[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]

This one is deleted.

[gar]

050621-1901 EST USA

bigal:

If your motors operate with two phases energized at once, and if each phase requires 2 amps, then you require 4 amps per motor, and 12 amps for three motors.

There should be no need for regulation, especially with a chopper drive.

If your drive is rated 80 v maximum input, then you need to analyze the drive and determine whether this is specmanship or a real valid figure for reliable operation.

With a chopper drive you could probably work fine at 60 volts input, and just lose a little in maximum speed capability.

Also on the emitter follower vs 78xx. Both would be classified as series pass regulators as differentiated from a shunt regulator. But, the means of controlling the regulator output voltage is quite different.

.

[bigal]

max input on drivers is 40V - output from transformer / rectifier = 40v

guess regulation is not needed, but since the parts are here, i may as well drop it in, if it is problamatical it is all of 10 mins to remover it.

[gar]

050622-0755 EST USA

bigal:

I do not know where 60 came from I should have said 30.

In other words whether the chopper drive is sourced from 40 or 30 volts will not matter in terms of low speed torque because the chopper will regulate the current to the motor, but a lower source voltage will degrade high speed performance.

.

[smarbaga]

the cct u posted looks like someone has put a zener on the ground leg of a 7800 series voltage regulator to raise the ground refference and therefore rasing the voltage regulators value.
- do u think someone could have replaced the original part and put the bx # in by mistake.
with a 7800 series reg, you would get the regulators value plus the zeners value on the output-
the resistors could be used to keep the zener regulating in a no load situation

[bigal]

how does a BDX33C become a 7800 series regulator? the BDX33C is a npn transistor, not a regulator. i will bear the zenner theroy in mind for future projects though, cheers.

[gar]

050623-1751 EST USA

bigal:

In a series pass regulator you fundamentally put an electronically adustable resistor between the input voltage source and the load where you want a regulated voltage. The transistor or whatever the device is is the variable resistor.

In the case of the NPN transistor with a fixed voltage applied to the base this occurs as a result of the characteristic of the transistor. In your drawing the fixed (approximately fixed under certain conditions) voltage is supplied from the Zener diode. The characteristic of the transistor that provides regulation is that a small current into the base can control a large current between the collector and emitter (for the transistor you specified this current gain is over 1000 to 1 for a fairly large operating range). Second this occurs when the base to emitter junction looks like a forward biased diode. There is only a small change in this base to emitter voltage for the large change in collector current. Let us assume it was constant at 1 volt. By the way the sum of all currents at a node (point) is zero. Thus, because of the large current gain from base to collector the output current is essentially equal to the collector current. A fixed voltage on the base puts the emitter voltage of an NPN transistor about 1 volt lower than the base and we get a fairly good regulator.

For a positive output regulator of the emitter follower type the transistor must be an NPN in the configuration shown in your drawing.

The 78xx regulator is a different anaimal. Internally it has a series pass transistor, but beside this there is a voltage reference source that has a more constant voltage than a standard simple Zener diode, and a high voltage gain difference amplifier to compare the output voltage to the reference voltage which is fedback to control the series pass regulator. ( In the emitter follower regulator you could consider the base to emitter juction as a difference amplifier of much lower voltage gain than in the 78xx.) The 78xx is essentially a more complex device and could use either an NPN or PNP transistor for the series pass depending upon design. Also may include some over current and temperature protection. The common lead, the one going to the negative side of the load, is where the internal reference gets its reference point from the low end of the load to control the output voltage. If a fixed voltage is applied to the regulator common relative to the load common, then by varing this fixed voltage you can change the output voltage of the regulator. The sum of the voltages in any closed loop is equal to zero.

Take these kind of devices and at lower currents play with these ideas. Get the manufacturers data sheets and study them. national.com for National Semiconductor. They make 78xx series and similar regulators, also may types of transistors. Get the data sheets for the 7805 and for your NPN.

.

[bigal]

having a few difficulties with this, got my board all finished but one module makes the motor whistle, and the module gets hot, takes alot of power etc..

weirdness...

any suggestions on what to look out for or should i buy another module (£30)

everything else on the board works... the SSR seems to conduct a bit even when off, which is weird...

datasheet for drivers:

http://www.selectronic.fr/includes_...on/GS_D200S.pdf

my PCB board:

http://www.cnczone.com/forums/attach...achmentid=8362

cheers people!

[Al_The_Man]

Selectronic Driver link did not work?
Al.