Help with a none audio power supply

[Kram]

Hello

I have been asked to make a 40v 2amp power supply and could do with a little help.

I am using a lm317HV to regulate the voltage but I think that this device will have difficulty supplying 2amps, the data sheet says that it will supply in excess of 1.5amps.

There is an application circuit in the data sheet that can increase the current capability using 3 Lm195 transistors in parallel.

All good so far untill I realised that i was having dificulty trying to figure out how 3 transistors can be connected in parallel. Can anyone do a quick drawing of how the LM195s should be connected.

Here is the circuit from the data sheet.

Thanks to anyone that can help.

Mark

[richie00boy]

errr.... just connect their bases together, their emitters together, and their collectors together.

Although I would just use a single TIP33C or something.

[sawreyrw]

richie00boy is right on how to connect them, but I would use the LM395s rather than a regular NPN, because they are short circuit proof. The LM395 is actually an IC that acts like an NPN power transistor, but it has very high current gain, some protection from overvoltage and overcurrent limiting/protection. I've tried to 'kill' these devices and the only way to do it is apply too much voltage in an inductive circuit. In other words, they can take a lot of abuse.

[macboy]

If you use the LM350 instead of the LM317, you will make your life easier. It can supply 3 A with adequate heatsinking. Remember however that you need to consider the voltage drop across the regulator and calculate the power dissipation. The 3 A rating is maximum, and not available under all conditions.

[Christer]

One shouldn't just parallel transistors without having some emitter degeneration to guarantee an even load sharing. On the other hand, for 2 A there shouldn't be any need to parallel transistors. There are plenty transistors that should be able to cope with this alone. Maybe that schematic was for very high output currents or something?

Edit:
I see now that it seems to be intended to be adjustable from some unknown voltage almost all the way down to around 1 V, so they may have paralleled the transistors for power handling. So what are you trying to build? An adjustable supply, or a fixed voltage one? The output votage matters a lot for the dissipation power in the transistor(s). What is the input voltage in your case?

[Kram]

Hello everyone and thanks for the help.

I would of used the LM350 but it is only capable of upto 35V out and my boss is demanding 40V for some unknown reason

the supply is to power 2 stepper motor drivers and 2 motors and reading the data sheet for the driver and the motor it would seem that it only requires approx 1.2amp with a supply voltage ranging from 24v to 40v. So the boss wants 40v and 2 amps making my life harder

sawreyrw i see you mention the LM395, well the LM395 and the LM195 are the same thing except that the lm195 is a mil spec part, RS and Farnell dont list the lm195 so of course because we dont have the Internet at work i spent ages wasting time looking through catalogues for alternatives to the lm195 and getting data sheets faxed to work. it took 2 seconds for me to find out that the lm395 and lm195 are the same thing when i got home and googled it.

oops small rant there. lol.

further reading of the LM317hv data sheet suggests that the device is capable of 2.2amp typical and 3.7amp max if Vin -Vout < 15V.

I am using a 18-0-18 transformer to get 50ish volts out. therefore my voltage differential is only 10v so it would seem that the extra bit of circuit probably isnt needed after all

anyway wish me luck and offer any advice if you can.

Thanks
Mark

[richie00boy]

What's the surge/startup current like on stepper motors?

Do you want the voltage to be variable or just fixed at 40V? Does it absolutely have to be dead on 40V?

[classd4sure]

Originally posted by richie00boy
What's the surge/startup current like on stepper motors?

Do you want the voltage to be variable or just fixed at 40V? Does it absolutely have to be dead on 40V?

http://www.solarbotics.net/library/p...f/motorbas.pdf

Good read..no.. great read!

"the supply is to power 2 stepper motor drivers and 2 motors and reading the data sheet for the driver and the motor it would seem that it only requires approx 1.2amp with a supply voltage ranging from 24v to 40v. So the boss wants 40v and 2 amps making my life harder"

Two motors going... X2 the current required, worst case. I trust you know exactly what the load is and are designing for worst case? If not just be a bad *** and go for 6 amps or something, a TIP should do.

[macboy]

further reading of the LM317hv data sheet suggests that the device is capable of 2.2amp typical and 3.7amp max if Vin -Vout < 15V.

No it doesn't. You are reading the "current limit" spec, which indicates the point at which the internal current limitting circuitry is activated to help prevent damage. It does not indicate the safe operating conditions. The 317 is capable of only 1.5 A, and even then only when the power dissipation is kept safe, and heatsinking is adequate.

[Kram]

thanks, excellent advice so far.

hopefully soon I will be able to better assess the actual current draw of the motors when our clients get round to sending out the hardware that the motors are supposed to run on.

Oh yeah the supply is fixed at 40v.

Thanks
Mark