what is this thing for?

[bigal]

Hi,
i am currently making a new control board for my cnc router (here ) but have hit a wierd part.

For the most i am copying the old board, however the power to the drivers (and thus to the steppers) is a bit weird..

They had a torroydal transformer (OP 25vac - 120VA) going into a recifier, going into a big smoothing cap - output 40VDC. good fo far, i was going to use it like that, however, they also had a strange circuit, using a BDX33C "Power Darlington TR" (DATASHEET )

and a potential divider on the base.

Heres the two circuits, first their one, then the one i was going to use.
(not the resistor on the potential divider measures 0.89Kohm (with broken track) but the colours read it as 1.8K - WTF???)

anyway, is there any advantage to doing it their way, or am i ok using the simpiler meathod (not using the darlington "NPN Epitaxial Transistor")



Cheers
Alex

EDIT: that diode is a BZX 55C 39

all this circuit seems to do is make that 1k resistor hot...

[gar]

050621-0736 EST USA

bigal:

It appears someone is making a voltage regulator. The transistor is configured as an emitter follower. This means the emitter voltage will be about 1 diode drop below the base voltage. It appears that the diode D3 is a 39 volt Zener diode which should give you about 38 volts output. But you do not have enough input voltage to get regulation. However, you may have more than 25 x 1.414 unloaded or at high input line voltage.

The 1 k resistor gets hot because you are dissipating about 1.5 watts.

The 1 k resistor is to provide a minimum load on the transistor.

There has to be enough base drive (from the 1.8 k to 0.89 k resistor) to give you your desired output current.

.

[bigal]

ok, i see now.
It is basically acting as a 78xx regulaor but for 38V... few more questions though:

1) since 78xx's only handle 1A, what current can his handle (it should handle about 6A..

EDIT: spec sheet says 10A, so thats cool.

2) If it doesnt have enough power to regulate (hence why it gives equal volts out as it does in, then if i pump , say 60V in, i will only get 40 out, but pump the 40V in i get 40V out.... in which case, since the transformer / recifier / capacitor only gives that out, do i need it?

would it be beneficial to impliment it (i am tight on PCB space, but could squeeze it in if i had to...)

BTW: i have 40V to regulate, >2 above regulated voltage...

[Al_The_Man]

What you need to ask yourself is, do you need regulation, most do not use regulated supplies for drivers. If you need to drop the voltage, look at removing toroid turns if necessary, ( a search here will show the method).
Al.

[bigal]

personally i would of left it out, but i just wanted to know if it is necciary, since i get 40V with or without it i wouldent think it necciary..

[drk]

In many cases the transhormer manufacturer takes in the loading effect when making the transformer , if you are using this power supply for motor drives, you dont need regulation, I havent used it for the three machines Ive made, and am not using for the fourth Im making now.

[bigal]

just noticed that the MAX on the drivers i am using is 40V, so i susspect they wanted to use the max but not go over it, so in case of any problems, they added this little circuit to prevent any damage (which would be alot) if anything happened..

[Al_The_Man]

I think you will find it will not work that way as the regulator will have to be kept forward biased all the time, so if the input voltage is low then it will not conduct at all. If you require it to regulate at 40v and the input to the regulator drops below this, then you get no output.
Al.

[bigal]

i was able to include it in the PCB design, so if i want, i can allways bypass it by shorting 2 pins... either way i am covering my bases..

[gar]

050621-1105 EST USA

bigal:

No the emitter follower does not operate like a 78xx. The 78xx is much more complex and has much better regulation, and some variations like the LM323 have over temperature protection. Maybe some 78xx do today.

Because a device like a transistor has a maximum collector current rating of 10 amps does not necessarily mean you can operate it with 10 amps flowing. A more important criteria is power dissipation in the transistor and maximum junction temperature. You do not want to operate at maximum junction temperature. You might find in some applications that the transistor rated at 10 amps could not be operated above 1 amp.

The cooler I can run a device the longer its life, and thus reliability is increased. You never want to design a circuit to continuously run at maximum ratings. For example for a resistor I do not generally run it higher than 50% of its rating. Also note this would be a function of the maximum ambient temperature you want to work at. If you put all your circuits in an enclosed box you might find ambients in the box of 150 to 170 deg F. In this case and the resistors I would need to study the parts further, but now I would probably use a criteria of 25%.

On the operation of the emitter follower. If the voltage on the zener drops below its breakdown voltage, then very little current flows thru the Zener. In this case you have a resistor from collector to base. There will be an equilibrium point where the voltage drop from collector to base is sufficient to produce enough base current to produce the required load current. This primarily will be a function of input voltage, collector base resistor, transistor beta at the operating point, and the load resistance.

Because a product has a certain rating does not mean that product can be operated at that value.

For the most part the use of an unregulated power supply is just fine for a motor power supply. If you had very large swings in input line voltage, like 95 to 135 v, then maybe a switching regulator or use of chopper drivers would be advised. When we design an industrial device for reliable operation we design to work continuously at 135 V input. In larger systems we put a Sola constant voltage transformer at the input. The Sola is an extremely rugged, reliable device, expensive, and runs hot. However, the Sola and the equipment connected to it survive lightening storms and other power problems very well.

Note: on a simple linear transformer, rectifier, capacitor power supply under fixed output load resistance the output voltage is approximately proportional to input line voltage.

.

[bigal]

thanks alot, i am working on the basis that because this was all present on the origional setup, it will all work ok on this setup... however, if i experiance problems, i simply unsolder a few things / cut them off and join a jumper / solder gap. the system is then unregulated. i understand about the maximum ratings, there is 3x 2amp motors and only if all 3 are running simutaniously will there be a 6amp draw, and since the drivers are choppers then i guess this helps too. anyway, thanks for the tips, i shall bear it in mind, i will be making the PCBs tonight and probably be all set up tomorrow!

[gar]

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