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

[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.

[Mark Hennessy]

Glad it arrived safely - quicker than I expected as well

It's annoying that the 7815 and 7915 have different pin-outs (the input and ground terminals are swapped). The 7815 has the nicely logical pinout of IN, GND, OUT, as viewed from left to right on the PCB. When it comes to the 79xx series, I always look them up to double-check. Once upon a time I had it locked in memory, but since getting ADSL at home, I'm getting lazy...

It's a clever little PCB - someone really thought that through. For the 78xx, you populate the components on the right, and the opposite is true for the 79xx. Of course, the 4700uF and bridge rectifier are needed in either case, along with the 470n in the middle (those caps aren't critical, BTW - anything between 100n and 1uF would be fine there). Note also that the connections at the bottom change according to the regulator fitted.

It's worth saying - and I hope this helps - that how to use that board might be obvious to some of us on here, but that's only because we've been using the chips for years, and know how to pander to their needs. But without the explicit instructions from RS, it's definitely confusing for someone who is relatively new to all this!

If it helps, you don't need to use such a large smoothing capacitor. 4700uF is massive. For your oscillator PCB, for example, you could get away with a tenth of that value. To determine the appropriate value, you could go through a whole load of maths, or you could follow rules of thumb or similar. In other words, it won't hurt to experiment a bit. The value has an effect on ripple and stuff, and by implication, how low the mains needs to dip before 100Hz ripple appears on the output (try that on your variac!). But do watch the voltage rating. Take the transformer voltage (say, 15V), add 20% to that, then multiply it by 1.4. That's very close to 25V, and a 25V cap would probably be OK, but it wouldn't hurt to go for 35V. Verify it once built by ramping your variac up to 250V or thereabouts.

Another thought - probably more of a "thought experiment" at this stage - is that you could build both boards identically. That's right - you can make two identical 15V supplies using 7815s, or even 7915 if that's what you have to hand*. Having done that, recall a point I made earlier - because both boards are producing 15 volts that is floating - both with respect to earth, and with respect to each other - then you can think of them as being batteries. So you can connect them in series to make -15, 0, +15.

(* Yes, I know you've got samples of both regulator types on their way, but it's still a useful concept to chew over during a quiet moment. In practice, it is sometime done because negative regulators don't always behave as perfect mirror images of their positive counterparts.)

Anyway, glad you're sorted for this project - and the next**

(** Hmm. A simple TDA2030 amplifier to "beef up" the output of the oscillator board is worth thinking about. And RS used to make a PCB for that IC: 434-576, it seems. I don't have any here, but I'm sure someone on the forum could come up trumps? Sorry - I'll shut up now!)

All the best,

Mark

[crustytv]

Thanks Jon & Mark for the wealth of information contained within your posts, its is much appreciated.

I forgot to order the bridge rectifiers I found these (below) which should be fine as the originals were rated at 200V 2A, I will place an order today. In the meantime if the regulators turn up today I guess so I can continue, I could temporarily arrange some 1N4004 diodes as a bridge, until the correct ones arrive.

http://www.ebay.co.uk/itm/3-KBPC102-3A- ... 9300&rt=nc

Just added the regulator pin-outs (below) so its all documented in this thread for ease.

[crustytv]

The two pots, 3-way wave selector switch and output BNC have now been wired in rather than croc clipped. It all functions ( excuse the pun) as expected. Frequency and amplitude can be adjusted, the 3-way selector switch is a little intermittent on one section, it was a junk box find so probably needs some cleaning.

I found some suitable knobs which came off a scrap piece of test equipment.I've found a suitable box to house it all with plenty of scope for me to extend the device features in the future. I am quite tempted to add a frequency display module as mentioned earlier in the thread but that's for another day, perhaps.

Only need to decide on the final choice of PSU, encapsulated or dual board PSU. The L7815CV turned up on Saturday, I'm still waiting for the L7915CV and the KBPC102 3A 200V bridge rectifiers, then I can construct the 15V rails. If I go with the dual board supplies I will need to source a suitable TX

[crustytv]

Without a schematic or plan can I confirm the pin-outs are located correctly in reference to the board.

[TVJON74]

Hi Chris,
I looked for the datasheet over the weekend, but I cant find it (if I even had it in the first place)!
I have just got off the phone to RS and I have been sent what they have.
I will email it to you shortly.

Cheers
Jon

[Mark Hennessy]

Without a schematic or plan can I confirm the pin-outs are located correctly in reference to the board.

Looks good to me

[crustytv]

Thanks to Jon a data sheet for the PSU.

[crustytv]

I've started a new section in the Technical Library under "Component Data & Information". Its called "RS Projects", I've loaded the RS data sheets used for this project. As and when other RS project sheets turn up I will load them as if they are becoming hard to source we need to preserve them.

Off topic but I've just noted there are over 6,209 service data items in the library now

[Terrykc]

Would this be useful, Chris? Sorry I didn't have it done when you were struggling earlier!

[Rallycoordinator]

My example of a 9 volt +ve regulator board built using a 7809 ic, plus other recycled components may help:

As only a very low current supply was required, I used a small heat sink that was to hand.

Mike

[crustytv]

Hi Mike,

Nice to see another example, thanks. I see you used a 6800uF 35V instead of the 4700uF. Mark explained that I could get away with a lot less for my generator, in fact a 10th of that. This is handy as I don't have any 4700uF but have a bucket load of 470uF so that's what I'm going to use.

I'm just waiting on the bridge rectifiers then the boards will be complete, oh and the project box to arrive.

The final piece of the puzzle is to find a TX, I found one online, 120 VA Toroidal Transformer with 0-30, 0-30 VAC which is £22

I could of course just go with the encapsulated PSU which is much neater and simpler. The fun and education lies in the twin PSU build so I'll run with that and make the final decision on what powers this project at the end.

[Mark Hennessy]

The final piece of the puzzle is to find a TX, I found one online, 120 VA Toroidal Transformer with 0-30, 0-30 VAC which is £22

Quick note of caution - don't use this one!

If you put 30V AC into a bridge rectifier, you'll end up with around 45V DC! And the 7x15 chips won't like that up 'up

From memory, the highest input voltage for those chips is 35 volts.

I'd recommend 15 or 18 volt secondaries. No point going much higher as all you'll do is cause the voltage regulator ICs to sweat needlessly (the power they dissipate is governed by the voltage drop across them).

Also, you won't need anything like 120VA - as little as 6VA would be fine. Sure, have a bit more if that's what's available, but obviously the larger you go, the more it costs and the more space it takes.

Something like this would be fine if you like torroidal transformers (I do!): http://cpc.farnell.com/multicomp/mcta01 ... dp/FF01539

Or if you prefer frame types, this is ideal, and good value for money: http://www.rapidonline.com/electrical-p ... 8v-88-5308

(suppliers chosen at random, for the sake of example. Equivalent devices will be carried by all the supplies at hopefully comparable prices)

The oscillator PCB only draws a couple of tens of milliamps, so the required VA is tiny - a couple at most.

Hope this helps,

Mark

[TVJON74]

If you can hang on a day or two Chris, I have a box full of transformers and may have something usable for you.

[crustytv]

Thanks for the advice Mark, I knew I didn't have a clue what I was looking for and you confirmed it

Hi Jon,

absolutely no rush I've got lots of TX's but typically out of TV's or radios and not suitable.

[Rallycoordinator]

I see you used a 6800uF 35V instead of the 4700uF. Mark explained that I could get away with a lot less for my generator, in fact a 10th of that. This is handy as I don't have any 4700uF but have a bucket load of 470uF so that's what I'm going to use.

The only reason for using a 6800uf capacitor was that it was to hand, having been salvaged from an old printer. As already stated a much lower value could have been used.

Part of the fun of DIY construction, from my viewpoint, is selecting (within safety constraints) suitable components from the recycled spares box.

Mike

[Mark Hennessy]

Part of the fun of DIY construction, from my viewpoint, is selecting (within safety constraints) suitable components from the recycled spares box.

Completely agree

[Alistair D]

Just been searching one of my junk sorry spares boxes and came across a suitable transformer. The spec is:

30Va torroidal.
Two separate 15V windings.
60mm dia x 30mm high.
Weight 415g.

Earlier Mark mentioned adding a TDA2030 output amplifier board if one turned up. The advantage of doing this is that the generator can work into much lower load impedances without the level of the output being dragged down. This transformer would be able to supply enough current for the TDA2030 as well. The PSU heatsinks and smoothing caps would need to be suitable for supplying up to 1 amp though.

FOC + postage

Al

[crustytv]

I made a slight boo boo on estimating the size of project box I was going to need.

It arrived today and is way too small, not sure what to go for now as I didn't want a huge great box, I need to give this some thought now.

The 3A rectifiers arrived so now the two PUS boards have been fitted out.