Golborne Vintage Radio

Ok...confession time, I've not built a SS power supply for many moons, having said that the LM338 is rated at 5 amps max, various articles on the web suggest that you can vary it's output from 1.2 volts up to 30 odd volts, most valve heaters run at under 1amp (there are exceptions) 30 volts DC input to the regulator (or there about)would give a variable DC output to cover most of the more common type of valve heaters.

You could also use the 30 volts output from the transformer to supply -ve grid bias via a rectifier, filter and pot + a voltmeter.

If that would work ok then it might be better to construct it as a separate bench supply, that way it's easy to use it for other jobs, others will probably know more about SS regulators than I, Mark H springs to mind.

Lawrence.

Well, having bagged things quickly and then wished I hadn't (there will always be another) I wouldn't. It doesn't say and I don't want to get drawn into too deep a study so I may have missed it, that the 220V output is a completely separate wdg (winding) and you would want that. And you don't need 100VA ... think how much a valve takes ! How big is the thing and is it over specified anyway (probably!).

As to the other piece of electronics it waxes lyrical about over current but being variable it doesn't say anything about over voltage. So there you are with a rare valve with a 1.5V heater and it suddenly loses control, nice light bulb for a few moments.

With a transformer, of some sort, and separate voltages its simple with little to go wrong, apart from selecting the wrong tap. As to setting a repeatable voltage than a Variac on the input will do that for you and be a useful piece of kit for other applications. AVO did it of course by having a mains Xformer with selectable input tapings with a switch. In conjunction with the meter. If you haven't seen one of these try to get someone to show you a MK1 or MK 2 (the only ones I know).

Gary

Good advice Gary.
Design the circuit first. Check with you guys it makes sense. Then, and only then, buy the components.
Much more sensible.
Thanks for getting my feet back on the ground.
Nick

It might be possible to protect from over voltage with a crowbar, not to well up on SS designs, selectable zeners and a thyristor to trip a circuit breaker maybe.

Lawrence.

Just keep it KISS. Use a tapped transformer and a useful elsewhere Variac + accurate meter, again an essential and item. Variacs turn up all the time for few pounds.

Gary

I too would vote for Gary's KISS approach. A variac feeding a suitable transformer and a decent meter. Just about bullet proof and dead easy to use. Just remember to leave the variac turned down to zero when not in use and check again before switching it on. I understand you can get cheap DVM modules which are ideal for the meter. I've not used them but I'm sure those who have will give relevant advice.

If the maximum heater voltage you'll need is 6.3V then use a 6.3V transformer, or something not much more than that. If you need much more than that consider something with taps. Dual 6V secondaries will get you to 12V. You won't need more than that unless you're testing valves designed for series heater chains.

The other occasion this approach won't work is for battery valves which need DC for their filaments. In this case might I suggest - wait for it - a battery OK, it won't give a strictly accurate voltage but unless you're doing some really critical testing it won't matter.

I'd go along with that...

The LM388T is a good voltage regulator - I have used them - but there are a lot of challenges. The main one being thermal design. Get that wrong and the IC might fail with a short between input and output, which means the unregulated input voltage (9V or more) will find its way to the heater. A crowbar circuit to protect against this would be quite complex.

Variac+mains TX is a lot safer.

Regarding the cheap DVM modules from eBay and other places, I brought a few a couple of years back, and it's fair to say that they exceeded my expectations. They're not as good as the £20+ Lascar types, but it's hard to complain for the price. The only real concerns I had related to the safety of one designed to run straight from the mains, but that's easily avoided in a project like this.

Quote: it waxes lyrical about over current but being variable it doesn't say anything about over voltage.

That brings to mind an HT supply I have used for curve tracing, 0 - 550V (as I recall) set by panel switches. Easy to step the anode voltage by exactly 50V and plot a curve each time.  

Now Fluke made lovely supplies like this, and I have two, but this one was made by some company I never heard of, in Rahway, New Jersey, a town better known for stinky oil refineries.  It works beautifully, unless the mains power is interrupted, in which case it comes back on at full voltage!  I fixed it by installing a time-delay relay so it would not come back on after a dropout.  I never did smoke any rare valve before I discovered the quirk, but I might have.

(29-07-2015, 09:38 PM)Mark Hennessy Wrote: [ -> ]I'd go along with that...

The LM388T is a good voltage regulator - I have used them - but there are a lot of challenges. The main one being thermal design. Get that wrong and the IC might fail with a short between input and output, which means the unregulated input voltage (9V or more) will find its way to the heater. A crowbar circuit to protect against this would be quite complex.

Variac+mains TX is a lot safer.

Regarding the cheap DVM modules from eBay and other places, I brought a few a couple of years back, and it's fair to say that they exceeded my expectations. They're not as good as the £20+ Lascar types, but it's hard to complain for the price. The only real concerns I had related to the safety of one designed to run straight from the mains, but that's easily avoided in a project like this.

Power supply choice aside, would this be ok for over voltage crowbar protection:

http://www.ti.com/lit/ds/symlink/lm431.pdf

Lawrence.

Well it wont go lower than 2.5V and you would have to use it with a DC supply, regs and all that. Not KISS to me.

Gary