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Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
Hey Guys,
I've been away for a few years now but I've been reading through the forums the last few days trying to find answers to a few questions that have been bouncing around in my brain. Most of them are pretty elementary but I've found that my knowledge of amplifiers fairly incomplete.
The first question, which spawned from an older thread about Mag Lev, what is the common practice for amplifying DC voltages. Let's say I want to power an electromagnet and I need 12v for a 12ohm coil to get 1 amp to flow through it but my supply voltage is only 6v. How would I go about amplifying the 6v input to a 12v output? From what I understand devices such as MOSFETs and Op Amps only amplify periodic signals inside their frequency response range but how would you go about amplifying a constant non-periodic signal?
My second question is, when people build mag lev devices. A couple of years ago I tried building a mag lev system that use repulsion rather than attraction to 'push' a weight with permanent magnets on it up into the air and keep it there. The problem I ran into time after time was that when the permanent magnets got too close to my electromagnet they overwhelmed its magnetic field and oppositely polarized the metal core of my electromagnet thus causing them to stick together rather than push apart.
Lastly, if anyone is still reading, is in this mag lev design ( he talks about operating his electromagnet at a particular frequency, wouldn't that create problems with back emf? and why would he not just run his electromagnet at a constant DC voltage and have his control circuit change that number up and down based on the feedback loop?
I appreciate the help if anyone is willing to explain some of these things to me because I seem to be missing a vital piece of information about amplifiers/EM.
I have no experience with Mag Lev, but to answer your first question: you would use/need a "Boost Converter". A boost converter chops up DC into pulses across an inductor and then captures the inductive kick for use. For example, you might put 6V at 2A in to it and get 12V at 1A out (theoretically, nothing is 100% efficient).
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
That did make me think of a cool idea, though. If you could make a feedback system, you could make it so the electromagnet is pulsed at a certain repetition rate, allowing whatever it is you're trying to float to stay at a certain level.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
blackgrunge wrote ...
The first question, which spawned from an older thread about Mag Lev, what is the common practice for amplifying DC voltages. Let's say I want to power an electromagnet and I need 12v for a 12ohm coil to get 1 amp to flow through it but my supply voltage is only 6v. How would I go about amplifying the 6v input to a 12v output? From what I understand devices such as MOSFETs and Op Amps only amplify periodic signals inside their frequency response range but how would you go about amplifying a constant non-periodic signal?
A typical amplifier circuit cannot produce an output voltage outside of the power supply rails, although many amplifiers can amplify DC signals as well as AC. Decreasing power supply voltage is easy: add a resistor or a linear regulator acting as a controlled variable resistor. Increasing it requires more work. A boost converter being a simple way, it uses the interruption of current through an inductor to cause a change in the polarity of the inductor's voltage (i.e. back EMF or inductive spike). Other techniques involve other switching power topologies or charge pumps. Power is conserved, so you you supply 12V @ 1A to a load then you must draw at least 2A from a 6V supply.
wrote ... Lastly, if anyone is still reading, is in this mag lev design ( he talks about operating his electromagnet at a particular frequency, wouldn't that create problems with back emf? and why would he not just run his electromagnet at a constant DC voltage and have his control circuit change that number up and down based on the feedback loop?
He has an antiparallel diode clamping the spike it sounds like. When the transistor is on current is increasing in the coil though it may eventually reach a maximum due to resistance of the coil or power supply limitations. When the transistor turns off the spike is clamped and current decays. Average current is related to the duty cycle of the switching transistor which is easily implemented with the TL494 chip.
Generating an adjustable DC voltage (in which case it's not really DC) would require more effort as it would require a full switching power supply, or really a class D switchmode amplifier, or otherwise a linear regulator which is more simple but dissipates substantial power as heat.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you have an electromagnet that needs 12v but only a 6v supply, the sensible thing would be to get a different electromagnet, or a different supply.
If you rewound your magnet with half the turns of wire, twice as thick, lo and behold it would be a 6V, 2A coil with the same magnetic strength as before.
Registered Member #1889
Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55
Mattski wrote ...
blackgrunge wrote ...
The first question, which spawned from an older thread about Mag Lev, what is the common practice for amplifying DC voltages. Let's say I want to power an electromagnet and I need 12v for a 12ohm coil to get 1 amp to flow through it but my supply voltage is only 6v. How would I go about amplifying the 6v input to a 12v output? From what I understand devices such as MOSFETs and Op Amps only amplify periodic signals inside their frequency response range but how would you go about amplifying a constant non-periodic signal?
A typical amplifier circuit cannot produce an output voltage outside of the power supply rails, although many amplifiers can amplify DC signals as well as AC. Decreasing power supply voltage is easy: add a resistor or a linear regulator acting as a controlled variable resistor. Increasing it requires more work. A boost converter being a simple way, it uses the interruption of current through an inductor to cause a change in the polarity of the inductor's voltage (i.e. back EMF or inductive spike). Other techniques involve other switching power topologies or charge pumps. Power is conserved, so you you supply 12V @ 1A to a load then you must draw at least 2A from a 6V supply.
wrote ... Lastly, if anyone is still reading, is in this mag lev design ( he talks about operating his electromagnet at a particular frequency, wouldn't that create problems with back emf? and why would he not just run his electromagnet at a constant DC voltage and have his control circuit change that number up and down based on the feedback loop?
He has an antiparallel diode clamping the spike it sounds like. When the transistor is on current is increasing in the coil though it may eventually reach a maximum due to resistance of the coil or power supply limitations. When the transistor turns off the spike is clamped and current decays. Average current is related to the duty cycle of the switching transistor which is easily implemented with the TL494 chip.
Generating an adjustable DC voltage (in which case it's not really DC) would require more effort as it would require a full switching power supply, or really a class D switchmode amplifier, or otherwise a linear regulator which is more simple but dissipates substantial power as heat.
That makes a lot more sense, controlling the power delivered to the electromagnet by meaning of average power rather than an adjustable voltage. Thank you all for your responses.
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Amplifier and EM Questions
