How can I amplify current only

Thread Starter

Electronics Student

Joined Oct 28, 2009
7
Am working with an optical isolator (ISO). My ISO source side (8051) is 5v +- and working properly. However, my load side only works with no load.

Load side... 4 C batteries (just over 6v), when I run this with resistor, transistor and no load, my output is 5.83v.. (thats ok). But I get 2mA...

How can I raise the output current to 330mA min - 380mA max from the output connection?

Am trying to hook up a motor with the above minimum current requirement

The motor works with direct connect to batteries, but you already knew that.

Any help or op-amp design submission would be greatly appreciated...

Thank you
 

SgtWookie

Joined Jul 17, 2007
22,201
Post a schematic of your circuit as it is now. That helps us help you more quickly, as there is much less of a chance of misunderstanding.
 

ELECTRONERD

Joined May 26, 2009
1,146
When tubes were used in the early 20th century, they were known as voltage amplifiers. Now we have transistors, which are current amplifiers. That being said, a proper transistor amplification circuit could be made to suit your needs.

But first, as SgtWookie said, a schematic would be helpful. ;)
 

Thread Starter

Electronics Student

Joined Oct 28, 2009
7
Sorry, was hoping I input enough info... I will post my schematic Saturday since I left it at school.

Any suggestions on how to post that? I'm brand new here and not sure how this forum works... yet....
 

SgtWookie

Joined Jul 17, 2007
22,201
I'm afraid that a single PNP transistor simply won't provide enough current.

The optoisolator's output might reasonably source/sink up to about 9mA, but that's about the limit. Using the standard forced beta of 10 for transistor saturation, you would only get up to about 90mA collector current before the transistor came out of saturation. Even if you "milked" that to beta=30 (an iffy proposition at best), you still fall short at 270mA. The transistor would be out of saturation, and would get fried in a big hurry.

You will either need to use a Darlington transistor and suffer the resulting 1/6 loss of voltage across the collector-emitter junction, or use a logic-level power MOSFET. A standard MOSFET would not be turned on fully with just 6v available.
 

SgtWookie

Joined Jul 17, 2007
22,201
.png format attachments are preferred over .jpg, as .png format files are relatively compact, and are not "lossy" as .jpg files are. .png format really is the best for posting schematics here.

In order to get to the "Manage Attachments" button, you must first click the "Go Advanced" button below the Quick Reply text box.
 

Thread Starter

Electronics Student

Joined Oct 28, 2009
7
Voltage loss is fine since the motor will run at 2v, but would prefer the speed of motor to run at 4v+. Am thinking I would need to cascade transistors to achieve the current desired. Not sure if there is a limit on that or the actual schematic to do so...

More info on Saturday.... thanks again
 

SgtWookie

Joined Jul 17, 2007
22,201
Here is a circuit similar in concept to CDRIVE's PNP transistor driver, but using a logic-level N-channel power MOSFET:



Note that Rload represents your 6v electric motor.
The diode must be present to take care of the motor's reverse EMF.

R2 prevents "ringing" on the MOSFET's gate when it is being charged from the 6v source.
R3 pulls the MOSFET gate low when the optoisolator's output is turned off.

This is not a high-speed circuit. Turn-on and turn-off times for the MOSFET will be rather slow due to the anemic current source capabilities of the optoisolator and the low discharge rate via R3.
 

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CDRIVE

Joined Jul 1, 2008
2,219
I'm afraid that a single PNP transistor simply won't provide enough current.

The optoisolator's output might reasonably source/sink up to about 9mA, but that's about the limit. Using the standard forced beta of 10 for transistor saturation, you would only get up to about 90mA collector current before the transistor came out of saturation. Even if you "milked" that to beta=30 (an iffy proposition at best), you still fall short at 270mA. The transistor would be out of saturation, and would get fried in a big hurry.

You will either need to use a Darlington transistor and suffer the resulting 1/6 loss of voltage across the collector-emitter junction, or use a logic-level power MOSFET. A standard MOSFET would not be turned on fully with just 6v available.
Check out the specs on the Zetex ZXTP25020DG. You may be surprised.
 

SgtWookie

Joined Jul 17, 2007
22,201
If you want some interesting capabilities using power opamps, download ST Microelectronics' datasheet for the L2722. It is available in an 8-pin DIP that contains two power opamps capable of sourcing/sinking up to 1A, and the manufacturer claims "low drop".

You will lose about 1v when sinking 400mA current, and about .5v when sourcing 400mA current, for a total of 1.5v. This is 1/4 of your available voltage.

However, this dual power opamp can act as a miniature H-bridge that can be directly controlled by a microcontroller using only two I/O pins, and only requires a voltage divider to act as a voltage reference. This will give your uC the capability of ordering the motor to run forward, reverse, or provide braking action. You will need a 100nF cap across the supply pins, and you may need to add a Bucherot cell on the output, along with a 1k resistor from the output to Vcc.

See Figure 8 on page 5.
 

SgtWookie

Joined Jul 17, 2007
22,201
Check out the specs on the Zetex ZXTP25020DG. You may be surprised.
Pretty nifty little transistors. Still, they give Vce(sat) specs using the standard forced beta=10 rule. You could go with the hFE of 200 or 300 or higher spec, but the Vce will be much higher than when it's saturated (probably around 1v to 1.5v); and as a consequence power dissipation in the transistor will increase considerably.

In order to get Vce down with an Ic that high, you really have to get the transistor saturated by supplying more base current.

The L2722's I mentioned are pretty nifty little power opamps; they'll still wind up dissipating 1/4 of the power as heat, but the trade-off is gaining a much greater degree of control over the motor in a pretty compact package.

Power MOSFETs are pretty hard to beat for low power dissipation.
 

CDRIVE

Joined Jul 1, 2008
2,219
According to their specs:

Vce(Sat) Ib=10mA Ic=1A Vce=150mV(Typ)

That's quite impressive.
 

SgtWookie

Joined Jul 17, 2007
22,201
According to their specs:

Vce(Sat) Ib=10mA Ic=1A Vce=150mV(Typ)

That's quite impressive.
I completely blew right by that after I saw the 1st spec; Ic=1A, Ib=100mA, Vce=50mV (typ)! Guess I should've read the next line, huh? ;)

That would be pretty good for our OP; power dissipation for that figure you gave would be only around 85mW. I tried it with the trusty old 2N2907A, and it didn't cut the mustard.
 

CDRIVE

Joined Jul 1, 2008
2,219
I completely blew right by that after I saw the 1st spec; Ic=1A, Ib=100mA, Vce=50mV (typ)! Guess I should've read the next line, huh? ;)
I put this transistor's data sheet in a special folder named WowPnPs! :D

If anyone knows where there's a PSpice model for it please posts it.
 

Thread Starter

Electronics Student

Joined Oct 28, 2009
7
This is to turn on a motor, then when the pulley the motor drives makes a complete rotation, a microswitch will tell our 8051 microprocessor to shut it off. This also resets our program and the timers.

If it helps, am making an automated pet food dispenser from a candy vending machine. If you want more details of the whole project let me know.
 

thatoneguy

Joined Feb 19, 2009
6,349
Two questions:

A) Will the motor need to turn an exact distance each time?

B) Does the motor need to be reversible?


If A or B, a stepper motor may be a better solution, if only B, then a stepper or an H-Bridge driver for brake and reverse on motor.
 
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