# Bomb-proof Multi Use Load Driver for My Benchtop

#### Sir Kit

Joined Feb 29, 2012
127
I often need a power amplifier to connect a function generator or sound card to a wide range of loads. For example, low resistance air coils, plate capacitors or primaries of step-up transformers. These projects usually involve a 12VDC, 1 or 2A single rail supply and waveforms within the sub-audio and audio range.

Instead of rebuilding for each occasion, I am looking for a "black box" that can be ready on the bench. The only necessary adjustments (via pots) would be gain and a DC offset to allow for both unipolar and bipolar input, at amplitudes between 1 and 5Vpp.

Because this is not a hi-fi application, simplicity and reliability are the prime objectives. Does anyone happen to know of a circuit that would best suit this type of application?

#### LowQCab

Joined Nov 6, 2012
1,913
Yepp ........
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#### Sir Kit

Joined Feb 29, 2012
127
499R resistor?? You can tell this is from a mfr's datasheet.

I was wondering about adapting it to 12VDC single rail supply, and then found the same schematic below that reveals -V as 0V. To complete, I would leave the current limit and shutdown indicator out and add a 10K gain pot in place of the fixed 9K.

Any other suggestions? Maybe a cheaper op amp with an emitter follower for the current? There are such generic designs online, but I would need to understand how to best implement the DC input level adjust and single rail supply, as described in my OP.

#### LowQCab

Joined Nov 6, 2012
1,913
The Gain must remain above 10 for Amplifier Stability, so don't mess with the Feedback Resistors.
Also, if you have any extremely Inductive Loads, it's a good idea to install
Diodes between the Supply Rails and the Output to prevent accidental Blue Smoke.
The Data Sheet covers how and why to install them.

Current capacity costs money, how many Dollars worth of Amps do you want ?

TLE-2301 = 1-Amp,
LM-675 = 3-Amps, for $5.oo, OPA-548 = 5-Amps, for$15.oo,

It sounds like you're still learning how these things work,
the most important parts are usually not shown in the schematic,
and that's the Bypass Capacitors.
They are generally all the same, and universal, but MUST be done right, no scrimping.
See the picture below.
The Ferrite Beads are only for extremely noisy environments.

This is a DC Amplifier with a Gain of 10.
you put 1 Volt in and get 10 Volts out, at a Current of up to 10-Amps.
You set the "Bias", or the "zero level" by adding a Pot to the Inverting Input, instead of tying it to Ground.
But you must maintain at least a Gain of 10 for Stability.
But a better solution, if you must Bias the Output,
is to use a second, much smaller Op-Amp, with the pot moved to the Input of the smaller Op-Amp,
this will add stability and accuracy to everything, and keep the Gain at 10.
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#### Sir Kit

Joined Feb 29, 2012
127
I was looking for a circuit to build up this weekend. I have a few LM675's direct from China, so I will apply your suggestions to those. The OPA series are expensive here abroad. Not a good way to make mistakes.

#### LowQCab

Joined Nov 6, 2012
1,913
One of the best things abut them is that they are manufactured without any Blue Smoke,
they have great self-protection circuitry built-in.