Basic Electronics - Bridge rectifiers

[Mark_h]

Right you clever lot....

I've got a cheepie cordless drill. The charger's packed up.
(Anyone got a spare charger for a performance power drill part# 9704)

The drill itself if perfectly useful and I have two batteries for it so a bit loathed to bin it all for the sake of a £5 transformer.

Took it to bits and I'm getting nothing out of the secondary side of the input transformer. Looks pretty simple in there

mains - Big transformer - circuit board.
Circuit board has
- 4 chunky diodes (assuming it's a full wave bridge rectifier)
- Few little resistors/caps
- a relay
- couple of LEDs to indicate charging/charged

Now I know the transformer's shafted as I've de-soldered it from the circuit board, nothing on the output side at all (yes, meter is set to AC) and it eventually gets a little bit warm suggesting the primary side is at least a circuit.

So If I remember back to my school days you need to multiply something by root 2 to get the other thing but can't remember what you multiplied.

I need to generate 17.5v DC
I have 240v AC coming into a transformer into a Full wave BR so what secondary AC voltage do I need?

What's the chances of a Hornby power supply (18v AC) being fed into the BR producing anything useful on the output. Charger says it charges 17.5v @ 1.5A to charge a 14.4v battery.

Don't want to go cannibalising a perfectly good Hornby PSU just to find it's not going to do the job.

If not where on earth does Joe public go to get a transformer when the original has no part number or spec on it?

[atassiedevil]

So you need an 18v 27va transformer

http://www.ebay.co.uk/itm/TRANSFORMER-50VA-2X-18V-Part-Number-CTFC50-18-/390657473444?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item5af4fffba4

or

here

http://www.ebay.co.uk/itm/Encapsulated-Mains-Insulated-PCB-Power-Transformers-230-6-9-12-15-18-24-VAC-V-AC-/180858536621?pt=UK_BOI_Electrical_Components_Suppl ies_ET&var=&hash=item2a1c0242ad

the 2nd option you want the 40va 18v one.

[JM2]

Bollox, nice long technical reply got lost.
18v ac will give 25v dc max no load after a "C" smoothed bridged - bit high (c/f 17v).
If original transformer is running (warm/hum), check for secondary failure. Either winding termination break or (under outer cover) thermal or overcurrent fuse where the winding itself is joined to the pig tails. Obviously if it is all burnt out forget that.
Reverse engineering is tricky - need more detail on charging characteristics.

[Mark_h]

Bollox, nice long technical reply got lost.
18v ac will give 25v dc max no load after a "C" smoothed bridged - bit high (c/f 17v).
If original transformer is running (warm/hum), check for secondary failure. Either winding termination break or (under outer cover) thermal or overcurrent fuse where the winding itself is joined to the pig tails. Obviously if it is all burnt out forget that.
Reverse engineering is tricky - need more detail on charging characteristics.

So ACv * 1.4 = BR DC Output?
Does this mean I'll be needing a 12-13v secondary winding to feed into the BR to get c17.5v DC out?

It's a real cheap Transformer. I've peeled back the outer insulation so I can see where the leads are fixed onto the windings. Got 0V on the secondary windings. The rest of it looks like it's been put together in a way it's not going to be take-apart-able. No signs or burning, doesn't hum, buzz or smell, is just a bit warm after being left on for a couple of hours.

If I go for the Hornby 18v AC transformer option I assume that'll give me in the region of 25v DC which may fry the Ni-Cads. Could I stick in a Zenna or similar to drop a few V's or is that being too simplistic?

[JM2]

Yeah, but, a 12v secondary may not give enough output when loaded.
It all depends. It needs more info, what cells, what type etc.
Where did 17.5v DC come from - if defacto then redesign the input to the charger part with a nice stiff 17.5v dc supply. [Okay, my guess for ten, stack of 12 cell ni-cads]
Too simplistic, I doubt it - some real fancy kit has chargers made of nothing more than the transformer, rectifier and maybe a resistor, but they're designed for a particular charging job (and for your you could do with knowing the starting parameters).
[Edit Yup, as long as rated high enough, an appropiate zenner would help]

[Mark_h]

Managed to find an over-voltage, under-powered transformer to see if it would work.

17v (should be c12v) and not enough VA but thought it'd be worth a bash.

Wired it in, got 16v on the charge pins at no load so was feeling a bit optimistic.

Shoved the battery on it and it dropped to zero volts at the DC connector and decked the AC input.

So either it was some sort of crow-bar protection on the hornby supply protecting it from the over-draw or something else is amiss on the charge circuit.

Guess it'll all be going on the ever growing "come in handy" pile while I keep my eye open for a replacement charger.