portable power supplies and dew heaters

Hi

I have just ordered a Kendrick dew heater (basic model) and objective band heater for an 11" scope. To power the scope and heater I recently purchased one of the power pack units from Halfords. The pack has a 20 amp hour rating, a built in 220v invertor if required and a trickle charger so that it can be left on the charge when not in use. It also has a built in circuit breaker and polarity alarm. It only has one cigarette lighter plug though so I bought a simple adapter with four plug sockets for it.
If I use both the scope and dew heater from the same psu, has anyone experienced electrical problems associated with sudden voltage drops or spikes when the heater system clicks in which could affect the mount electronics (CGE). Should I get a seperate supply for the heater? Maplins are selling 17 amp hour jump start packs for 33 Euro at the moment.

John O'Mahony

John - I know nothng about amps, amp/hours etc but I know you can calculate the power requirements of your battery if you know the power requirements of the equipment attached to it.

That said, I use the Kendrick 33 amp/hr battery (four cig sockets) and I have a premier controller and the AP1200 mount attached. When the dewheater kicks in, the voltage drops alright (by a few tenths) but doesn't affect the mount.

I think others here would be able to advise you better.

Dave

John,

I don;t think you will have any problems with voltage. If you check most PSU's you will find widely varying voltages. Modern electronic equipment deals with this well. What you really need to think about is the amount of power the battery can deliver. Dew removal systems draw a lot of current. You can check the Kendrick site for specific power consumption details for your new gear. If memory serves me right an 11" tape can draw up to 4amps. If this is true your 20amp-hour battery would last 5 hours max.

The thing is you shouldn't drain lead acid batteries down to zero too often so in practise you will probably only get a couple of hours out of it. Also the more you recharge it the less capicity it will charge up to each time. The CGE doesn't draw too much power when just tracking but if you are slewing around a lot, it can draw over 1amp. This further reduces the amount of time you will get off one battery.

I bought a plug in PSU rated at 10amps from maplins. This will power all the gear I need and more besides. If you are ioperating from home this would be a far better alternative to a battery.

John,

Also if you have any worries about voltage damaging your equipment you can use a voltage regulator between the power supply and the equipment to ensure a steady supply to the target equipment. The little circuit below is an example of how to use it. It looks far more complicated that it is. The diode D1 also ensures that you wont damage anything if you manage to get the polarity wrong one day.



PS. The circuit above is for 12V equipment drawing up to 1amp. DO NOT USE it where more amps or anything other than 12V is required. If you wanted to use a circuit like this with equipment which draws anything other than the obove power. You would just need to replace IC1 with a regulator rated for whatever you need. You would probably need to replace C1 and C2 too. The datasheet for the regulator would tell you which capacitors are needed.[/img]

PPS as I write this it looks as if my website is down hence the missing circuit diagram. Should be back soon.

Thanks Dave

The kendrick site claims about 2.5 amps when on full capacity. The controller can be switched to 40% of this capacity if required so this would take the overall load down to an acceptable level on most evenings. Failing that I can either get an extra power pack or use one of the mains plugs with a cigarette socket I saw in Maplins yesterday (10 amps unregulated-possibly the one you mentioned-the regulation may not be so critical for a passive device). Thanks for the circuit diagram-this will come in handy. Time to borrow my fathers soldering irons and oscilliscopes again after 20 years.

cheers
John