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Controlling Separate Source with Low Power Signal

mason5059

Oct 15, 2014
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Hi,

I am having a problem with a senior design project that I was hoping to be solved by some of the experts on the forums. What I need to do is have a voltage signal from a DAQ (basically a micro controller), and have it control a separate voltage source. The DAQ outputs 0-5VDC (that can be controlled in the programming) at a maximum of .02A, and I want it to be able to PROPORTIONALLY control a separate DC source with this signal that would provide more current at the same voltage (up to a maximum of 8A). The type of separate power supply is not important, as long as it is able to be proportionally controlled by the low-amperage signal from the DAQ, and can provide the current. The DAQ cannot provide the current I need to control the load, which is an electronic brake that operates from 0-12V. Any ideas/suggestions would be greatly appreciated, thanks for your time.
 

Gryd3

Jun 25, 2014
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You just want a FET or Darlington transistor
I believe the op wants a linear control of the 0-12V output with a 0-5V input.
Your suggestion would latch to either 0 or 12V out unless it was biased and operated in the linear region, but that would make it get very hot.
Do you have a trick up your sleeve?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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You want a high power unity gain buffer then.
Adam
I agree. High power unity gain buffers, or buffers with gain, are readily available but pricy.

Check out this web site for Apex Microelectronics. We use their PA-12 hybrid power op-amps to drive coils and high voltage transformers for ion-beam scanning. They can be configured with gain, so a 0 to 5 V input will produce a 0 to 12 V high current output, up to 15 A for the PA-12, but there are other models available.

You will need to purchase sockets and heatsinks, and be very careful to follow the Apex instructions on how to apply their op-amps. Expect to spend several hundred dollars. The alternative is to build your own if you have the design skills.
 

Gryd3

Jun 25, 2014
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I agree. These are readily available but pricy. Check this web site for Apex Microelectronics. We use their PA-12 hybrid power op-amps to drive coils and high voltage transformers for ion-beam scanning. You will need to purchase sockets and heatsinks, and be very careful to follow the Apex instructions on how to apply their op-amps. Expect to spend several hundred dollars. The alternative is to build your own if you have the design skills.
Depending on the OP's application, would PWM be an option?
The 0-5V input could control the duty cycle which could switch the 12V output on/off... using a little filtering and you get a close enough DC output depending on the application.
 

KrisBlueNZ

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Here's a circuit that should do what you want.

270837.001.GIF

It's a unity gain voltage follower with high-current output transistors (QP1, QP2).

The 0~5V input voltage feeds into the non-inverting input (pin 3) of op-amp U1A. U1A will control its output in order to keep its inverting input (pin 2) at the same voltage as pin 3. It does this by varying its output voltage, to vary the current flow through the output transistors, to achieve a matching voltage.

The section with QD1 and QP1 can be repeated more times if necessary but I think two will be fine. The reason for having two separate output paths is just to split the current to reduce the power dissipation in the individual transistors. You could argue that it's not even necessary, but 8A is quite a lot of current, even for a 2N3055 which is rated for 15A.

The final output transistors (QP1 and QP2) are powered from a separate rail, to reduce power dissipation in them. Power dissipated can be calculated by multiplying the current flowing through the transistor (up to 4A each if two are used) by the voltage across the transistor, which is equal to the 6~12V rail voltage minus the voltage across the load. The 6~12V rail must be at least 6V to cover the collector-emitter saturation voltage of the output transistors, but voltage above 6V will unnecessarily increase the power dissipation in the transistors.

Assuming the load is resistive (as well as inductive, no doubt, but assuming the current has stabilised), the load's DC resistance will be 5V / 8A = 0.625Ω. If the output voltage is 3V it will draw 4.8A which is 2.4A per power transistor (assuming you use two). If the 6~12V rail is 6V there will be 3V across each transistor and each transistor will dissipate 7.2W but if the 6~12V rail is 12V, there will be 9V across each transistor and each transistor will dissipate 21.6W. That's OK but it means you'll need a bigger heatsink.

D1 and D2 protect the power transistors against back EMF from the load, which I assume is inductive.

If you don't already have a suitable power supply, I recommend using a PC power supply. You'll have to increase the output voltage on the 5V rail to 6V; this may be possible using the voltage adjustment trimpot, otherwise you may need to modify it. There are lots of articles on this topic on the net.

Edit:

Can you please clarify the output voltage range you need. You said:
... that would provide more current at the same voltage ...
... but then you said...
... the load, which is an electronic brake that operates from 0-12V.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Depending on the OP's application, would PWM be an option?
The 0-5V input could control the duty cycle which could switch the 12V output on/off... using a little filtering and you get a close enough DC output depending on the application.

This could be a very viable and inexpensive approach, depending on what kind of electrical response time is needed. In our bearing test lab we have two variable speed motor drives used for bearing load testing (I don't know if they are testing the bearings or testing the lubricant). A three-phase PWM variable frequency drive system controls the motors. Works fine, lasts a long time, does a little, costs a lot. Made in Germany. Somewhere between there and a senior project's time and money budget should be an answer.

The brake may be part of a negative-feedback control system. It may also have some appreciable inductance which could make PWM control difficult at 8 A if the switching frequency were several kilohertz. I think it's worth a shot, but we need more information concerning what kind of load impedance the brake presents to a 0 to 12 V at 8 A drive signal. I am assuming the 8 A occurs at 12 V and we are only required to provide about 100 W to the brake. Perhaps PWM at, say, 20 kHz would not require any filtering, depending on the dynamics of the electrical brake. And there is the minor issue that a PWM controller cannot go to zero duty cycle.

I know very little about electronic brakes. I suspect this one uses a rotating clutch plate that varies the braking torque in proportion to a magnetic field or current that pulls the clutch plate against an abrasive braking surface. Or it could be a Prony Brake, a variable tension band around a braking cylinder, tension controlled by who knows what. A company a few miles north of Dayton once had a contract to test Space Shuttle landing gear brakes using such an arrangement. Kind of awesome the amount of heat they had to get rid of to simulate a real landing. Anyway, electronic brakes come in a wide range of sizes and flavors. Need more info to propose a PWM solution. Plus, remember this is someone's senior project so time and money are of the essence! I don't think the OP wants to make his senior design project an exercise in design and construction of the PWM electronic brake driver.
 

Arouse1973

Adam
Dec 18, 2013
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I believe the op wants a linear control of the 0-12V output with a 0-5V input.
Your suggestion would latch to either 0 or 12V out unless it was biased and operated in the linear region, but that would make it get very hot.
Do you have a trick up your sleeve?
I think Collin means an emitter follower version, but the problem with this is might be the voltage drop.
Adam
 

hevans1944

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Jun 21, 2012
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@Arouse1973: In post #8 @KrisBlueNZ proposed what is essentially "an emitter follower" with negative feedback to ensure unity gain. This circuit will provide unity gain and 8 A or more of load current. If it is necessary to translate 0 to 5 V input into 0 to 12 V output, more gain can be accommodated by replacing 10 kΩ resistor, RC, with a voltage divider connected to pin 2 of LM358, U1A.

This looks like the fastest, least expensive, solution to the OP's problem, IMHO of course. Comments from the OP appreciated.
 

mason5059

Oct 15, 2014
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Hi Everyone,

VERY sorry I never responded. I got the alert for the first response, but never received any more alerts so I tries to enlist the help of some professors and tried several methods over the last two weeks. I HAVE rolled around the idea of using PWM to drive a MOSFET, but have no way of controlling the pulses using the DAQ. @KrisBlueNZ, I really like the circuit you made, and if it's ok with you I'd like to ask some questions about it over PM. It may not be until later (other senior design work to be done) but I really appreciate the help. Thanks again everyone, and again I'm very sorry I didn't realize this was all on here. I really appreciate the help.
 

KrisBlueNZ

Sadly passed away in 2015
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No problem. You can ask any questions about my circuit on this thread.
 
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