# Super diode (active rectification) question

#### molecool

Joined Oct 24, 2011
32
I wonder why it even works... starting voltage 2VDC?? 120mA current should be doable with a MOSFET, though.

Sorry, I have to call it a day.

Cheers

edit: we are all cross-posting, LOL
See above - I tested this thing and it runs gangbusters at 1.5V (about 6000 rpm) - and it even turns at about half that at 0.7V. So that quoted starting voltage is b.s. based on what I'm seeing. I do have a voltage/ohm meter - unfortunately it's tricky to measure the resistance of a motor properly due to back EMF and stuff. If you guys know a simple procedure let me know and I'll do it.

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#### Ron H

Joined Apr 14, 2005
7,014
Do you just want a motor that runs when 1.5V is applied, or do you have some more exotic requirements? I read your original post, but I can't imagine what your application might be.

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#### molecool

Joined Oct 24, 2011
32
Do you just want a motor that runs when 1.5V is applied, or do you have some more exotic requirements? I read your original post, but I can't imagine what your application might be.
Good! ;-)

Yes, I just want that motor to run when the voltage is applied at the proper polarity. Now if I could keep the voltage at near 1.5V level for an extended amount of time at the cost of the AAA's half life time then I'm fine with that. But again, that latter part would be more of an extra feature.

#### Ron H

Joined Apr 14, 2005
7,014
Good! ;-)

Yes, I just want that motor to run when the voltage is applied at the proper polarity. Now if I could keep the voltage at near 1.5V level for an extended amount of time at the cost of the AAA's half life time then I'm fine with that. But again, that latter part would be more of an extra feature.
I've been working on an idea. What is the voltage when it is the wrong polarity? Is it -1.5V?

#### The Electrician

Joined Oct 9, 2007
2,834
Good! ;-)

Yes, I just want that motor to run when the voltage is applied at the proper polarity. Now if I could keep the voltage at near 1.5V level for an extended amount of time at the cost of the AAA's half life time then I'm fine with that. But again, that latter part would be more of an extra feature.
Trying to maintain the output voltage at 1.5 V during the discharge of the battery substantially complicates things. How much money can you spend on a non-regulating design? How much for a regulating design?

#### praondevou

Joined Jul 9, 2011
2,942
There are a lot of readymade solutions for step-up converters working with extrem low input voltages. Is this of any interest to you?
However it will also cost you a few dollars (maybe around $6). Example: The LM2623 works down to 0.8V and gives you an adjustable output voltage. #### Ron H Joined Apr 14, 2005 7,014 There are a lot of readymade solutions for step-up converters working with extrem low input voltages. Is this of any interest to you? However it will also cost you a few dollars (maybe around$6).

Example: The LM2623 works down to 0.8V and gives you an adjustable output voltage.
That's a neat part, but it won't handle the reverse voltage that he wants to be able to apply.

I would really like to know why his supply voltage might be of either polarity.

#### molecool

Joined Oct 24, 2011
32
I've been working on an idea. What is the voltage when it is the wrong polarity? Is it -1.5V?
Yes, it's the very same current just with the polarity reversed.

#### molecool

Joined Oct 24, 2011
32
Trying to maintain the output voltage at 1.5 V during the discharge of the battery substantially complicates things. How much money can you spend on a non-regulating design? How much for a regulating design?
I can spend a reasonable budget on developing this, but the eventual component cost should be below a buck in higher volume. So I guess that's a non-starter...

#### Ron H

Joined Apr 14, 2005
7,014
I can spend a reasonable budget on developing this, but the eventual component cost should be below a buck in higher volume. So I guess that's a non-starter...
I have a design that does what you want (I think), but not for less than a buck.

#### molecool

Joined Oct 24, 2011
32
I have a design that does what you want (I think), but not for less than a buck.
What's the component cost (roughly)?

#### Ron H

Joined Apr 14, 2005
7,014
What's the component cost (roughly)?
Here's the schematic. You can figure out the cost.
This circuit is a takeoff on the Joule Thief, which is a blocking oscillator which will drive an LED with a single cell battery, and will work until the battery is pretty much shot. Here, the blocking oscillator generates 10 volts to drive the gate of the MOSFET.
The motor will slow down as the voltage drops.
My simulation shows it to be about 99% efficient at 1.5V with a 100Ω (150mA) load. At 1V, the simulated efficiency is still about 98%.
Google "joule thief" to find more construction details.

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#### molecool

Joined Oct 24, 2011
32
Here's the schematic. You can figure out the cost.
This circuit is a takeoff on the Joule Thief, which is a blocking oscillator which will drive an LED with a single cell battery, and will work until the battery is pretty much shot. Here, the blocking oscillator generates 10 volts to drive the gate of the MOSFET.
The motor will slow down as the voltage drops.
My simulation shows it to be about 99% efficient at 1.5V with a 100Ω (150mA) load. At 1V, the simulated efficiency is still about 98%.
Google "joule thief" to find more construction details.
Ron - are you available for a little contract gig? I would prefer to pay someone to complete this per my specifications plus I would also like to keep the motor running at constant rpm until the battery is almost shot. There are also other considerations which I cannot disclose on an open board. I hope this does not violate forum rules otherwise I encourage the admin to delete this comment.

#### Ron H

Joined Apr 14, 2005
7,014
Ron - are you available for a little contract gig? I would prefer to pay someone to complete this per my specifications plus I would also like to keep the motor running at constant rpm until the battery is almost shot. There are also other considerations which I cannot disclose on an open board. I hope this does not violate forum rules otherwise I encourage the admin to delete this comment.
See my PM.

#### molecool

Joined Oct 24, 2011
32

#### thatoneguy

Joined Feb 19, 2009
6,359
For "Super Simple", I'm thinking of the cheap Joule Theif or similar simple boost circuit, which runs batteries completely dead (down to 0.6V).

The problem would be winding the coil big enough to handle the current, in addition to an LED or similar component to keep the voltage low. Actually, since the current supply is low from a non-efficient boost so it could be hooked up directly.

Maybe worth a shot if you have spare motors and some time to wind the coil.

#### Ron H

Joined Apr 14, 2005
7,014
For "Super Simple", I'm thinking of the cheap Joule Theif or similar simple boost circuit, which runs batteries completely dead (down to 0.6V).

The problem would be winding the coil big enough to handle the current, in addition to an LED or similar component to keep the voltage low. Actually, since the current supply is low from a non-efficient boost so it could be hooked up directly.

Maybe worth a shot if you have spare motors and some time to wind the coil.
Do you think that would keep the average current relatively constant as the battery is depleted?
Besides, in order to get an average current of ≈50mA with a pulsed load (driven by the blocking oscillator), battery resistance would probably become problematic as it begins to die.
Of course, I have never been accused of being an optimist.

#### thatoneguy

Joined Feb 19, 2009
6,359
Do you think that would keep the average current relatively constant as the battery is depleted?
Besides, in order to get an average current of ≈50mA with a pulsed load (driven by the blocking oscillator), battery resistance would probably become problematic as it begins to die.
Of course, I have never been accused of being an optimist.
I wasn't thinking about constant current/RPM, just that it would turn one way and run from a AA battery as cheap as possible.

There are many drawbacks to the suggestion, as you pointed out. If it is just to vibrate, and not at a specific frequency, the pulsed input current wouldn't be a huge issue, it would still run. I missed it if there are other requirements besides making a pager motor run with only a AAA battery having polarity protection.

#### Ron H

Joined Apr 14, 2005
7,014
Well, the circuit I posted doesn't compensate for battery voltage droop either, but molecool would like it if it did.

#### thatoneguy

Joined Feb 19, 2009
6,359
Well, the circuit I posted doesn't compensate for battery voltage droop either, but molecool would like it if it did.
My apologies, I had left the window open for a while and forgot to refresh before adding the info, so I missed 3 hours of responses prior to mine.