forgot their password or even their user name.

Doesn't look like we're going to get any further clarification.
I went back to the "information" section and found that xxL is student. Perhaps they were overwhelmed. nd I think that I recall seeing a mechanical method of always turning generator in the same direction no matter which way the drive turns. That would be interesting to see. The plan would be to examine some of the electrical publications from the 20's and the 30's, back when more folks were more mechanically inclined. There was a lot of interesting stuff done back then.

#### jrjong11

Joined Nov 11, 2020
4
Below is the LTspice simulation of a MOSFET bridge rectifier:
Note there is very little voltage drop (just do to the MOSFET on-resistance) when the voltage is above the MOSFET's threshold voltage (about 1V for the MOSFETs shown).
Below that voltage the current is carried by the intrinsic MOSFET substrate diodes.

If the voltage is above 20V peak, then additional circuitry (Zeners and resistors) must be added
https aka ms remoteconnect to keep the MOSFET's gain-source voltage below that level.

View attachment 223377
Thanks for the detailed learning guide mate.