The Sinclair QL Forum

Hi All,

Im brand new to the world of the QL and have acquired a QL PSU which is said to be in working order but befroe i plug it in I put a meter across the primary winding (live and nuteral on the plug). I get a reading of around 66ohms once it settles down which may be ok, but i dont have any experience of working with QL power supplies to reference that against.

Does this reading sound about right or would someone mind taking a measurement of a known working PSU they have and posting it here ?

Cheers
Shaun

Same as mine, about 67ohm.

Ah cool,

Thanks for taking the time to check that for me,

I feel confident about pluggin it in now,
Cheers
Shaun

Remember that DC resistance measurements take no account of the reactance - or effecive resistance to a 50Hz sign wave - which could be considerably higher than that figure

The resistance figure tends to suggest a fair number of windings are present which will mean a large inductance. A winding with 500mH inductance would give a reactance of 157 Ohms.

Pr0f wrote: Sun Dec 04, 2022 7:44 am ... A winding with 500mH inductance would give a reactance of 157 Ohms.

...And that would be very low for a primary reactance of that size transformer. The primary inductance will easily be in the tens of H region.

ones' complement wrote: Sat Dec 03, 2022 4:54 pmSame as mine, about 67ohm.

Why does your inductance meter say 93.5R? Just curious why DC resistance would be different between meters.

Ruptor wrote: Mon Dec 05, 2022 9:17 pm

ones' complement wrote: Sat Dec 03, 2022 4:54 pmSame as mine, about 67ohm.

Why does your inductance meter say 93.5R? Just curious why DC resistance would be different between meters.

The meter would be applying a constant DC voltage. The component tester (using ATmega328) would, I assume, be repeatedly applying a slow varying (or step) voltage to judge inductance, so the extra resistance value is from reactance. At 50Hz the reactance would add considerably to the effective resistance.

ones' complement wrote: Mon Dec 05, 2022 10:22 pmThe meter would be applying a constant DC voltage. The component tester (using ATmega328) would, I assume, be repeatedly applying a slow varying (or step) voltage to judge inductance, so the extra resistance value is from reactance. At 50Hz the reactance would add considerably to the effective resistance.

My inductance meter returns the identical resistance to a multimeter on a simple inductor but when I tried to measure the QL transformer inductance across the mains plug live & neutral it thinks it is a transistor. Perhaps the QL transformer has some protection or filter screwing things up.