Help me debug power up please

Hi,

Powering up my project. In the schematic below I measure the following.

An output of 14.2V a.c. from the transformer.
An output of 12.06V from the + & - pins on the bridge rectifier?
An output of 8.72V from the - pin on the rectifier and Pin 3 on the LM7812?

The following components are not in circuit but should make no difference
- LM741
- Relay
- Probe (thermistor)
- Temp Pot

Cheers
Andrew

PS: The board is consistent with the schematic according to EAGLE

An output of 14.2V a.c. from the transformer.
An output of 12.06V from the + & - pins on the bridge rectifier?
An output of 8.72V from the - pin on the rectifier and Pin 3 on the LM7812?

Click to expand...

hi Andrew,
For reading #3, which point on your circuit is the -V lead of the meter connected too.??
In other words when you measure with the meter which line is the reference 0V/Common.?

Hi Eric,

the solder joint where the - pinout is on the bridge rectifier.

Cheers
Andrew

Will be offline for an hour, need to get to the electronics store.

You don't appear to have any smoothing capacitor on your power supply and so the regulator is seeing full wave rectified AC and not smooth DC. Your circuit is effectively being turned on and off 100 times per second.

Mike.

Andrew Leigh said:

Will be offline for an hour, need to get to the electronics store.

Click to expand...

hi,
If the view of the 7812 is shown looking thru the pcb, is the 7812 the correct way around.? see 3rd image

Also another possibility is you have a different pin out version of the 7812, see 2nd image.

The 1st image is showing the 0V and +V copper, it looks OK to me.?
As Pommie says where are the smoothing caps.?

Thanks boys,

it has been said that to assume is to make and "ass" out of "u" and "me". It would appear as if my regulator has diffferent pinouts to any other regulator I have used. I normally use a LM78XX series. This is a L7812CV which appears to be different?? On the data sheet they show the same pinouts as the LM series (1=input, 2=Ground, 3=output) yet in their application notes they show pin2 as being out and pin 3 as ground??? Can you please clarify?

Eric the view is from the component side looking through the board.

Mike, aaahhh to be on the steep part of the learning curve. Your rebuke is taken, any ideas for the value?

Well looks like a new board for me, the relay package I used I cannot find locally and of course I need to make provision for a smoothing cap. Thanks

Andrew

PS: Would not mind dispensing with the transformer, a bit bulky and overkill. Anyway to get 220V down to 12 differently?

hi,
What are driving with the 5VA psu project.?

Sorry about the delay in responding, but it took some time to do 100 press ups for stupidly not noticing the caps were missing from you circuit.

You could move that fuse holder to along side the transformer and use the free space for a couple of 100uF and 100nF's.
IIRC is RSA there is no fuse in the mains plug,, so why not put the fuse in the mains LINE on the pcb.??

Hi Eric,

back to the old caravan again. The existing control has a hysteresis of ± 7.5°C, established through testing. This makes it impossible to use, fridge on full in the day freezes overnight due to the large change in ambient temperature, one can't keep changing the setting. With this unit I plan on having a temperature of 4°C ±1°C. I wrote a two part article for a local publication regarding absorption fridges. The data I collected was from a 3 week holiday in December. Max ambient 51.5°C, minimum 9.2°C. At night the lettuce and tomato's would on occasion freeze, more importantly, in the day my beers could drift up to 12°C UNACCEPTABLE.

This circuit has three pots to set temperature range, hysteresis and the fridge temperature. The circuit switches on the relay when the set point is reached (via a thermistor). The relay drives the 180W 220V heating element.

The relay coil draws 40mA. The 5VA is a chronic overkill but they were readily to hand. I would not think that anymore than 100mA is required to power this puppy.

Cheers
Andrew

hi,
Do you have a circuit for the existing control, perhaps we could make it more sensitive.?

I did have problems with my caravan fridge in RSA.
As you say first thing in a morning almost everything is frozen, by lunchtime when the van hits the low 40C's the ice starts to drip.!

I did improve it a lot by adding an extractor fan near the 'hot spots' on the fridge which draws the warm air out of the fridge casing, during the day and blows it outside thur the existing louvres.

I'm still using the fridges original thermostat.

Hi Eric,

the existing control is that thermo mechnical type with the bulb and capilliary tune filled with gas. The pressure in this contraption then activates a diaphram with activates the switch.

I did a pretty comprehensive study which resulted in me installing a special cowl on the vent plates. This was fitted with a fan that was thermally controlled. The fan I set to come on at about 28°C and gradually increased speed until about 45°C when it was at full revs. I got this circuit of the net from an Australian site (Elliott Sound Products). I also altered the venting and ducting of the baffles around the fridge (which were non existent). All the changes gave me back a further 8°C cooling.

It was only then, once she was really performing, did the poor performance of the controller manifest. I noticed on gas that there was less of a problem as the flame remained the same at all time for a given setting and did not switch on and off causing a lead and a lag in the cycle.

I know this circuit works, it is being used by the author. Unfortunately I build by numbers and rely heavily on replicating known working circuits. I have no ability to engineer electronics but do understand some really basic stuff.

Once I have tweaked it I will use fixed resistors for two of the pots and keep only one pot for dialing in the temperature. Would like to dispense with the relay though.

Thanks for the help
Andrew