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two in one: How to work with transistors and how to use a 24v system in a car and a 12v? Answered

1: So I have been playing with transistors, awesome little woks of science aren't they!?  Well everything I have done has been arduino based with the same voltage switching as the transistor's input.  Now of course there has to be a way to send more voltage across than is used to do the switching, but how do you add a ground and prevent that high voltage from entering the controlling circuit.  The basic idea is using an arduino to operate a transistor which controls a 12v DC light source.  Lets assume the arduino can't handle 12 volts and can only get up to 5v which is the highest supplied voltage.  Can a simple resistor attached to the emitter side work? I still feel like the voltage potential on the emitter would be too high for this to work, but I am new to transistors, any wiring diagram recommendations.  I have been through amplifier diagrams, but its too confusing to pick out just what is going on with the transistor, and the most important factor of why.

2: I was looking at aircraft lamps today for the sake of improved vehicle lighting, its quite popular around here, but the majority are 24 volts, at least they are on the brighter end of the spectrum.  Now I know you could use two 12 volt batteries and get the 24 volts and also just pull isolated power from one, (i think) to run the 12v system.  The problem I see is when applying voltage from a 24 volt alternator, It could potentially bring the 12v system voltage above desired levels right?  There isn't a high power easy 12v limiter is there either.  So would these require two completely separate systems along with 3 batteries or are there other ways of handling this task which allude me.

Bonus: for all those light lovers check out these incandescent beasts!: http://www.normanlamps.com/index.php/cPath/473_53
Oh no, now I realized these are PS type, there goes those radical fun ideas.  Would this mean probe start or pulse start, and if so how do they work?



Best Answer 4 years ago

Ahh Isolation is your desire,

Opto-Fets can run from a uP with PWM for a DC circuit and 1000 volt isolation.

If you want AC circuit isolation ( Third Pic ) and don't worry about the 470 ohm 1/2 watt resistor because as soon as current flows through it the main Triac turns on dropping the voltage to 1.5 peak volts !


any concept of simple electronics I had has now been ruined by these isolators. I have always gotten by without the understanding of pin types and just gone with a transistor is a transistor using minimal voltages and power levels, but wow. So what I take from this (middle picture) is that pin 1 would be wired to an arduino PWM pin. pin 2 would go to the arduino ground. Pin 4 would be wired to the positive side of the transistor to supply power to be switched in the opto-fet, pin 6 would then switch the transistor. But with the lack of any ground I don't know that this would work, but then again I don't know much more than there is a control pin, a + and a - on transistors. Also, why on earth would anyone bother with trying to make an amp using transistors when it seems that these opto-fets could allow much cooler, more precise, cheaper, and smaller amplification. I guess as of now there aren't exactly high power devices, but would it be that hard to engineer a photo-resistor which could handle 100 watts, or even 10 watts could revolutionize the world of sound right? A completely isolated, but also exact copy of the original sound, it could at least pass as a hi-fi gimic right?

But on the final note, how would I wire this for a DC 100-200 v source. The pottential for closing the circut on unrectified dc is just too much to worry about when a big cap and rectifier can solve the issue.

I Present you jj what I consider the HOLY GRAIL of MOSFET driving.

The TLP5991B ...

A 20ma LED driven photovoltaic cell that supplies an isolated 7 VDC at 24 uA just enough to drive a 500 VDC MOSFET.

Enjoy an unknown opto component, you could make this on your own !



To isolate a control circuit you use optocouplers like in this circuit.

Remote C 7b.bmp

The transistor does not, as you see, isolate the high voltage side from the low, in the sense that its not impossible to something nasty to the low volt side if something goes wrong. That said, if your circuit is done right, and the transistor is rated correctly (and then some) for the load, there is no issue driving a high voltage load with a logic signal. The maximum voltage drop from base to emitter is 0.6V.

While you might get slightly more than 12V across the battery driving your loads when its charging - you can see 14.7 volts "normally" on a charging battery, you can design around that. The major issue you need to be aware of is the phenomenon of "load dumping", when the output of the alternator rises to a hundred volts, transiently. You'd protect your circuit with a transorb + capacitors.,

I should have provided more details, but it sounds like it could work with even 200 volts? I'm looking at a 300 + rated transistor for this so definitely far more than required. So even that voltage, which could arc shouldn't require anything between the ground on the board and the emitter? I know it make take a couple resistors to get up to switching such a high voltage, like any amplifier.

I'd be inclined to optoisolate. Working on high voltages in an amateur setting could be a recipe for disaster.

high voltages in any setting is a recipe for disaster haha, I can't figure out if an opto isolator is a type of switch or if voltage on one side is induced to the other somehow.

Its essentially an LED and a transistor inside a box. When the LED is on, the transistor turns on. The box is designed so that it would take a couple thousand volts (or more) to jump over the internal gaps, so it isolates.

If you're used to working on high voltages, you can handle all the gotchas and the issues.....but you might only get one chance to learn...

I assume it would need to be used to operate another transistor for it to handle the power requirements?

1. As you know the arduino is limited on it's current output. The best way to use an arduino to turn on and off a 12V or higher source is to use a relay. You'll use a transistor to provide the 5V and >500mA the coil on the relay will need to switch. Now if you want to use a larger transistor you can. As long as it will trigger at 5V and can handle the 12V. Just tie the grounds together. You won't have any problems with voltage differences. But you might run into ground loops depending on what your doing. With lights it shouldn't be an issue.

2. It may be best to have a saparate pair of 12V batteries for your 24 volt system. So you'll have to find a way to add a 24V alternator to your engine. I can't think of a way to wire the batteries in series for the lights and in parallel for a slightly stronger 12V alternator to keep them charged.

I should have provided more details, I am actually looking at something like a DC light dimmer which is why a transistor is required, and it will probably be operating at more around 100-200 volts.


4 years ago

Simply use a N type MOSFET of sufficient current ability as your light.

Mosfets usually can handle 60VDC but you can run 400VDC just as easily.


....we should point out this is NOT isolating either. The only way to ISOLATE the high from the low voltage is to use "optoisolators"