Roband Vareco 33-10 PSU

[timewave]

My old Roband PSU has failed and I wonder if anyone has a circuit diagram.
It's a continuously variable 0-33V, variable current limit and adjustable overvoltage level unit.

I've checked the obvious and replaced 2 of the 4 2N3055 pass transistors which were suspect, but the internal 20A fuse still blows.

It's a very well protected unit and short of removing each semiconductor and capacitor for individual testing I'm stuck.

There are no signs of any components overheating.

I've carried out numerous searches, but no joy.

Cheers

John

[timewave]

OK, I've removed a shorted diode which appears to feed the gate of a thyristor. Without the diode in circuit the PSU functions ok for about 45 seconds and then the O/P volts slowly decay to zero.

Unfortunately I dont recognise the diode markings which are S2M3 and 7439.The manufacturers symbol looks like Motorola.

Any ideas?

Thanks

John

[kalee20]

I don't know this unit at all, but is it a 33V 10A unit? If so, that's a pretty big demand for a linear power supply.

It is possible that the mains transformer has several secondary tappings, and as you wind up the input voltage, just before the linear pass transistors run out of headroom, the transformer secondary taps are changed. Doing this minimises the dissipation in the series pass transistors at low output voltages and high currents.

If this is the case, then the tap changes could well be done by thyristors trigered 'on' only when necessary. The thyristor you mention could be one of them.

This may provide an insight ito the internal operation - linear power supplies can use some clever tricks, and if you have to trace out the circuit, don't be surprised if you find something like this.

Incidentally, you say the output voltage falls to zero in 45 seconds. Does applying a load affect the speed it falls at?

[timewave]

Thanks for the reply.
.
Yes it's a 33V 10A unit that has served me well for about 30 years.

There is one 45V winding on the transformer which is the main supply and there are two other low current windings (40V and 25v both at 50mA),these go to small bridge rectifiers on the PCB.

Applying a load drops the O/P voltage immediately but remember this is without the offending diode in circuit.

Thanks

John

[timewave]

More info.

The thyristor with the faulty diode connected to its gate is probably the crowbar protection circuit which would account for the 20A fuse being taken out.

Rectification seems to be done by 2 X thyristors on the positive side and 2 X stud type diodes on the negative side.The smoothing capacitor is 15,000uF.
I presume the thyristors are gated on the +ve half cycles and the -ve side diodes function in the normal fashion. How the gating is done is a mystery at the moment but I get the feeling that this is where the problem lies.
I'm guessing that the 2 low current windings come into play here, along with their associated brige rectifiers.

I have no experience of this type of rectification so if anyone can steer me in the right direction I'd really appreciate the advice.

Thanks

John

[Brian R Pateman]

I've moved this from "Information Wanted" as we now seem to be discussing what the fault might be.

Regards,

[KeithsTV]

I would think that the two thyristors in the rectifier are a pre regulator to reduce the input to the main regulator at low voltage settings. This would reduce the dissipation of the main regulator.

Check the gate drive to the thyristors. it should alter as the output voltage is changed.

HTH

Keith

[timewave]

It's bridge!!

Now that I've had chance to look more closely I realise that I've been suckered.

However I'm still not sure how those thyristors are controlled.

[KeithsTV]

That does look like some form of pre-regulator. The thyristors would be triggered after the peak of the AC input. This has the effect of reducing the voltage across 15000uF cap which will reduce the volt drop across and hence the dissipation of the series transistors. The later they are triggered in the cycle the lower the voltage across the cap. There should be a trigger circuit which is connected to one of the secondary windings on the transformer.

Are you sure that the diode across the output is the correct way round? It should be reverse biased at all times. It's purpose is to stop the output going negative with an inductive load.

The thyristor on the output is, as has been stated before, a crowbar which shorts out the output if, for example, the series transistors go short circuit.

Keith

[timewave]

Thanks for the comments Keith.

I have eventually traced the PCB back to a unijunction trigger circuit.Associated with it was a BAX13 high speed switching diode which was low resistance both ways.I've wacked in a 1N4148 and the PSU is now up and running.

Regarding the original faulty diode.The diode is between the unregulated and the regulated +ve rails so the regulated rail is always going to be more negative wrt the unregulated rail.Could the original have been included maybe to protect against higher voltages applied to the O/P. ( Battery charging?)

It took me a while but I got there and I feel like I've had an upper body workout.The PSU weighs 10Kg and I've lifted it and turned it over quite a few times this weekend.
I feel the need to celebrate!

Thanks all

John

[KeithsTV]

Glad to hear it's working now. We have something similar at work- almost a 2 man lift. We also have 0-30v 10A switch mode supplies which can be lifted with one hand.

Perhaps these earlier PSUs came with a free weight lifting course

Keith

[kalee20]

Well done for sorting the fault and fixing your unit.

Yes, the thyristors are working as a sort of pre-regulator circuit, exactly as KeithsTV says.

The faulty diode is normally reverse biased, but is there for protection in case a voltage is applied to the output. Battry charging is one example - connecting two power supplies in parallel is another. Without the diode, if the power supply is switched off but you stick a stiff voltage across the output, the series pass transistors would have their collectors at zero (because the big reservoir capacitor is not charged) but the emitters would be positive. More than a few volts of this kills them by base-emitter breakdown! So the reverse clamp diode is added. It needs to be a chunky diode, because under the abnormal conditions, it must pass enough current to drag up the reservoir capacitor's voltage to (nearly) equal the imposed output voltage.

[Tim]

I have a Roband bench PSU at home capable of 2x 60 volts at 5 amps per side!!(Variable current limit 0-5 amps etc.) That IS a two man lift! I was thinking of fitting some handles to make it easier to carry( or is that stagger?). Maybe I augt to consider wheels!!
Good gear Roband, I was impressed with the quality the one and only time I had the top off mine.
Trouble is, because it's up in the loft I only use it when I have to.
Well done for finding that fault.

[timewave]

Thanks everyone.

I have some hefty diodes so I will pop one in and that should do it.It's been a good exercise and I've learned a few things on the way.
All I need now is the circuit diagram and I will be happy.

I thought you could get anything and everything on the WWW but there seems to be very little reference to Roband equipment.

Cheers

John