RS Function Generator RS 435-226: RS 434-289: RS 591-124

[crustytv]

The presets I ordered turned up today as did the multi-turn pot from Norman

Can anyone shove me in the right direction, I've powered it up and voltages seem ok however when I probe the pins for the various waveforms I'm not getting anything other than a raggedy sine across all, which cannot be adjust by any of the presets or pots. Its even there when the unit is not powered. I'm not sure I'm wiring in the pots correctly or if I'm doing some thing even more stupid my brain is not in the right place for other reasons but I'm trying to occupy myself.

[Mark Hennessy]

What frequency is this sine wave?

And you are using a split PSU?

[Alistair D]

As Mark has pointed out the supply needs to be + and -15V. The 0V pins being the common terminal.

The ideal complement to the PCB would have been the R.S. ready made 78/79 series regulator PCBs.

Al

[crustytv]

I'm only feeding in 15V from the bench PSU, told you I know diddle squat when it comes to this project.

I've now got the three waveforms though, Jeffrey was right about the grounding and one wire had come off the mult-turn. The square wave is pretty bizzare but I've not set-up the presets and I guess the power needs sorting before I proceed anyway, not sure how I do that though.

Waveforms present

Bench PSU feeding 15V to the device.

[crustytv]

Probe is correctly compensated.

I was oblivious to the -15 and had no idea what Mark was talking about when he mentioned split PSU, now I see what you all mean.

[crustytv]

Dug out another PSU, applied the -15V and now the Square wave is sorted.

[Alistair D]

Al

[crustytv]

Further numptiness but where would I find a power supply that will provide +15V & -15V

[Mark Hennessy]

It would be very easy to DIY it, and it would be a great learning experience. Would be happy to help...

If you don't want to do the design, this Velleman kit looks great: http://www.theelectronicsshop.co.uk/con ... /p293.html - just add a mains transformer. Good price too...

At work, I have a suitable PSU - probably an RS kit - that has everything including the mains TX mounted on the PCB. I've had it for years; will never use it. Yours for postage.

[crustytv]

it would be a great learning experience

Hi Mark,

Can we discuss what is involved in a DIY effort? Description and diagrams would help lodge it in the ageing grey matter, I can then decide if I feel up to the challenge. If I decide to wimp out I could then take you up on the kind offer of the suitable PSU.

I've always avoided home build, its a confidence thing, but If I can tackle old Tellies then I should stop wimping

I've run through the set up as per the RS sheet, its looking good now. One thing I'm having trouble with is figuring out how I hook in the 4K7 level pot. I have four pins 0V, o/p, a, b.

[Mark Hennessy]

Hi Chris,

Taking the last question first:

(a) goes to the wiper of the pot
(b) goes to the "top" of the pot - the end that the wiper is at when turned fully clockwise.
(0V) goes to the other end of the track - the end that the wiper is at when turned fully clockwise.

(o/p) is the output, which goes to whatever connector you're using. E.g., if using a BNC socket, it'll be the centre pin.
(0V) is also used as the output ground connector - the outer screen of the BNC.

To the power supply...

Ideally, it would be built in the same box as the oscillator because that makes it an "all in one" product. But it could be separate if you prefer.

It starts with the input connector (personally, I much prefer an IEC inlet rather than a captive mains lead). Next would be a fuse, followed by a switch, and then the mains transformer. The hardest part about all that it making it safe - mostly heat-shrink and shrouding and grounding - that's common sense that you already have in spades

The transformer doesn't need to be big or expensive - the current draw of the oscillator is tiny. A small 6VA frame transformer would be fine - the sort of thing that most of us would have in our junk box if you don't already have such a thing yourself. The secondary would need to be either centre-tapped, or have dual secondary windings - most transformers carried by RS, Farnell, CPC, etc are made that way, so no problem.

I'd recommend that the transformer is either "15-0-15" (i.e. 30V with a centre tap), or "0-15, 0-15", which means two separate windings, each of which being 15V. Arguably, 18 instead of 15 volts might be better here...

All that said, because the current demand is so very low, you could get away with a transformer with a single secondary winding (I'd go for 18V in this case). Or, a "wall wart" could be used if it has an AC output of about 18V - these are convenient because you don't need to do any mains wiring, but that might not be the easiest thing to find? All options to be investigated as and when...

The transformer can connect directly to the Velleman kit, and that would be job-done! The regulators won't need a heat sink, so it's really very straightforward.

But if you're feeling confident, then you can build the rest of it yourself on Veroboard or even tagstrip. Here's what you need:

Rectification --> Smoothing --> Regulation

It's worth looking at the datasheet for the Velleman kit, as there is a schematic in there. http://www.velleman.co.uk/manuals/k8042.pdf

The rectification is 4 diodes (D1 to D4). Smoothing is done by C1 and C2. And regulation is done by VR1 and VR2. This kit uses LM317 and LM337 regulators, which have the advantage of being adjustable. If you were doing it yourself, you'd probably be better off using the 7815 and 1915 regulators - that will simplify things and usefully reduce the component count.

Some years back I wrote an article about the basics of power supplies, and it deals with everything apart from regulation. It might be useful background reading, as it expands on the safety and practical aspects, and also looks at rectification and smoothing. But it's not essential to fully understand all the theory at this stage - if you're anything like me, you might prefer to learn by doing, and power supplies are a good way to do that. Basically, once you've got the primary (high voltage) bits assembled and safely insulated, then you can experiment and learn in reasonable safety with the secondary outputs. You can even use a low-powered lamp limiter if you're worried about blowing something up.

Hope this helps,

Mark

[crustytv]

Thanks for all your advice Mark, it is all taken on board and very informative. I hope you won't mind but I let my mind wander and it ended up playing. This little lash-up will probably have you or others raising your hands in horror. I found a little TX in the spares a 240V 0-6V 0-6V and decided to build a 555 charge pump. I have generated the -ve rail, so it worked and I thought I could have my +ve and -ve rails but there's a but, read on.

555 Charge Pump Description

The 555 is connected as an astable multivibrator operating at around 1KHz. During the timer output at pin 3 goes positive, the series C3 capacitor charges through the diode D1. When the output switches to ground, C3 cap discharges through D2 and charges C4 to a negative voltage. As a result of this the voltage at the junction of the anode of D2 and cathode of C4 will be always negative with respect to the ground. The output will deliver about 50mA.

As I mentioned above the TX is a 240V in and 0-6v 0-6v out. I expected to have +6V and -6v I've ended up with +6VAC and -12VDC. The photo shows below the fluke reading the voltage out of the charge pump and the Yellow meter is reading the voltage in. Not sure what I have to do now, how do I get the +6VAC to be +12VDC.

I reasoned I had two options, one I could use a single winding shared for both rails, tapping off before the charge pump for the +ve. The second option I thought was to use one winding for the +ve and one winding for the -ve,. That seems to have all gone to pot and I think I'm getting myself in a pickle, need to go read some more.

[Rallycoordinator]

Separate supplies
---------------------
1: Proper -ve supply, with centre tapped transformer, and a bridge with +ve and -ve regulators

2: As above but with separate windings, rects etc. Essentially 2 separate PSUs. If you use +ve regs on both of them just watch out that both regs are actually sourcing current. They can't sink current.

If you can find two discontinued RS 434-289 boards (which will match your function generator board), this could form the ideal basis for a dual -/+ 15 volt supply.


Mike

[Alistair D]

A variation on Jeffrey's rail splitter, connect 2x12V1.3W zeners in series then connect a 120 ohm resistor from the zener to the +ve of a 30V supply. Connect the other end of the zener chain to the supply negative. The centre point of the 2 diodes will become the 0V connection to the PCB. Connect the zener/resistor junction to the PCB +ve and the PCB -ve to the supply -ve. You may need to put a small electrolytic across each zener for stability.

Al

[Mark Hennessy]

Hi Chris,

No, I'm not about to raise my hands in horror because I'd hate to discourage anyone from experimenting and learning. OK, it might seem to be an "unconventional" way to achieve the objective, and it might or might not end up being the final solution, but it's the journey that is important.

As you might already know, these charge-pumps can either be used as voltage "doublers", or to provide a voltage of "equal magnitude" but opposite polarity (the latter is the one you have, of course). The classic MAX232 has a pair of charge pumps to produce plus and minus 10V from the incoming 5V rail so that it can generate reasonable RS-232 signals. They are built into most flash memory chips, and are often used to drive LEDs. Definitely a useful technology.

Things to watch for:

- First, the output voltages are approximate at best. So that's why I used quote marks above. There are losses in the diodes and switches.
- Also, they aren't terribly well regulated. That means the voltage "sags" as the load current is increases.
- And, any "switching" supply will have ripple on the output.

Whether any of these matter simply depend on your application. The TL084 is perhaps not bothered, but the 8038 might be, and as a result it might well be that you would need to provide some sort of regulation after the charge pumps. That's what I did with a 25V generator I made some years back.

---

But before getting too involved with the charge pump, it might be worth thinking about the rectification stage.

If you feed 6V AC into a full-wave rectifier that is followed by a smoothing capacitor, you'd expect to get about 8.5V DC. This is because AC is generally quoted and measured in RMS terms (unless otherwise stated), but the rectifier will produce the peak voltage. For a sine wave, the magic number is 1.4 (which is the square-root of 2), so 6 times 1.4 is 8.5.

This is approximate because there are losses in the diodes (0.7V in each, and for a bridge rectifier, two are in series at once). But against that, the stated 6V from the transformer is at full load - when at less than full load, the voltage rises. Perhaps by 20% for a small frame transformer like the one you're playing with. Easy enough to measure...

It's probably worth spending a few minutes experimenting with rectification strategies (if you wanted, you could try the ideas I've put on my article). From experience, it's a subject that trips people up, and there are one or two tricky concepts in there. When I run the power supply course at work, normally at least one person blows up a capacitor!

---

If you wanted to use the charge-pump approach with your transformer, here's one option:

- Connect the two 6V AC windings in series to give 12V AC.
- Rectify and smooth that. You should have approximately 16-17V DC.
- Feed that into a doubling charge pump that makes approximately 30V DC
- Feed that into an inverting charge pump that makes approximately -30V DC
- Feed these two unregulated rails into a pair of voltage regulator ICs (such as the 7815 and 1915) which will make +/- 15V

The regulators will clean up the switching noise, and they will ensure that the generator has a rock-steady pair of 15V rails. It might seem that we are making very high voltages only to "waste" that in the ICs, but remember, these are optimistic voltages - they will be less in practice.

The main drawback with this approach is that you can't use a 555 to make the -30V rail. By the time you've worked out how best to tackle that, you might as well have brought the correct mains transformer! But then, you wouldn't have learnt anything

Another way:

- Take 6V AC from one of the secondary windings, rectify and smooth it to give approx 8V DC
- Feed this into a 555 voltage doubler to get approx 16V
- Feed this into a 7812 to make 12V (you need at least 3V more than the output for the regulator to do its job)
- Make an exact copy of all this, powered from the other 6V AC winding
- Join the two outputs together to make a split supply. This works because the two circuits are totally separate from each other.

This approach has the advantage of working with the 555 timer, so you've basically done the bulk of the work (you just need to reconfigure the diodes and capacitors in the output to make a doubler rather than an inverter). The only thing to check is that the function generator PCB will be happy at +/-12V



There is another, simpler way:

You can make voltage doublers that work on AC. These just require extra diodes and capacitors - no 555s required. Have a look: http://en.wikipedia.org/wiki/Voltage_doubler

- Connect the two 6V AC windings in series to give 12V AC.
- Feed this into a voltage doubler to get approximately 20-30V DC
- Feed that into an inverting charge pump to get approx -20V DC
- Feed these two unregulated rails into a pair of voltage regulator ICs to make +/- 15V

Again, the 555 won't work here, but a simple circuit using transistors could be devised if you want to explore further.



Or:

- Take 6V AC from one of the secondary windings, rectify and smooth it using a voltage doubler to get approx 16V DC
- Feed this into a 7812 to make 12V
- Make an exact copy of all this, powered from the other 6V AC winding
- Join the two outputs together to make a split supply, as before

Of all the options, I think this is probably the one I'd pick...



Of course, there is another, much more complex option:

A pair of switched-mode DC-DC converters could do this equally well. And you won't need the final step of regulation:

- Connect the transformer windings any way you like to give either 6 or 12V AC
- Rectify and smooth this
- Use a "Boost" converter to step this up to 15V DC
- Use a "Buck-Boost" converter to make -15V DC

Now these are quite different in operation to flying capacitor charge pumps - they store energy in an inductor rather than a capacitor. Luckily, the inductor is not critical - especially for low-power operation. There are many different chips out there that will do the necessary switching and regulation - try playing with the applet on this page to get some ideas: http://www.ti.com/ww/en/simple_switcher/



But ultimately, I'd regard all of this as a (valuable) learning experience first and foremost. Generally, a designer would only go to the complexity of a switching converter when some other constraint forces them to - obvious examples being battery operation, or a need for high efficiency. This is especially true for an analogue application, where noise might be an issue.

I hope these notes are of interest. Apologies for where I teach you to suck eggs, and equal apologies for when I've gone over your head (please tell me if I do either).

Oh, and when you've eventually had enough of all this, I did find another suitable PSU in the attic. It's an encapsulated lump that is intended to be soldered to a PCB, but I'm sure it could be clamped or secured with double-sided tape or similar. It takes in 240V and gives +/-15V regulated at 200mA IIRC. It's made by RS. If interested, I'll bring it downstairs, test it, and post all the details. It's probably better than the first one I suggested, and I'm sure I'll never need it.

All the best,

Mark

[Refugee]

I used the circuit in figure 4 here to run a 12 volt fan from 6.3 volts.
http://en.wikipedia.org/wiki/Voltage_doubler
It runs directly from the heater supply in the amp it is cooling and floats either side of the center tapped winding so both connections to the fan must not touch the chassis. No problem.

If you build two and add them to both windings you will get two isolated DC outputs of between 12 and 15 volts from the transformer you have got there.

[Mark Hennessy]

Mark has suggested several useful approaches. However if I was in Chris's position I'd go for simplicity.

Just for avoidance of doubt, that is my recommendation also. But there is a lot of fun to be had experimenting, and sometimes, to learn something, you do need a project in mind. I often do. The techniques explored might not make it into the final function generator, but they might well provide the perfect solution to some other problem in the future

[crustytv]

Hi Chris,

Oh, and when you've eventually had enough of all this, I did find another suitable PSU in the attic. It's an encapsulated lump that is intended to be soldered to a PCB, but I'm sure it could be clamped or secured with double-sided tape or similar. It takes in 240V and gives +/-15V regulated at 200mA IIRC. It's made by RS. If interested, I'll bring it downstairs, test it, and post all the details. It's probably better than the first one I suggested, and I'm sure I'll never need it.

All the best,

Mark

Hi Mark,

I would like to take you up on the offer of that PSU.

I will be exploring all the other options presented by you and others as a good educational experience but at a leisurely pace so as to not overwhelm myself. For now would just like to get this function Gen housed and running. I still cannot get this 4.7K gain pot to work I know the pot is fine but what's odd is I don't appear to have any output on the OP pin on the PCB. Need to look into this when my head settles as for personal reasons, I'm not firing on all cylinders.

I do appreciate each and everyone's input and please don't think I've ignored it if I've not responded or tried them all yet.

[Mark Hennessy]

Hi Mark,

I would like to take you up on the offer of that PSU.

Hi Chris,

It's an RS 591-124. It measures 89 by 63 by 31mm approx. Once I've had a tidy-up, I'll wake it up slowly on the variac and make sure it's OK, and all being well, I should be able to get it posted on Tuesday. Send me a PM with your address...

Cheers,

Mark

[Terrykc]

... I've run through the set up as per the RS sheet, its looking good now. One thing I'm having trouble with is figuring out how I hook in the 4K7 level pot. I have four pins 0V, o/p, a, b ...

Mark has already answered this but I wonder if this makes it clearer?

I'm not surprised you were confused! The o/p pin doesn't go to the pot at all - and is incorrectly marked +15V on the schematic, to make matters worse - and the a & b connections appear on the schematic as 3 & 6 with the 0V connection not being marked at all!

As for the PSU, why don't you try the easy approach as shown in Mark's article?

http://www.markhennessy.co.uk/articles/ ... al_psu.gif

The left hand one is simplest, you probably have all the bits for a quick lash-up and should get results immediately - a Rolls Royce version can always be substituted later on!

[crustytv]

Mark has already answered this but I wonder if this makes it clearer?

Indeed he did and I followed his instructions but I still don't have any output when I connect the scope to the o/p connection. Not to worry I will get there in the end and its probably something silly I've done.

[TVJON74]

Hi Chris,

Just found 2 of these while trying to tidy up the workshop!

RS PSU PCB's

Yours if you would like them.
You just need a handful of bits and you will have your -&+ power supply

[crustytv]

Thanks to the generosity of Mark and Jon I have a couple of options for power and to further experiment with this generator build.

Removing my two bench PSU's from duty I hooked up the encapsulated PSU Mark sent me, as expected it all works perfectly well. A nice neat tidy solution for the -15/+15 rail requirement. I need to find a project box for this now.

Function Generator being supplied by the encapsulated PSU

Meanwhile as a parallel task, the two RS boards Jon sent arrived. These require building, I've ordered two Voltage regulators an L7815CV (+15V) and a L7915CV (-15V) hopefully they should be here soon.

I've been trying to find online a datasheet/ circuit and build instructions for the RS 434-289 to get component types and voltage values, thus far I've been unable to find anything. I understand I have to build both boards, one will provide the +ve rail whilst the other the -ve rail.

I'm a little unsure, do you populate the side dependant on if its the -ve (79xx) or +ve (78xx) board or do you populate both sides? Am I making that/myself clear? What TX should I use to power both boards and how?

RS 434-289

[TVJON74]

Hi Chris,
I will look to see if I still have the data sheet for it tomorrow.
I think for the positive supply fit the 7815 device to the PCB (three central holes by the heatsink marking, with the tab facing the outside. Then the 4k7, 0.47uF and the 0.1uf caps in the top right hand corner. The 4700uF cap and bridge go in the middle of the PCB as marked. The COM 7800 and OP 7800/7900 are your output pins. For the negative supply fit 7915 device and fit the parts to the top left hand corner. The 4700uF cap and bridge go in the middle of the PCB as marked. COM 7900 and OP 7800/7900 are your output pins.
I hope that makes some kind of sense to you??

[TVJON74]

From a quick look at the datasheet, you need a minimum of 17.7v dc input to maintain the 15v output. I think I would probably use a transformer with a 15v secondary, this will give around 21v dc so you have a bit of head room.