Circuit Design Help? rectifier-voltage peak detector.

hi, I have a newbie question.
I am trying to step up the voltage output, out of series of peltier devices. I can get 85 mV out of my peltiers and I want to step it up to around 3V. for that I ordered this device off the ebay: http://www.ebay.com/itm/Joule-Thief-No-toroid-inductor-SMD-DC-DC-/161080170207?ssPageName=ADME:X:RTQ:US:1123
it steps up the DC input but it outputs a digital pulse, and since I need a DC output, I thought that I would filter it by a full bridge rectifier (or peak detector), so I tried these two circuits:
1- http://en.wikipedia.org/wiki/File:RC_Filter.png
2- http://www.allaboutcircuits.com/vol_3/chpt_3/5.html
since morning I have been tweaking these circuits to achieve my goal but I get nothing on the output side. am I doing something wrong? Do you guys have any suggestion? it seems that input is way too small for this configuration to work.
thanks

I can get 85 mV out of my peltiers and I want to step it up to around 3V.

That is not enough voltage to drive a DC / DC converter.

Why are you using a peltier device as a voltage generator, they are normally used the other way round as a heat pump. You actually need to use a thermocouple pile for this, which has lots of junctions in series giving you a higher input voltage. However you need quite a thermal gradient to get enough power to drive anything.

it steps up the DC input but it outputs a digital pulse,

You sure of that?

so I tried these two circuits

These circuits need at least 0.7V into them to get the diode to conduct.

The basic theory behind Joule Thief converters, requires a minimum voltage of approx 0.5V to get them to oscillate.
They use a single transistor as a self oscillating converter and with only 85 mv for input , its extremely difficult to make a DC - Dc converter that will work.

Linear Technology makes the LTC3108, which is a DC-DC converter that does just what you want. It can produce 3 V or 5 V from as little as 20 mV input. The chips need a special step-up transformer and have their own rectification and power management circuitry built in. There is a guy who sells PCB modules based on the chip and I've bought a couple from him. You can generate 5V from the heat of your hand from a single, 127 element Peltier module (provided there is a heat sink on the other side, i.e. cooling fins). However, the total power output is very low and sufficient only for very low duty cycle devices, like remote sensors that send a radio burst every minute or two.

Here is his web site [OBSOLETE] LTC3108 Energy harvesting breakout board – Martin's corner on the web
Linear Tech also sells a breakout board but I think they want $150 for it.

You can actually get 5V out of a Peltier but they need to be manufactured using high temperature solders and other components. These modules are called "thermoelectric generators" and are typically run at 250 C on one side and 100 C on the other. There is an outfit that sells backpacking cook pots that use boiling water to cool the device, which generates about 5V at 1 amp, enough to charge an iPhone. Also about $150.

Edit: Don't try to run standard Peltier modules (intended for cooling) hot enough to generate several volts. Most aren't designed for temperatures above about 85 C and will be destroyed if overheated. You can buy high temperature TEG modules from several manufacturers on line for typically $35 apiece. I'm working with a couple I bought from Marlow Industries, http://www.marlow.com , but there are also these folks http://www.tecteg.com/modules.php?id=6 (and others).

Grumpy_Mike:
Why are you using a peltier device as a voltage generator, they are normally used the other way round as a heat pump. You actually need to use a thermocouple pile for this, which has lots of junctions in series giving you a higher input voltage.

I am trying to harvest body temperture and and I am using series of these: 2 x TEC1 12705 Thermoelectric Cooler TEC Peltier Thermo Cooling Generators | eBay
so you are saying that thermocouples are better in producing voltages (comparing to peltier devices)?

jremington:
Linear Technology makes the LTC3108, which is a DC-DC converter that does just what you want. It can produce 3 V or 5 V from as little as 20 mV input.
Here is his web site http://harizanov.com/wiki/wiki-home/ltc3108-energy-harvesting-breakout-board/

Thanks jremington, LTC3108 was what I was looking for. But this guy is not selling these pcb modules anymore, do you know anywhere else that I can get this stuff? or any schematics that I can build it myself?

jremington:
Linear Tech also sells a breakout board but I think they want $150 for it.

can you send me a link to this breakout, I am just curious…

jremington:
You can actually get 5V out of a Peltier but they need to be manufactured using high temperature solders and other components. These modules are called “thermoelectric generators” and are typically run at 250 C on one side and 100 C on the other.

are these “thermoelectric generators” commercially available for purchase?

Thanks again

I googled "ltc3108 breakout" and one of the top few hits was this: Home | Femtogen Technologies
The Linear Tech breakout was equally easy to find on their web site. LTC3108 Datasheet and Product Info | Analog Devices but the price is ridiculous.

I posted two links for companies selling TEG modules in my first post. Did you try them? As I stated, I bought a couple of high temp TEG modules from Marlow Industries for about $35 each. I had to sign a form agreeing not to export them. I suppose these modules would somehow aid terrorists.