90vdc power supply

[mark pirate]

Hi all, I am going to build a 90vdc power supply for my ever increasing battery set collection, has anyone got a circuit?

I have a suitable tx and can probably find most parts lying around my workshop

Cheers,
Mark.

[Darren-UK]

Yes, have a read of this .

[mark pirate]

Thanks for that Darren, but what I want for now is a simple unregulated PSU using a 240-90v tx that I have in my bits bin and a diode bridge, smoothing caps etc, similar to some used in mains/battery sets in the fifties. My bench PSU provides LT.

Cheers,
Mark.

[FRANK.C]

Hi Mark
This is a simple power supply that I constructed to run my battery sets off.

It basically uses a string of 10V Zeners. Adjust R2 according to your transformer. The relay was just a small 12V one that I had in the junk box and I used it as an over current trip. Once the current through it reaches approximately 20mA the contacts will change over. R3 provides a holding current. The electroltic capacitor across the relay coil stops surges from tripping it and also keeps the coil energized as the contacts change over.

Sorry the diagram isn't very clear

frank

[Herald1360]

Strangely enough, a 240:90 transformer isn't a very good starting point for an unregulated 90V supply. By the time it's rectified and fed to a capacitor, it'll give nearer to 130V than 90. However a simple shunt regulator with a few cheap zeners can take care of this....

Circuit below should be OK for up to about 15mA @ 90V ish before it drops out of regulation. The zeners are 400mW 22V types, the 8.2K resistor represents about a 12mA dc load. (Put the TX 90V secondary where the Sine source is.)

Cheers

Chris

[MALC SCOTT]

Hi, factory built eliminators often appear on ebay for little money. Keep an eye out. Cheers, Malc.

[John Robson]

What is it your powering, and what do you estimate the current consumption
will be ?. Have a go at measuring it.

Regulation isn't always important, so long as the voltage is approximately near
the required voltage. For example 85 volt.

If you know that your equipment will allow for this, then you can vastly reduce
the scale and the amount of components required, and keep it simple.

[mark pirate]

Thanks to all, I have taken copies of the circuits kindly supplied by Frank and Chris, time now to rummage through my many boxes of bits
to see if I have all the parts to make a start.

As I have four sets waiting to be powered up, there is no better time than while I have a (relatively) clear bench .

Cheers all,
Mark.

[paulsherwin]

You can bodge something together from whatever you have to hand, since you only need 60-90V DC at maybe 10mA at the end of it. Old VCR transformers often have several windings which can be connected together to get maybe 50VAC offload, which will be fine. Alternatively you can connect two transformers of different voltages back-to-back to get the right AC voltage, or use a voltage doubler or tripler with a 20-40V AC supply.

The 1.5V or 7.5V LT supply is much more of a challenge

Paul

[Tim]

I made one with an 80-0-80@80mA/15-0-15@400mA transformer( I had a few lying around) and a PL82 just for fun! Haven't sorted out a current limit yet though.
Doesn't have an LT supply , but a 1.5 volt HP2 is handy for that.

[KeithsTV]

I'm in the process of building a 90v & 7.5v supply to power a BAC31. I used components that were to hand including a Decca Bradford mains transformer. The rectified voltages from this would provide the required voltages for the regulators so I opted for a doubler on the outputs of the transformer. The doubler need not be used if a suitable transformer is available.

I decided to use regulators with discrete components as this allows the current limit to be set at levels appropriate for the HT and LT current requirements. The HT limit is set at approx 30mA and the LT limit is set to 65mA, more than enough for the BAC31. The LM317 type of regulator commonly used for LT regulators has a typical current limit of an amp or more.

The 90v series pass transistor is a high voltage FET commonly used in switch mode power supplies and almost any high voltage N channel FET can be used. Both the pass transistors will need heatsinks although they do not dissipate much heat.

Keith

[Robert Darwent]

Hi Keith,

Thanks for sharing your psu circuit diagrams with us all, very interesting! For those of us not sure of our circuit design abilities, myself included , could you point out which resistor(s) are performing the current limiting?

I was thinking of modifying the LT design to be variable also (1.4v to 7.5v) and to be able to set the maximum current allowed to different values as well. As you will be aware, for example, some 2v LT supplies need to be far in excess of the 65mA you have set. What would you expect the realistic maximum LT current attainable to be with this design simply by altering the appropriate resistor values?

Many thanks!

[ppppenguin]

Originally Posted by rrdmpb

For those of us not up on circuit design, myself included , could you point out which resistor(s) are performing the current limiting?

The 22R resistor in series with the output sets the HT current limit. As the current rises, the voltage across the resistor starts to turn on the BC184. This tends to turn off the output FET. It will be a fairly soft limit because the transistor doesn't turn on suddenly at a fixed voltage but this doesn't matter. The 10R resistor does a similar job in the LT supply.

The LT supply, as it stands, won't go below 5.1V, the zener voltage. You could use a lower voltage zener but not by much. This is because the 2K2 resistor in the emitters of the long tail pair needs a reasonable voltage across it.

For an LT supply that goes down to 1.5V you may as well use an IC regulator such as the LM317. I like Keith's discrete design but for LT I feel the chip will win. I'm sure there are low cost chips that also have variable current limit but can't recall them immediately.

[KeithsTV]

OK here's a short and hopefully simple explaination of the functions of the circuit. I'll refer to the LT circuit but the principles are applicable to the HT circuit.

The output voltage is set by the 1.5k and 4.7k resistors across the output and the 5.1v zener by the formula ((1.5 + 4.7)/4.7)x 5.1v. This sets the minimum output voltage to approx 5.1v. To get an output voltage less than 5.1v either reduce the zener voltage or put a potential divider across the zener. (I'll draw out a circuit to show what I mean and post it later). I chose a 5.1v zener for two reasons, I had some available and the temperature coefficient of zeners around 5 to 6v is close to zero so the output voltage won't drift much with temperature.

The current limit is set by the 10 ohm resistor in the emitter of the TIP121. As the output current increases the voltage across the resistor increases until it reaches approximately 0.65v. This turns on the BC184 which reduces the base current in the TIP121 and reduces the output voltage. The current limit is set by the formula 0.65/10. To increase the current limit decrease the value of the resistor.

As the current output increases the power dissipation in the TIP121 will increase. In this case with approximately 17v from the rectifier and an output voltage of 7v there will be 10v across the transistor, at 50mA this would result in the transistor dissipating 500mW. For this dissipation a small heatsink would be required. If the output current were to increase the dissipation in the transistor would increase.

I have a circuit for a 2v regulator I designed to use with a radio with 2v filaments This has a current limit of about an amp. I'll dig it out and post it.

I've just seen Jeffreys post and yes an IC design for low voltages does have advantages but I didn't have any available. Also the typical LM317 type will only go down to 1.25v. Designing a variable power supply that can range from 0v to 30v at up to 2A is quite an art but power supplies, such as these, which operate over a relatively small range of voltage and current is much simpler.

Hope that helps

Keith

[ppppenguin]

Originally Posted by KeithsTV

... Also the typical LM317 type will only go down to 1.25v. Designing a variable power supply that can range from 0v to 30v at up to 2A is quite an art but power supplies, such as these, which operate over a relatively small range of voltage and current is much simpler.

The LM317 etc will only go down to 1.25V because the internal voltage reference is a bandgap device which is typically about 1.2V. Going down to 0V is quite tricky. A negative auxiliary supply helps! However you shouldn't need less than 1.5V for valve heaters unless you plan to use some of those unusual early mains ones with 0.8V filaments at rather high currents.

At high currents, one problem is getting rid of all the heat dissipated in the regualtor when you want low voltage and high current at the same time. This is sometimes solved with a switchmode pre-regulator which adds complexity and electrical noise.

[KeithsTV]

I've started a new thread on a 2v power supply in the components and circuits section as this is in danger of drifting off thread from the original topic of 90v power supplies in this section.

Keith

[Robert Darwent]

Keith, Jeffrey,

Thank you for your explanation's of the circuit action involved with the two power supply diagrams. I have been playing around with the LM317 chip recently and have successfully built a couple of LT psu's using it, however I was interested by the current limiting offered by Keith's circuits. You've both given me something new to experiment with!

Many thanks again!